improve ergonomics for the derive macro generated api

.gitignore

@@ -1,2 +1,3 @@
 /target
 Cargo.lock
+.DS_Store

Cargo.toml

@@ -3,6 +3,8 @@ name = "bad-optics"
 version = "0.1.0"
 edition = "2021"
 
-# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+[workspace]
+members = ["bad-optics-derive"]
 
 [dependencies]
+bad-optics-derive = { path = "./bad-optics-derive" }

README.md

@@ -0,0 +1,60 @@
+# bad optics
+
+ergonomic lenses in rust
+
+`bad-optics` implements the haskell concept of lenses, prisms, and traversals in rust
+
+it does *not* implement the operators, as it's not really a thing we can do in rust
+
+you will need nightly for this library to work, as it uses a bunch of unstable features. they're technically not *critical*, but i think not having them reduces the ergonomics too much
+
+does bringing lenses into rust actually make sense? probably not, but it was fun to implement so who can say
+
+## example
+
+```rust
+use bad_optics::{
+    lenses::{over, set},
+    prelude::*,
+};
+
+fn main() {
+    let a = ((1, 2), 3);
+
+    // use view to access inside the tuple
+    let res = view(_0, a);
+    assert_eq!(res, (1, 2));
+
+    let res = view(_1, a);
+    assert_eq!(res, 3);
+
+    // you can combine lenses
+    let lens = _0 + _1;
+
+    // use the view function to access
+    let res = view(lens, a);
+    assert_eq!(res, 2);
+
+    // you can also call the lens as a function
+    let res = lens(a);
+    assert_eq!(res, 2);
+
+    // call the over function to modify the value
+    let a = over(lens, a, |v| v + 1);
+    assert_eq!(a, ((1, 3), 3));
+
+    // call the set function to set the value
+    let a = set(lens, a, 5);
+    assert_eq!(a, ((1, 5), 3));
+
+    // you can also call the lens as a function to modify the value
+    let res = lens(a, |v| v + 1);
+    assert_eq!(res, ((1, 6), 3));
+}
+```
+
+## how to use
+
+bad-optics provides some of the lenses, prisms, and traversals defined in `lens`. i'm still trying to add more, so if there's one you need and it's missing from here, feel free to open a PR
+
+if you don't know how lenses work, this is not really gonna be a tutorial, you should read [this](https://hackage.haskell.org/package/lens-tutorial-1.0.3/docs/Control-Lens-Tutorial.html) first instead. the general idea is that they are first-class getters and setters

bad-optics-derive/Cargo.toml

@@ -0,0 +1,12 @@
+[package]
+name = "bad-optics-derive"
+version = "0.1.0"
+edition = "2021"
+
+[lib]
+proc-macro = true
+
+[dependencies]
+syn = {version = "1.0", features = ["extra-traits"]}
+quote = "1.0"
+proc-macro2 = "1.0"

bad-optics-derive/src/lib.rs

@@ -0,0 +1,168 @@
+use std::collections::HashMap;
+
+use proc_macro2::{Span, TokenStream};
+use quote::quote;
+use syn::{parse_macro_input, DataStruct, DeriveInput, Field, Ident, Lit, Meta, Type, Visibility};
+
+// TODO add attributes to rename the lens/module/skip making lenses/idk
+
+#[proc_macro_derive(Optics, attributes(mod_name))]
+pub fn derive(input: proc_macro::TokenStream) -> proc_macro::TokenStream {
+    // Parse the input tokens into a syntax tree
+    let input = parse_macro_input!(input as DeriveInput);
+
+    let name = &input.ident;
+    let mod_name = Ident::new(&get_mod_name(&input), Span::call_site());
+    let container_name = Ident::new(&get_container_name(&input), Span::call_site());
+
+    let expanded = match input.data {
+        syn::Data::Struct(s) => expand_struct(s, name, &mod_name, &container_name),
+        syn::Data::Enum(_) => todo!("not yet implemented for prisms"),
+        syn::Data::Union(_) => panic!("this macro does not work on unions"),
+    };
+
+    // Hand the output tokens back to the compiler
+    proc_macro::TokenStream::from(expanded)
+}
+
+fn get_mod_name(input: &DeriveInput) -> String {
+    for i in &input.attrs {
+        if let Ok(Meta::NameValue(meta)) = i.parse_meta() {
+            if let Some(ident) = meta.path.get_ident() {
+                if ident == "mod_name" {
+                    if let Lit::Str(a) = meta.lit {
+                        return a.value();
+                    }
+                }
+            }
+        }
+    }
+
+    input.ident.to_string().to_lowercase()
+}
+
+fn get_container_name(input: &DeriveInput) -> String {
+    for i in &input.attrs {
+        if let Ok(Meta::NameValue(meta)) = i.parse_meta() {
+            if let Some(ident) = meta.path.get_ident() {
+                if ident == "container_name" {
+                    if let Lit::Str(a) = meta.lit {
+                        return a.value();
+                    }
+                }
+            }
+        }
+    }
+
+    format!("{}LensContainer", input.ident.to_string())
+}
+
+fn expand_struct(
+    data: DataStruct,
+    name: &Ident,
+    mod_name: &Ident,
+    container_name: &Ident,
+) -> TokenStream {
+    let fields = match &data.fields {
+        syn::Fields::Named(n) => n.named.iter(),
+        syn::Fields::Unnamed(_) => todo!(),
+        syn::Fields::Unit => todo!(),
+    }
+    .filter(|f| matches!(f.vis, Visibility::Public(_)));
+
+    let lens_funcs = fields
+        .clone()
+        .map(|field| {
+            let fname = field.ident.as_ref().unwrap();
+            let ty = &field.ty;
+            quote! {
+                pub fn #fname() ->
+                    bad_optics::lenses::Lens<
+                        bad_optics::lenses::lens_with_ref::LensWithRef<
+                            bad_optics::lenses::Lens<
+                                bad_optics::lenses::lens::FuncLens<#name, #ty>
+                            >,
+                            bad_optics::lenses::Lens<
+                                bad_optics::lenses::to::ToRefInner<#name, #ty>
+                            >,
+                            #name
+                        >
+                    >
+                {
+                    bad_optics::field_lens_with_ref!(#name, #fname)
+                }
+            }
+        })
+        .collect::<TokenStream>();
+
+    let group_impls = group_by_type(fields)
+        .into_iter()
+        .map(|(ty, fields)| {
+            let lenses = fields
+                .into_iter()
+                .map(|field| {
+                    let fname = field.ident.unwrap();
+                    quote! {
+                        bad_optics::field_lens_with_ref!(#name, #fname),
+                    }
+                })
+                .collect::<TokenStream>();
+
+            quote! {
+                impl bad_optics::has_lens::HasLensOf<#ty> for #name {
+                    fn get() ->
+                          Vec<
+                              bad_optics::lenses::Lens<
+                                  bad_optics::lenses::lens_with_ref::LensWithRef<
+                                    bad_optics::lenses::Lens<
+                                        bad_optics::lenses::lens::FuncLens<#name, #ty>
+                                        >,
+                                      bad_optics::lenses::Lens<
+                                        bad_optics::lenses::to::ToRefInner<#name, #ty>
+                                        >,
+                                    #name
+                                  >
+                                >
+                            >
+                    {
+                        vec![
+                            #lenses
+                        ]
+                    }
+                }
+            }
+        })
+        .collect::<TokenStream>();
+
+    quote! {
+        mod #mod_name {
+            use super::*;
+
+            pub struct #container_name;
+
+            impl HasLens for #name {
+                type Lenses = #container_name;
+            }
+
+            impl #container_name {
+                #lens_funcs
+            }
+
+            #group_impls
+        }
+    }
+}
+
+fn group_by_type<'a>(fields: impl Iterator<Item = &'a Field>) -> Vec<(Type, Vec<Field>)> {
+    let mut map = HashMap::<Type, Vec<Field>>::new();
+
+    for field in fields {
+        if let Some(f) = map.get_mut(&field.ty) {
+            f.push(field.clone());
+        } else {
+            map.insert(field.ty.clone(), vec![field.clone()]);
+        }
+    }
+
+    map.into_iter().collect()
+}

examples/convoluted.rs

@@ -0,0 +1,48 @@
+use bad_optics::prelude::*;
+
+#[derive(Debug, PartialEq, Clone)]
+struct MyStruct {
+    hey: (u8, (u8, [Option<i32>; 5])),
+}
+
+fn main() {
+    // make a lens that accesses `hey` in `MyStruct`
+    let hey = lens(
+        |hello: MyStruct| hello.hey,
+        |mut hello: MyStruct, v| {
+            hello.hey = v;
+            hello
+        },
+    );
+
+    // the thing we want to access
+    let thing: (MyStruct, &'static str) = (
+        MyStruct {
+            hey: (1, (2, [None, Some(1), Some(2), None, Some(4)])),
+        },
+        "hello",
+    );
+
+    let array_lens = _0 // access the first element in the tuple
+              + hey // access hey
+              + _1 // access the second element in the tuple
+              + _1; // access the second element in the tuple
+
+    assert_eq!(array_lens(thing.clone()).len(), 5);
+
+    let lens = array_lens
+              + each // access each element of the [Option<i32>; 5] array
+              + _Some; // access the ones that are Some;
+
+    assert_eq!(lens(thing.clone()), vec![1, 2, 4]);
+
+    assert_eq!(
+        lens(thing, |v| v + 10),
+        (
+            MyStruct {
+                hey: (1, (2, [None, Some(11), Some(12), None, Some(14)])),
+            },
+            "hello",
+        )
+    );
+}

examples/derive.rs

@@ -0,0 +1,56 @@
+use bad_optics::prelude::*;
+
+#[macro_use]
+extern crate bad_optics;
+
+// the Optics derive macro will implement lenses for every public field in the struct
+// it makes a module named whatever the struct is called, but in lower case
+// you can rename the generated module by adding `#[mod_name = "other_name"]` to the struct
+
+#[derive(Optics, Clone, Debug)]
+pub struct MyStruct {
+    pub field1: String,
+    pub field2: String,
+
+    pub field3: u8,
+    _field4: u8,
+}
+
+fn main() {
+    let o = MyStruct {
+        field1: "first field".to_string(),
+        field2: "second field".to_string(),
+        field3: 12,
+        _field4: 1,
+    };
+
+    // we can manually get lenses for each field
+    // note that it's a function that returns a lens
+    //
+    // these lenses work for `MyStruct` with `view` and `over`,
+    // and for `&MyStruct` with `view`
+    let field1 = <MyStruct as HasLens>::Lenses::field1();
+    // short macro for convenience, expands to the line above
+    let field2 = lens!(MyStruct::field2);
+
+    // the lenses work normally as any other lens :)
+    assert_eq!(field1(&o), "first field");
+    assert_eq!(field2(&o), "second field");
+
+    // we can get a vec with all the lenses that match a type
+    let string_lenses = <MyStruct as HasLensOf<String>>::get();
+    assert_eq!(string_lenses.len(), 2);
+
+    // since _field4 is private, there's no lens for it
+    let u8_lenses = <MyStruct as HasLensOf<u8>>::get();
+    assert_eq!(u8_lenses.len(), 1);
+
+    // short macro for convenience, expands to the line above
+    let _u8_lenses = lenses!(MyStruct::u8);
+
+    let mut o = o;
+    for lens in string_lenses {
+        o = lens(o, |s| s.to_ascii_uppercase());
+    }
+    dbg!(o);
+}

examples/main.rs

@@ -0,0 +1,38 @@
+use bad_optics::{
+    lenses::{over, set},
+    prelude::*,
+};
+
+fn main() {
+    let a = ((1, 2), 3);
+
+    // use view to access inside the tuple
+    let res = view(_0, a);
+    assert_eq!(res, (1, 2));
+
+    let res = view(_1, a);
+    assert_eq!(res, 3);
+
+    // you can combine lenses
+    let lens = _0 + _1;
+
+    // use the view function to access
+    let res = view(lens, a);
+    assert_eq!(res, 2);
+
+    // you can also call the lens as a function
+    let res = lens(a);
+    assert_eq!(res, 2);
+
+    // call the over function to modify the value
+    let a = over(lens, a, |v| v + 1);
+    assert_eq!(a, ((1, 3), 3));
+
+    // call the set function to set the value
+    let a = set(lens, a, 5);
+    assert_eq!(a, ((1, 5), 3));
+
+    // you can also call the lens as a function to modify the value
+    let res = lens(a, |v| v + 1);
+    assert_eq!(res, ((1, 6), 3));
+}

examples/struct_fields.rs

@@ -0,0 +1,27 @@
+use bad_optics::prelude::*;
+
+#[derive(Debug, PartialEq, Clone)]
+struct MyStruct {
+    hey: (u8, (u8, i32)),
+}
+
+fn main() {
+    // make a lens for Hello
+    let hey = lens(
+        |hello: MyStruct| hello.hey,
+        |mut hello: MyStruct, v| {
+            hello.hey = v;
+            hello
+        },
+    );
+
+    let my_struct = MyStruct { hey: (1, (2, -3)) };
+
+    // the thing we want to access
+    let thing = (my_struct, "hello");
+
+    // a lens that targets the -3 inside my_struct
+    let lens_that_targets_the_i32 = _0 + hey + _1 + _1;
+
+    assert_eq!(lens_that_targets_the_i32(thing), -3);
+}

src/combinations.rs

@@ -1,41 +1,337 @@
-use crate::{Lens, LensOver, LensTrait, LensView};
+use crate::{
+    lenses::{Lens, LensOver, LensView},
+    prisms::{Prism, PrismPreview},
+    traversals::{Traversal, TraversalOver, TraversalTraverse},
+};
+use std::ops::Add;
 
+#[derive(Clone, Copy)]
 pub struct Combination<A, B>(A, B);
-impl<A, B> LensTrait for Combination<A, B> {}
 
-impl<A, B> std::ops::Add<Lens<B>> for Lens<A>
-where
-    A: LensTrait,
-    B: LensTrait,
-{
+// additions
+
+// lens + lens
+impl<A, B> const Add<Lens<B>> for Lens<A> {
     type Output = Lens<Combination<Lens<A>, Lens<B>>>;
 
     fn add(self, rhs: Lens<B>) -> Self::Output {
         Lens(Combination(self, rhs))
     }
 }
+// prism + prism
+impl<A, B> const Add<Prism<B>> for Prism<A> {
+    type Output = Prism<Combination<Prism<A>, Prism<B>>>;
 
-impl<A, B, T> LensView<T> for Combination<A, B>
+    fn add(self, rhs: Prism<B>) -> Self::Output {
+        Prism(Combination(self, rhs))
+    }
+}
+// traversal + traversal
+impl<A, B> const Add<Traversal<B>> for Traversal<A> {
+    type Output = Traversal<Combination<Traversal<A>, Traversal<B>>>;
+
+    fn add(self, rhs: Traversal<B>) -> Self::Output {
+        Traversal(Combination(self, rhs))
+    }
+}
+// traversal + lens
+impl<A, B> const Add<Lens<B>> for Traversal<A> {
+    type Output = Traversal<Combination<Traversal<A>, Traversal<Lens<B>>>>;
+
+    fn add(self, rhs: Lens<B>) -> Self::Output {
+        Traversal(Combination(self, rhs.to_traversal()))
+    }
+}
+// lens + traversal
+impl<A, B> const Add<Traversal<B>> for Lens<A> {
+    type Output = Traversal<Combination<Traversal<Lens<A>>, Traversal<B>>>;
+
+    fn add(self, rhs: Traversal<B>) -> Self::Output {
+        Traversal(Combination(self.to_traversal(), rhs))
+    }
+}
+// traversal + prism
+impl<A, B> const Add<Prism<B>> for Traversal<A> {
+    type Output = Traversal<Combination<Traversal<A>, Traversal<Prism<B>>>>;
+
+    fn add(self, rhs: Prism<B>) -> Self::Output {
+        Traversal(Combination(self, rhs.to_traversal()))
+    }
+}
+// prism + traversal
+impl<A, B> const Add<Traversal<B>> for Prism<A> {
+    type Output = Traversal<Combination<Traversal<Prism<A>>, Traversal<B>>>;
+
+    fn add(self, rhs: Traversal<B>) -> Self::Output {
+        Traversal(Combination(self.to_traversal(), rhs))
+    }
+}
+// lens + prism
+impl<A, B> const Add<Prism<B>> for Lens<A> {
+    type Output = Traversal<Combination<Traversal<Lens<A>>, Traversal<Prism<B>>>>;
+
+    fn add(self, rhs: Prism<B>) -> Self::Output {
+        Traversal(Combination(self.to_traversal(), rhs.to_traversal()))
+    }
+}
+// prism + traversal
+impl<A, B> const Add<Lens<B>> for Prism<A> {
+    type Output = Traversal<Combination<Traversal<Prism<A>>, Traversal<Lens<B>>>>;
+
+    fn add(self, rhs: Lens<B>) -> Self::Output {
+        Traversal(Combination(self.to_traversal(), rhs.to_traversal()))
+    }
+}
+
+// trait impls for Combination
+
+// lens + lens
+impl<A, B, T> LensView<T> for Combination<Lens<A>, Lens<B>>
 where
     A: LensView<T>,
     B: LensView<A::Field>,
 {
     type Field = B::Field;
 
-    fn view(thing: T) -> Self::Field {
-        B::view(A::view(thing))
+    fn view(&self, thing: T) -> Self::Field {
+        B::view(&self.1 .0, A::view(&self.0 .0, thing))
     }
 }
-
-impl<A, B, T> LensOver<T> for Combination<A, B>
+impl<A, B, T> LensOver<T> for Combination<Lens<A>, Lens<B>>
 where
     A: LensOver<T>,
     B: LensOver<A::Field>,
 {
-    fn over<F>(thing: T, f: F) -> T
+    fn over<F>(&self, thing: T, f: F) -> T
     where
         F: FnOnce(Self::Field) -> Self::Field,
     {
-        A::over(thing, |b| B::over(b, f))
+        A::over(&self.0 .0, thing, |b| B::over(&self.1 .0, b, f))
+    }
+}
+
+// prism + prism
+impl<A, B, T> PrismPreview<T> for Combination<Prism<A>, Prism<B>>
+where
+    A: PrismPreview<T>,
+    B: PrismPreview<A::Field>,
+{
+    type Field = B::Field;
+
+    fn preview(&self, thing: T) -> Option<Self::Field> {
+        A::preview(&self.0 .0, thing).and_then(|a| B::preview(&self.1 .0, a))
+    }
+
+    fn review(&self, thing: Self::Field) -> T {
+        A::review(&self.0 .0, B::review(&self.1 .0, thing))
+    }
+}
+
+// traversal + traversal
+// lens + traversal
+// traversal + lens
+// prism + traversal
+// traversal + prism
+// prism + lens
+// lens + prism
+impl<A, B, T> TraversalTraverse<T> for Combination<Traversal<A>, Traversal<B>>
+where
+    A: TraversalTraverse<T>,
+    B: TraversalTraverse<A::Field>,
+{
+    type Field = B::Field;
+
+    fn traverse(&self, thing: T) -> Vec<Self::Field> {
+        let a = A::traverse(&self.0 .0, thing);
+        a.into_iter()
+            .flat_map(|v| B::traverse(&self.1 .0, v))
+            .collect()
+    }
+}
+impl<A, B, T> TraversalOver<T> for Combination<Traversal<A>, Traversal<B>>
+where
+    A: TraversalOver<T>,
+    B: TraversalOver<A::Field>,
+{
+    fn over<F>(&self, thing: T, mut f: F) -> T
+    where
+        F: FnMut(Self::Field) -> Self::Field,
+    {
+        A::over(&self.0 .0, thing, |b| B::over(&self.1 .0, b, &mut f))
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::{
+        lenses::{_0, _1},
+        prisms::_Some,
+        traversals::each,
+    };
+
+    #[test]
+    fn can_view_lens_combination() {
+        let a = ((1, 2), 3);
+
+        let lens = _0 + _1;
+        let a = lens(a);
+        assert_eq!(a, 2);
+    }
+
+    #[test]
+    fn can_over_lens_combination() {
+        let a = ((1, 2), 3);
+
+        let lens = _0 + _1;
+        let a = lens(a, |v| v + 1);
+        assert_eq!(a, ((1, 3), 3));
+    }
+
+    #[test]
+    fn can_combine_prisms() {
+        let thing = Some(Some(3));
+
+        // combine two traversals
+        let res = (_Some + _Some)(thing, |v| v + 1);
+        assert_eq!(res, Some(Some(4)));
+    }
+
+    #[test]
+    fn can_combine_traversals() {
+        let array = [vec![1, 2], vec![3, 4]];
+
+        // combine two traversals
+        let res = (each + each)(array, |v| v + 1);
+        assert_eq!(res, [vec![2, 3], vec![4, 5]]);
+    }
+
+    #[test]
+    fn can_combine_traversal_with_lens() {
+        let array = [(1, 2), (3, 4), (5, 6)];
+
+        // combine a traversal with a lens
+        let t = each + _0;
+
+        // traverse
+        let res = t(array);
+        assert_eq!(res, vec![1, 3, 5]);
+
+        // over
+        let res = t(array, |v| v + 1);
+        assert_eq!(res, [(2, 2), (4, 4), (6, 6)]);
+    }
+
+    #[test]
+    fn can_combine_lens_with_traversal() {
+        let array = [(1, 2), (3, 4), (5, 6)];
+
+        // combine a traversal with a lens
+        let t = _0 + each;
+
+        // traverse
+        let res = t(array);
+        assert_eq!(res, vec![1, 2]);
+
+        // over
+        let res = t(array, |v| v + 1);
+        assert_eq!(res, [(2, 3), (3, 4), (5, 6)]);
+    }
+
+    #[test]
+    fn can_combine_prism_with_traversal() {
+        let array = [Some(1), None, Some(3), None, Some(5)];
+
+        // combine a traversal with a lens
+        let t = each + _Some;
+
+        // traverse
+        let res = t(array);
+        assert_eq!(res, vec![1, 3, 5]);
+
+        // over
+        let res = t(array, |v| v + 1);
+        assert_eq!(res, [Some(2), None, Some(4), None, Some(6)]);
+    }
+
+    #[test]
+    fn can_combine_traversal_with_prism() {
+        let array = Some([1, 2, 3]);
+
+        // combine a traversal with a lens
+        let t = _Some + each;
+
+        // traverse
+        let res = t(array);
+        assert_eq!(res, vec![1, 2, 3]);
+
+        // over
+        let res = t(array, |v| v + 1);
+        assert_eq!(res, Some([2, 3, 4]));
+
+        let array: Option<[i32; 3]> = None;
+        // traverse
+        let res = t(array);
+        assert_eq!(res, vec![]);
+
+        // over
+        let res = t(array, |v| v + 1);
+        assert_eq!(res, None);
+    }
+
+    #[test]
+    fn can_combine_prism_with_lens() {
+        let thing = Some((1, 2));
+
+        // combine a traversal with a lens
+        let t = _Some + _0;
+
+        // NOTE: combination of a prism and a lens is a traversal
+        //
+        // > The optic kind resulting from a composition is the least upper bound (join)
+        // > of the optic kinds being composed, if it exists.
+        // > The Join type family computes the least upper bound given two optic kind tags.
+        // > For example the Join of a Lens and a Prism is an AffineTraversal.
+        //
+        // from: https://hackage.haskell.org/package/optics-0.4/docs/Optics.html
+
+        // traversal
+        let res = t(thing);
+        assert_eq!(res, vec![1]);
+
+        // over
+        let res = t(thing, |v| v + 1);
+        assert_eq!(res, Some((2, 2)));
+    }
+
+    #[test]
+    fn can_combine_lens_with_prism() {
+        let thing = (Some(1), 2);
+
+        // combine a traversal with a lens
+        let t = _0 + _Some;
+
+        // NOTE: combination of a lens and a prism is a traversal
+        // see can_combine_prism_with_lens for more info
+
+        // traversal
+        let res = t(thing);
+        assert_eq!(res, vec![1]);
+
+        // over
+        let res = t(thing, |v| v + 1);
+        assert_eq!(res, (Some(2), 2));
+    }
+
+    #[test]
+    fn can_combine_as_const() {
+        use crate::lenses::first::_0Inner;
+        use crate::lenses::Lens;
+        const LENS: Lens<super::Combination<Lens<_0Inner>, Lens<_0Inner>>> = _0 + _0;
+
+        let thing: ((i32, i32), i32) = ((1, 2), 3);
+
+        let r: i32 = LENS(thing);
+
+        assert_eq!(r, 1);
     }
 }

src/fns.rs

@@ -0,0 +1,185 @@
+use crate::{
+    lenses::{Lens, LensOver, LensView},
+    prisms::{Prism, PrismPreview},
+    traversals::{Traversal, TraversalOver, TraversalTraverse},
+};
+
+// lens view
+impl<L, A> std::ops::FnOnce<(A,)> for Lens<L>
+where
+    L: LensView<A>,
+{
+    type Output = L::Field;
+
+    extern "rust-call" fn call_once(self, args: (A,)) -> Self::Output {
+        L::view(&self.0, args.0)
+    }
+}
+impl<L, A> std::ops::FnMut<(A,)> for Lens<L>
+where
+    L: LensView<A>,
+{
+    extern "rust-call" fn call_mut(&mut self, args: (A,)) -> Self::Output {
+        L::view(&self.0, args.0)
+    }
+}
+impl<L, A> std::ops::Fn<(A,)> for Lens<L>
+where
+    L: LensView<A>,
+{
+    extern "rust-call" fn call(&self, args: (A,)) -> Self::Output {
+        L::view(&self.0, args.0)
+    }
+}
+
+// lens over
+impl<L, A, F> std::ops::FnOnce<(A, F)> for Lens<L>
+where
+    L: LensOver<A>,
+    F: FnOnce(L::Field) -> L::Field,
+{
+    type Output = A;
+
+    extern "rust-call" fn call_once(self, args: (A, F)) -> Self::Output {
+        L::over(&self.0, args.0, args.1)
+    }
+}
+impl<L, A, F> std::ops::FnMut<(A, F)> for Lens<L>
+where
+    L: LensOver<A>,
+    F: FnOnce(L::Field) -> L::Field,
+{
+    extern "rust-call" fn call_mut(&mut self, args: (A, F)) -> Self::Output {
+        L::over(&self.0, args.0, args.1)
+    }
+}
+impl<L, A, F> std::ops::Fn<(A, F)> for Lens<L>
+where
+    L: LensOver<A>,
+    F: FnOnce(L::Field) -> L::Field,
+{
+    extern "rust-call" fn call(&self, args: (A, F)) -> Self::Output {
+        L::over(&self.0, args.0, args.1)
+    }
+}
+
+// traversal traverse
+impl<L, A> std::ops::FnOnce<(A,)> for Traversal<L>
+where
+    L: TraversalTraverse<A>,
+{
+    type Output = Vec<L::Field>;
+
+    extern "rust-call" fn call_once(self, args: (A,)) -> Self::Output {
+        L::traverse(&self.0, args.0)
+    }
+}
+impl<L, A> std::ops::FnMut<(A,)> for Traversal<L>
+where
+    L: TraversalTraverse<A>,
+{
+    extern "rust-call" fn call_mut(&mut self, args: (A,)) -> Self::Output {
+        L::traverse(&self.0, args.0)
+    }
+}
+impl<L, A> std::ops::Fn<(A,)> for Traversal<L>
+where
+    L: TraversalTraverse<A>,
+{
+    extern "rust-call" fn call(&self, args: (A,)) -> Self::Output {
+        L::traverse(&self.0, args.0)
+    }
+}
+
+// traversal over
+impl<L, A, F> std::ops::FnOnce<(A, F)> for Traversal<L>
+where
+    L: TraversalOver<A>,
+    F: FnMut(L::Field) -> L::Field,
+{
+    type Output = A;
+
+    extern "rust-call" fn call_once(self, args: (A, F)) -> Self::Output {
+        L::over(&self.0, args.0, args.1)
+    }
+}
+impl<L, A, F> std::ops::FnMut<(A, F)> for Traversal<L>
+where
+    L: TraversalOver<A>,
+    F: FnMut(L::Field) -> L::Field,
+{
+    extern "rust-call" fn call_mut(&mut self, args: (A, F)) -> Self::Output {
+        L::over(&self.0, args.0, args.1)
+    }
+}
+impl<L, A, F> std::ops::Fn<(A, F)> for Traversal<L>
+where
+    L: TraversalOver<A>,
+    F: FnMut(L::Field) -> L::Field,
+{
+    extern "rust-call" fn call(&self, args: (A, F)) -> Self::Output {
+        L::over(&self.0, args.0, args.1)
+    }
+}
+
+// prism preview
+impl<L, A> std::ops::FnOnce<(A,)> for Prism<L>
+where
+    L: PrismPreview<A>,
+{
+    type Output = Option<L::Field>;
+
+    extern "rust-call" fn call_once(self, args: (A,)) -> Self::Output {
+        L::preview(&self.0, args.0)
+    }
+}
+impl<L, A> std::ops::FnMut<(A,)> for Prism<L>
+where
+    L: PrismPreview<A>,
+{
+    extern "rust-call" fn call_mut(&mut self, args: (A,)) -> Self::Output {
+        L::preview(&self.0, args.0)
+    }
+}
+impl<L, A> std::ops::Fn<(A,)> for Prism<L>
+where
+    L: PrismPreview<A>,
+{
+    extern "rust-call" fn call(&self, args: (A,)) -> Self::Output {
+        L::preview(&self.0, args.0)
+    }
+}
+
+// prism over
+impl<L, A, F> std::ops::FnOnce<(A, F)> for Prism<L>
+where
+    A: Clone,
+    L: PrismPreview<A>,
+    F: FnMut(L::Field) -> L::Field,
+{
+    type Output = A;
+
+    extern "rust-call" fn call_once(self, args: (A, F)) -> Self::Output {
+        L::over(&self.0, args.0, args.1)
+    }
+}
+impl<L, A, F> std::ops::FnMut<(A, F)> for Prism<L>
+where
+    A: Clone,
+    L: PrismPreview<A>,
+    F: FnMut(L::Field) -> L::Field,
+{
+    extern "rust-call" fn call_mut(&mut self, args: (A, F)) -> Self::Output {
+        L::over(&self.0, args.0, args.1)
+    }
+}
+impl<L, A, F> std::ops::Fn<(A, F)> for Prism<L>
+where
+    A: Clone,
+    L: PrismPreview<A>,
+    F: FnMut(L::Field) -> L::Field,
+{
+    extern "rust-call" fn call(&self, args: (A, F)) -> Self::Output {
+        L::over(&self.0, args.0, args.1)
+    }
+}

src/lenses/fields.rs

@@ -0,0 +1,26 @@
+#[macro_export]
+macro_rules! field_lens {
+    ($type:ident, $field:ident) => {
+        $crate::lenses::lens(
+            |v: $type| v.$field,
+            |mut u: $type, v| {
+                u.$field = v;
+                u
+            },
+        )
+    };
+}
+
+#[macro_export]
+macro_rules! field_lens_with_ref {
+    ($type:ident, $field:ident) => {
+        $crate::lenses::lens_with_ref(
+            |v: $type| v.$field,
+            |mut u: $type, v| {
+                u.$field = v;
+                u
+            },
+            |v: &$type| v.$field.clone(),
+        )
+    };
+}

src/lenses/fifth.rs

@@ -0,0 +1,108 @@
+use crate::lenses::{Lens, LensOver, LensView};
+
+#[derive(Clone, Copy)]
+pub struct _4Inner;
+pub const _4: Lens<_4Inner> = Lens(_4Inner);
+
+macro_rules! make_tuples {
+    ($f:ident, ( $( $v:ident ),* ), ( $( $t:ident ),* ) ) => {
+        impl< $($t,)* > LensView<( $($t,)* )> for _4Inner {
+            type Field = T;
+
+            fn view(&self, ( $($v,)* ): ($($t,)*)) -> Self::Field {
+                $f
+            }
+        }
+        impl< $($t,)* > LensOver<( $($t,)* )> for _4Inner {
+            fn over<F>(
+                &self,
+                mut tup: ($($t,)*),
+                f: F
+            ) -> ( $($t,)* )
+            where
+              F: FnOnce(Self::Field) -> Self::Field
+            {
+                tup.4 = f(tup.4);
+                tup
+            }
+        }
+
+        impl<'a, $($t,)* > LensView<&'a ( $($t,)* )> for _4Inner {
+            type Field = &'a T;
+
+            fn view(&self, ( $($v,)* ): &'a ($($t,)*)) -> Self::Field {
+                $f
+            }
+        }
+        impl<'a, $($t,)* > LensView<&'a mut ( $($t,)* )> for _4Inner {
+            type Field = &'a mut T;
+
+            fn view(&self, ( $($v,)* ): &'a mut ($($t,)*)) -> Self::Field {
+                $f
+            }
+        }
+    };
+}
+
+make_tuples!(t, (_u, _v, _w, _x, t), (U, V, W, X, T));
+make_tuples!(t, (_u, _v, _w, _x, t, _y), (U, V, W, X, T, Y));
+make_tuples!(t, (_u, _v, _w, _x, t, _y, _z), (U, V, W, X, T, Y, Z));
+make_tuples!(t, (_u, _v, _w, _x, t, _y, _z, _a), (U, V, W, X, T, Y, Z, A));
+make_tuples!(
+    t,
+    (_u, _v, _w, _x, t, _y, _z, _a, _b),
+    (U, V, W, X, T, Y, Z, A, B)
+);
+make_tuples!(
+    t,
+    (_u, _v, _w, _x, t, _y, _z, _a, _b, _c),
+    (U, V, W, X, T, Y, Z, A, B, C)
+);
+// not doing more cause i'm lazy, open a pr if you need more :)
+
+macro_rules! make_arrays {
+    ($f:ident, $n:expr, [$( $v:ident ),*]) => {
+      impl<T> LensView<[T; $n]> for _4Inner {
+            type Field = T;
+
+            fn view(&self, [ $($v,)* ]: [T; $n]) -> Self::Field {
+                $f
+            }
+        }
+      impl<T> LensOver<[T; $n]> for _4Inner {
+            fn over<F>(
+              &self,
+              tup: [T; $n],
+              fun: F
+            ) -> [T; $n]
+            where
+              F: FnOnce(Self::Field) -> Self::Field
+            {
+              let [$($v,)*] = tup;
+              let $f = fun( $f );
+              [$($v,)*]
+            }
+        }
+
+      impl<'a, T> LensView<&'a [T; $n]> for _4Inner {
+            type Field = &'a T;
+
+            fn view(&self, [ $($v,)* ]: &'a [T; $n]) -> Self::Field {
+                $f
+            }
+        }
+      impl<'a, T> LensView<&'a mut [T; $n]> for _4Inner {
+            type Field = &'a mut T;
+
+            fn view(&self, [ $($v,)* ]: &'a mut [T; $n]) -> Self::Field {
+                $f
+            }
+        }
+    };
+}
+
+make_arrays!(t, 5, [_a, _b, _c, _d, t]);
+make_arrays!(t, 6, [_a, _b, _c, _d, t, _e]);
+make_arrays!(t, 7, [_a, _b, _c, _d, t, _e, _g]);
+make_arrays!(t, 8, [_a, _b, _c, _d, t, _e, _g, _h]);
+make_arrays!(t, 9, [_a, _b, _c, _d, t, _e, _g, _h, _i]);

src/lenses/first.rs

@@ -1,20 +1,21 @@
-use crate::{Lens, LensOver, LensTrait, LensView};
+use crate::lenses::{Lens, LensOver, LensView};
 
+#[derive(Clone, Copy)]
 pub struct _0Inner;
 pub const _0: Lens<_0Inner> = Lens(_0Inner);
-impl LensTrait for _0Inner {}
 
 macro_rules! make_tuples {
     ($f:ident, ( $( $v:ident ),* ), ( $( $t:ident ),* ) ) => {
       impl< $($t,)* > LensView<( $($t,)* )> for _0Inner {
             type Field = T;
 
-            fn view(( $($v,)* ): ($($t,)*)) -> Self::Field {
+            fn view(&self, ( $($v,)* ): ($($t,)*)) -> Self::Field {
                 $f
             }
         }
-      impl< $($t,)* > LensOver<( $($t,)* )> for _0Inner {
+        impl< $($t,)* > LensOver<( $($t,)* )> for _0Inner {
             fn over<F>(
+                &self,
                 mut tup: ($($t,)*),
                 f: F
             ) -> ( $($t,)* )
@@ -26,17 +27,17 @@ macro_rules! make_tuples {
             }
         }
 
-      impl<'a, $($t,)* > LensView<&'a ( $($t,)* )> for _0Inner {
+        impl<'a, $($t,)* > LensView<&'a ( $($t,)* )> for _0Inner {
             type Field = &'a T;
 
-            fn view(( $($v,)* ): &'a ($($t,)*)) -> Self::Field {
+            fn view(&self, ( $($v,)* ): &'a ($($t,)*)) -> Self::Field {
                 $f
             }
         }
-      impl<'a, $($t,)* > LensView<&'a mut ( $($t,)* )> for _0Inner {
+        impl<'a, $($t,)* > LensView<&'a mut ( $($t,)* )> for _0Inner {
             type Field = &'a mut T;
 
-            fn view(( $($v,)* ): &'a mut ($($t,)*)) -> Self::Field {
+            fn view(&self, ( $($v,)* ): &'a mut ($($t,)*)) -> Self::Field {
                 $f
             }
         }
@@ -56,12 +57,13 @@ macro_rules! make_arrays {
       impl<T> LensView<[T; $n]> for _0Inner {
             type Field = T;
 
-            fn view([ $($v,)* ]: [T; $n]) -> Self::Field {
+            fn view(&self, [ $($v,)* ]: [T; $n]) -> Self::Field {
                 $f
             }
         }
       impl<T> LensOver<[T; $n]> for _0Inner {
             fn over<F>(
+              &self,
               tup: [T; $n],
               fun: F
             ) -> [T; $n]
@@ -77,14 +79,14 @@ macro_rules! make_arrays {
       impl<'a, T> LensView<&'a [T; $n]> for _0Inner {
             type Field = &'a T;
 
-            fn view([ $($v,)* ]: &'a [T; $n]) -> Self::Field {
+            fn view(&self, [ $($v,)* ]: &'a [T; $n]) -> Self::Field {
                 $f
             }
         }
       impl<'a, T> LensView<&'a mut [T; $n]> for _0Inner {
             type Field = &'a mut T;
 
-            fn view([ $($v,)* ]: &'a mut [T; $n]) -> Self::Field {
+            fn view(&self, [ $($v,)* ]: &'a mut [T; $n]) -> Self::Field {
                 $f
             }
         }

src/lenses/fourth.rs

@@ -0,0 +1,110 @@
+use crate::lenses::{Lens, LensOver, LensView};
+
+#[derive(Clone, Copy)]
+pub struct _3Inner;
+pub const _3: Lens<_3Inner> = Lens(_3Inner);
+
+macro_rules! make_tuples {
+    ($f:ident, ( $( $v:ident ),* ), ( $( $t:ident ),* ) ) => {
+        impl< $($t,)* > LensView<( $($t,)* )> for _3Inner {
+            type Field = T;
+
+            fn view(&self, ( $($v,)* ): ($($t,)*)) -> Self::Field {
+                $f
+            }
+        }
+        impl< $($t,)* > LensOver<( $($t,)* )> for _3Inner {
+            fn over<F>(
+                &self,
+                mut tup: ($($t,)*),
+                f: F
+            ) -> ( $($t,)* )
+            where
+              F: FnOnce(Self::Field) -> Self::Field
+            {
+                tup.3 = f(tup.3);
+                tup
+            }
+        }
+
+        impl<'a, $($t,)* > LensView<&'a ( $($t,)* )> for _3Inner {
+            type Field = &'a T;
+
+            fn view(&self, ( $($v,)* ): &'a ($($t,)*)) -> Self::Field {
+                $f
+            }
+        }
+        impl<'a, $($t,)* > LensView<&'a mut ( $($t,)* )> for _3Inner {
+            type Field = &'a mut T;
+
+            fn view(&self, ( $($v,)* ): &'a mut ($($t,)*)) -> Self::Field {
+                $f
+            }
+        }
+    };
+}
+
+make_tuples!(t, (_u, _v, _w, t), (U, V, W, T));
+make_tuples!(t, (_u, _v, _w, t, _x), (U, V, W, T, X));
+make_tuples!(t, (_u, _v, _w, t, _x, _y), (U, V, W, T, X, Y));
+make_tuples!(t, (_u, _v, _w, t, _x, _y, _z), (U, V, W, T, X, Y, Z));
+make_tuples!(t, (_u, _v, _w, t, _x, _y, _z, _a), (U, V, W, T, X, Y, Z, A));
+make_tuples!(
+    t,
+    (_u, _v, _w, t, _x, _y, _z, _a, _b),
+    (U, V, W, T, X, Y, Z, A, B)
+);
+make_tuples!(
+    t,
+    (_u, _v, _w, t, _x, _y, _z, _a, _b, _c),
+    (U, V, W, T, X, Y, Z, A, B, C)
+);
+// not doing more cause i'm lazy, open a pr if you need more :)
+
+macro_rules! make_arrays {
+    ($f:ident, $n:expr, [$( $v:ident ),*]) => {
+      impl<T> LensView<[T; $n]> for _3Inner {
+            type Field = T;
+
+            fn view(&self, [ $($v,)* ]: [T; $n]) -> Self::Field {
+                $f
+            }
+        }
+      impl<T> LensOver<[T; $n]> for _3Inner {
+            fn over<F>(
+              &self,
+              tup: [T; $n],
+              fun: F
+            ) -> [T; $n]
+            where
+              F: FnOnce(Self::Field) -> Self::Field
+            {
+              let [$($v,)*] = tup;
+              let $f = fun( $f );
+              [$($v,)*]
+            }
+        }
+
+      impl<'a, T> LensView<&'a [T; $n]> for _3Inner {
+            type Field = &'a T;
+
+            fn view(&self, [ $($v,)* ]: &'a [T; $n]) -> Self::Field {
+                $f
+            }
+        }
+      impl<'a, T> LensView<&'a mut [T; $n]> for _3Inner {
+            type Field = &'a mut T;
+
+            fn view(&self, [ $($v,)* ]: &'a mut [T; $n]) -> Self::Field {
+                $f
+            }
+        }
+    };
+}
+
+make_arrays!(t, 4, [_a, _b, _c, t]);
+make_arrays!(t, 5, [_a, _b, _c, t, _d]);
+make_arrays!(t, 6, [_a, _b, _c, t, _d, _e]);
+make_arrays!(t, 7, [_a, _b, _c, t, _d, _e, _g]);
+make_arrays!(t, 8, [_a, _b, _c, t, _d, _e, _g, _h]);
+make_arrays!(t, 9, [_a, _b, _c, t, _d, _e, _g, _h, _i]);

src/lenses/identity.rs

@@ -1,18 +1,19 @@
-use crate::{LensOver, LensTrait, LensView};
+use crate::lenses::{Lens, LensOver, LensView};
 
-#[allow(non_camel_case_types)]
-pub struct id;
+#[derive(Clone, Copy)]
+pub struct IdInner;
+#[allow(non_upper_case_globals)]
+pub const id: Lens<IdInner> = Lens(IdInner);
 
-impl LensTrait for id {}
-impl<T> LensView<T> for id {
+impl<T> LensView<T> for IdInner {
     type Field = T;
 
-    fn view(thing: T) -> Self::Field {
+    fn view(&self, thing: T) -> Self::Field {
         thing
     }
 }
-impl<T> LensOver<T> for id {
-    fn over<F>(thing: T, f: F) -> T
+impl<T> LensOver<T> for IdInner {
+    fn over<F>(&self, thing: T, f: F) -> T
     where
         F: FnOnce(Self::Field) -> Self::Field,
     {

src/lenses/lens.rs

@@ -0,0 +1,45 @@
+use std::sync::Arc;
+
+use crate::lenses::{Lens, LensOver, LensView};
+
+type Getter<T, U> = dyn Fn(T) -> U;
+type Setter<T, U> = dyn Fn(T, U) -> T;
+
+#[derive(Clone)]
+pub struct FuncLens<T, U>(pub(crate) Arc<Getter<T, U>>, pub(crate) Arc<Setter<T, U>>);
+
+impl<T, U> LensView<T> for FuncLens<T, U> {
+    type Field = U;
+
+    fn view(&self, thing: T) -> Self::Field {
+        (self.0)(thing)
+    }
+}
+impl<T: Clone, U> LensOver<T> for FuncLens<T, U> {
+    fn over<F>(&self, thing: T, f: F) -> T
+    where
+        F: FnOnce(Self::Field) -> Self::Field,
+    {
+        let v = f((self.0)(thing.clone()));
+        (self.1)(thing, v)
+    }
+
+    fn set(&self, thing: T, v: Self::Field) -> T {
+        (self.1)(thing, v)
+    }
+}
+
+/// Makes a lens that implements `LensView<T>` and `LensOver<T>` with the provided functions
+pub fn lens_from_arc<T, U>(
+    getter: Arc<Getter<T, U>>,
+    setter: Arc<Setter<T, U>>,
+) -> Lens<FuncLens<T, U>> {
+    Lens(FuncLens(getter, setter))
+}
+/// Makes a lens that implements `LensView<T>` and `LensOver<T>` with the provided functions
+pub fn lens<T, U>(
+    getter: impl Fn(T) -> U + 'static,
+    setter: impl Fn(T, U) -> T + 'static,
+) -> Lens<FuncLens<T, U>> {
+    Lens(FuncLens(Arc::new(getter), Arc::new(setter)))
+}

src/lenses/lens_with_ref.rs

@@ -0,0 +1,83 @@
+use crate::lenses::{
+    lens,
+    lens::FuncLens,
+    lens_from_arc,
+    to::{to_ref, to_ref_from_arc, ToRefInner},
+    Lens, LensOver, LensView,
+};
+use std::sync::Arc;
+
+#[derive(Clone, Copy)]
+/// Lens that works on both a T and &T
+pub struct LensWithRef<A, B, T>(A, B, std::marker::PhantomData<T>);
+
+impl<'a, A, B, T> LensView<T> for LensWithRef<Lens<A>, B, T>
+where
+    T: Clone,
+    A: LensView<T>,
+{
+    type Field = A::Field;
+
+    fn view(&self, thing: T) -> Self::Field {
+        A::view(&self.0 .0, thing)
+    }
+}
+impl<'a, A, B, T> LensOver<T> for LensWithRef<Lens<A>, B, T>
+where
+    T: Clone,
+    A: LensOver<T>,
+{
+    fn over<F>(&self, thing: T, f: F) -> T
+    where
+        F: FnOnce(Self::Field) -> Self::Field,
+    {
+        A::over(&self.0 .0, thing, f)
+    }
+}
+
+impl<'a, A, B, T> LensView<&'a T> for LensWithRef<A, Lens<B>, T>
+where
+    T: Clone,
+    A: LensView<T>,
+    B: LensView<&'a T>,
+{
+    type Field = B::Field;
+
+    fn view(&self, thing: &'a T) -> Self::Field {
+        B::view(&self.1 .0, thing)
+    }
+}
+
+type Getter<T, U> = dyn Fn(T) -> U;
+type Setter<T, U> = dyn Fn(T, U) -> T;
+
+/// Makes a lens that implements `LensView<T>` and `LensOver<T>` with the provided functions
+pub fn lens_with_ref_from_arc<T, U>(
+    getter: Arc<Getter<T, U>>,
+    setter: Arc<Setter<T, U>>,
+    getter_ref: Arc<dyn Fn(&T) -> U>,
+) -> Lens<LensWithRef<Lens<FuncLens<T, U>>, Lens<ToRefInner<T, U>>, T>>
+where
+    T: Clone,
+{
+    Lens(LensWithRef(
+        lens_from_arc(getter, setter),
+        to_ref_from_arc(getter_ref),
+        Default::default(),
+    ))
+}
+/// Makes a lens that implements `LensView<T>` and `LensOver<T>` with the provided functions
+pub fn lens_with_ref<T, U>(
+    getter: impl Fn(T) -> U + 'static,
+    setter: impl Fn(T, U) -> T + 'static,
+    getter_ref: impl Fn(&T) -> U + 'static,
+) -> Lens<LensWithRef<Lens<FuncLens<T, U>>, Lens<ToRefInner<T, U>>, T>>
+where
+    T: Clone,
+{
+    Lens(LensWithRef(
+        lens(getter, setter),
+        to_ref(getter_ref),
+        Default::default(),
+    ))
+}

src/lenses/mod.rs

@@ -1,7 +1,261 @@
-mod identity;
+pub mod fields;
+
+pub mod identity;
 pub use identity::id;
 
-mod first;
+pub mod first;
 pub use first::_0;
-mod second;
+pub mod second;
 pub use second::_1;
+pub mod third;
+pub use third::_2;
+pub mod fourth;
+pub use fourth::_3;
+pub mod fifth;
+pub use fifth::_4;
+
+pub mod to;
+pub use to::{to, to_from_arc};
+pub mod lens;
+pub use lens::{lens, lens_from_arc};
+pub mod lens_with_ref;
+pub use lens_with_ref::{lens_with_ref, lens_with_ref_from_arc};
+
+/// Wrapper type
+#[derive(Clone, Copy)]
+#[repr(transparent)]
+pub struct Lens<L>(pub(crate) L);
+
+/// For lenses that allow viewing
+pub trait LensView<T> {
+    type Field;
+
+    fn view(&self, thing: T) -> Self::Field;
+}
+
+/// For lenses that allow setting
+pub trait LensOver<T>: LensView<T> {
+    fn over<F>(&self, thing: T, f: F) -> T
+    where
+        F: FnOnce(Self::Field) -> Self::Field;
+
+    fn set(&self, thing: T, v: Self::Field) -> T {
+        Self::over(self, thing, |_| v)
+    }
+}
+
+impl<L, T> LensView<T> for Lens<L>
+where
+    L: LensView<T>,
+{
+    type Field = L::Field;
+
+    fn view(&self, thing: T) -> Self::Field {
+        L::view(&self.0, thing)
+    }
+}
+impl<L, T> LensOver<T> for Lens<L>
+where
+    L: LensOver<T>,
+{
+    fn over<F>(&self, thing: T, f: F) -> T
+    where
+        F: FnOnce(Self::Field) -> Self::Field,
+    {
+        L::over(&self.0, thing, f)
+    }
+}
+
+pub fn view<T, L: LensView<T>>(lens: L, thing: T) -> L::Field {
+    L::view(&lens, thing)
+}
+pub fn set<T, L: LensOver<T>>(lens: L, thing: T, v: L::Field) -> T {
+    L::set(&lens, thing, v)
+}
+pub fn over<T, L: LensOver<T>>(lens: L, thing: T, f: impl FnOnce(L::Field) -> L::Field) -> T {
+    L::over(&lens, thing, f)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn view_first_from_tuple() {
+        let a = (1, 2);
+        assert_eq!(view(_0, a), 1);
+
+        let a = (1, 2);
+        assert_eq!(view(_0, &a), &1);
+
+        let mut a = (1, 2);
+        assert_eq!(view(_0, &mut a), &mut 1);
+
+        let a = (1, 2, 3);
+        assert_eq!(view(_0, a), 1);
+
+        let a = (1, 2, 3);
+        assert_eq!(view(_0, &a), &1);
+    }
+
+    #[test]
+    fn view_first_from_tuple_mut_works() {
+        let mut a = (1, 2);
+        *view(_0, &mut a) += 1;
+
+        assert_eq!(a, (2, 2));
+    }
+
+    #[test]
+    fn set_first_from_tuple() {
+        let a = (1, 2);
+        let a = set(_0, a, 3);
+        assert_eq!(a, (3, 2));
+    }
+
+    #[test]
+    fn over_first_from_tuple() {
+        let a = (1, 2);
+        let a = over(_0, a, |v| v + 1);
+        assert_eq!(a, (2, 2));
+    }
+
+    #[test]
+    fn over_first_from_array() {
+        let a = [1, 2, 3, 4];
+
+        let a = over(_0, a, |v| v + 1);
+        assert_eq!(a, [2, 2, 3, 4]);
+    }
+
+    #[test]
+    fn second() {
+        let a = (1, 2);
+        let a = over(_1, a, |v| v + 1);
+        assert_eq!(a, (1, 3));
+
+        let a = [1, 2, 3, 4];
+        let a = over(_1, a, |v| v + 1);
+        assert_eq!(a, [1, 3, 3, 4]);
+    }
+
+    #[test]
+    fn call_as_funcs() {
+        let a = (1, 2);
+        assert_eq!(_0(a), 1);
+
+        let a = (1, 2);
+        assert_eq!(_0(a, |v| v + 1), (2, 2));
+
+        let a = ((1, 2), 3);
+
+        let res = _0(a);
+        assert_eq!(res, (1, 2));
+
+        let lens = _0 + _1;
+
+        let res = lens(a);
+        assert_eq!(res, 2);
+        let res = lens(a, |v| v + 1);
+        assert_eq!(res, ((1, 3), 3));
+    }
+
+    #[derive(Debug, PartialEq, Clone)]
+    struct Hello {
+        hey: u8,
+    }
+
+    #[test]
+    fn can_use_to() {
+        // making a getter
+        let l = to(|hello: Hello| hello.hey);
+
+        let hello = Hello { hey: 8 };
+        assert_eq!(l(hello), 8);
+    }
+
+    #[test]
+    fn can_make_lens_for_field() {
+        // making a lens
+        let l = crate::field_lens!(Hello, hey);
+
+        let hello = Hello { hey: 8 };
+        assert_eq!(l(hello), 8);
+
+        let hello = Hello { hey: 8 };
+        assert_eq!(l(hello, |v| v + 1), Hello { hey: 9 });
+    }
+
+    #[test]
+    fn can_make_lens_with_ref_for_field() {
+        // making a lens
+        let l = crate::field_lens_with_ref!(Hello, hey);
+
+        let hello = Hello { hey: 8 };
+        // as ref
+        assert_eq!(l(&hello), 8);
+        // as move
+        assert_eq!(l(hello.clone()), 8);
+
+        assert_eq!(l(hello, |v| v + 1), Hello { hey: 9 });
+    }
+
+    #[test]
+    fn can_make_lens_out_of_funcs() {
+        // making a lens
+        let l = lens(
+            |hello: Hello| hello.hey,
+            |mut hello: Hello, v: u8| {
+                hello.hey = v;
+                hello
+            },
+        );
+
+        let hello = Hello { hey: 8 };
+        assert_eq!(l(hello), 8);
+
+        let hello = Hello { hey: 8 };
+        assert_eq!(l(hello, |v| v + 1), Hello { hey: 9 });
+    }
+
+    #[test]
+    fn can_make_lens_out_of_to() {
+        // we first use to, and then use that to make a full lens
+
+        let l = to(|hello: Hello| hello.hey);
+        let l = l.make_lens(|mut hello: Hello, v: u8| {
+            hello.hey = v;
+            hello
+        });
+
+        let hello = Hello { hey: 8 };
+        assert_eq!(l(hello), 8);
+
+        let hello = Hello { hey: 8 };
+        assert_eq!(l(hello, |v| v + 1), Hello { hey: 9 });
+    }
+
+    #[test]
+    fn convoluted_example() {
+        #[derive(Debug, PartialEq, Clone)]
+        struct Hello2 {
+            hey: (u8, (u8, i32)),
+        }
+
+        // make a lens for Hello
+        let l = lens(
+            |hello: Hello2| hello.hey,
+            |mut hello: Hello2, v| {
+                hello.hey = v;
+                hello
+            },
+        );
+
+        let thing = Hello2 { hey: (1, (2, -3)) };
+        let thing = (thing, "hello");
+
+        let lens_that_targets_the_i32 = _0 + l + _1 + _1;
+
+        assert_eq!(lens_that_targets_the_i32(thing), -3);
+    }
+}

src/lenses/second.rs

@@ -1,20 +1,21 @@
-use crate::{Lens, LensOver, LensTrait, LensView};
+use crate::lenses::{Lens, LensOver, LensView};
 
+#[derive(Clone, Copy)]
 pub struct _1Inner;
 pub const _1: Lens<_1Inner> = Lens(_1Inner);
-impl LensTrait for _1Inner {}
 
 macro_rules! make_tuples {
     ($f:ident, ( $( $v:ident ),* ), ( $( $t:ident ),* ) ) => {
-      impl< $($t,)* > LensView<( $($t,)* )> for _1Inner {
+        impl< $($t,)* > LensView<( $($t,)* )> for _1Inner {
             type Field = T;
 
-            fn view(( $($v,)* ): ($($t,)*)) -> Self::Field {
+            fn view(&self, ( $($v,)* ): ($($t,)*)) -> Self::Field {
                 $f
             }
         }
-      impl< $($t,)* > LensOver<( $($t,)* )> for _1Inner {
+        impl< $($t,)* > LensOver<( $($t,)* )> for _1Inner {
             fn over<F>(
+                &self,
                 mut tup: ($($t,)*),
                 f: F
             ) -> ( $($t,)* )
@@ -26,17 +27,17 @@ macro_rules! make_tuples {
             }
         }
 
-      impl<'a, $($t,)* > LensView<&'a ( $($t,)* )> for _1Inner {
+        impl<'a, $($t,)* > LensView<&'a ( $($t,)* )> for _1Inner {
             type Field = &'a T;
 
-            fn view(( $($v,)* ): &'a ($($t,)*)) -> Self::Field {
+            fn view(&self, ( $($v,)* ): &'a ($($t,)*)) -> Self::Field {
                 $f
             }
         }
-      impl<'a, $($t,)* > LensView<&'a mut ( $($t,)* )> for _1Inner {
+        impl<'a, $($t,)* > LensView<&'a mut ( $($t,)* )> for _1Inner {
             type Field = &'a mut T;
 
-            fn view(( $($v,)* ): &'a mut ($($t,)*)) -> Self::Field {
+            fn view(&self, ( $($v,)* ): &'a mut ($($t,)*)) -> Self::Field {
                 $f
             }
         }
@@ -56,12 +57,13 @@ macro_rules! make_arrays {
       impl<T> LensView<[T; $n]> for _1Inner {
             type Field = T;
 
-            fn view([ $($v,)* ]: [T; $n]) -> Self::Field {
+            fn view(&self, [ $($v,)* ]: [T; $n]) -> Self::Field {
                 $f
             }
         }
       impl<T> LensOver<[T; $n]> for _1Inner {
             fn over<F>(
+              &self,
               tup: [T; $n],
               fun: F
             ) -> [T; $n]
@@ -77,14 +79,14 @@ macro_rules! make_arrays {
       impl<'a, T> LensView<&'a [T; $n]> for _1Inner {
             type Field = &'a T;
 
-            fn view([ $($v,)* ]: &'a [T; $n]) -> Self::Field {
+            fn view(&self, [ $($v,)* ]: &'a [T; $n]) -> Self::Field {
                 $f
             }
         }
       impl<'a, T> LensView<&'a mut [T; $n]> for _1Inner {
             type Field = &'a mut T;
 
-            fn view([ $($v,)* ]: &'a mut [T; $n]) -> Self::Field {
+            fn view(&self, [ $($v,)* ]: &'a mut [T; $n]) -> Self::Field {
                 $f
             }
         }

src/lenses/third.rs

@@ -0,0 +1,99 @@
+use crate::lenses::{Lens, LensOver, LensView};
+
+#[derive(Clone, Copy)]
+pub struct _2Inner;
+pub const _2: Lens<_2Inner> = Lens(_2Inner);
+
+macro_rules! make_tuples {
+    ($f:ident, ( $( $v:ident ),* ), ( $( $t:ident ),* ) ) => {
+        impl< $($t,)* > LensView<( $($t,)* )> for _2Inner {
+            type Field = T;
+
+            fn view(&self, ( $($v,)* ): ($($t,)*)) -> Self::Field {
+                $f
+            }
+        }
+        impl< $($t,)* > LensOver<( $($t,)* )> for _2Inner {
+            fn over<F>(
+                &self,
+                mut tup: ($($t,)*),
+                f: F
+            ) -> ( $($t,)* )
+            where
+              F: FnOnce(Self::Field) -> Self::Field
+            {
+                tup.2 = f(tup.2);
+                tup
+            }
+        }
+
+        impl<'a, $($t,)* > LensView<&'a ( $($t,)* )> for _2Inner {
+            type Field = &'a T;
+
+            fn view(&self, ( $($v,)* ): &'a ($($t,)*)) -> Self::Field {
+                $f
+            }
+        }
+        impl<'a, $($t,)* > LensView<&'a mut ( $($t,)* )> for _2Inner {
+            type Field = &'a mut T;
+
+            fn view(&self, ( $($v,)* ): &'a mut ($($t,)*)) -> Self::Field {
+                $f
+            }
+        }
+    };
+}
+
+make_tuples!(t, (_u, _v, t), (U, V, T));
+make_tuples!(t, (_u, _v, t, _w), (U, V, T, W));
+make_tuples!(t, (_u, _v, t, _w, _x), (U, V, T, W, X));
+make_tuples!(t, (_u, _v, t, _w, _x, _y), (U, V, T, W, X, Y));
+make_tuples!(t, (_u, _v, t, _w, _x, _y, _z), (U, V, T, W, X, Y, Z));
+// not doing more cause i'm lazy, open a pr if you need more :)
+
+macro_rules! make_arrays {
+    ($f:ident, $n:expr, [$( $v:ident ),*]) => {
+      impl<T> LensView<[T; $n]> for _2Inner {
+            type Field = T;
+
+            fn view(&self, [ $($v,)* ]: [T; $n]) -> Self::Field {
+                $f
+            }
+        }
+      impl<T> LensOver<[T; $n]> for _2Inner {
+            fn over<F>(
+              &self,
+              tup: [T; $n],
+              fun: F
+            ) -> [T; $n]
+            where
+              F: FnOnce(Self::Field) -> Self::Field
+            {
+              let [$($v,)*] = tup;
+              let $f = fun( $f );
+              [$($v,)*]
+            }
+        }
+
+      impl<'a, T> LensView<&'a [T; $n]> for _2Inner {
+            type Field = &'a T;
+
+            fn view(&self, [ $($v,)* ]: &'a [T; $n]) -> Self::Field {
+                $f
+            }
+        }
+      impl<'a, T> LensView<&'a mut [T; $n]> for _2Inner {
+            type Field = &'a mut T;
+
+            fn view(&self, [ $($v,)* ]: &'a mut [T; $n]) -> Self::Field {
+                $f
+            }
+        }
+    };
+}
+
+make_arrays!(t, 3, [_a, _b, t]);
+make_arrays!(t, 4, [_a, _b, t, _c]);
+make_arrays!(t, 5, [_a, _b, t, _c, _d]);
+make_arrays!(t, 6, [_a, _b, t, _c, _d, _e]);
+make_arrays!(t, 7, [_a, _b, t, _c, _d, _e, _g]);

src/lenses/to.rs

@@ -0,0 +1,52 @@
+use std::sync::Arc;
+
+use crate::lenses::{Lens, LensView};
+
+use super::lens::FuncLens;
+
+#[derive(Clone)]
+pub struct ToInner<T, U>(Arc<dyn Fn(T) -> U>);
+
+impl<T, U> LensView<T> for ToInner<T, U> {
+    type Field = U;
+
+    fn view(&self, thing: T) -> Self::Field {
+        (self.0)(thing)
+    }
+}
+
+/// Makes a lens that implements `LensView<T>` with the provided function
+pub fn to_from_arc<T, U>(f: Arc<dyn Fn(T) -> U>) -> Lens<ToInner<T, U>> {
+    Lens(ToInner(f))
+}
+/// Makes a lens that implements `LensView<T>` with the provided function
+pub fn to<T, U>(f: impl Fn(T) -> U + 'static) -> Lens<ToInner<T, U>> {
+    Lens(ToInner(Arc::new(f)))
+}
+
+impl<T, U> Lens<ToInner<T, U>> {
+    /// Makes a full lens that implements `LensView<T>` and `LensOver<T>` with the provided functions
+    pub fn make_lens(self, setter: impl Fn(T, U) -> T + 'static) -> Lens<FuncLens<T, U>> {
+        Lens(FuncLens((self.0).0, Arc::new(setter)))
+    }
+}
+
+#[derive(Clone)]
+pub struct ToRefInner<T, U>(Arc<dyn Fn(&T) -> U>);
+
+impl<T, U> LensView<&T> for ToRefInner<T, U> {
+    type Field = U;
+
+    fn view(&self, thing: &T) -> Self::Field {
+        (self.0)(thing)
+    }
+}
+
+/// Makes a lens that implements `LensView<T>` with the provided function
+pub fn to_ref_from_arc<T, U>(f: Arc<dyn Fn(&T) -> U>) -> Lens<ToRefInner<T, U>> {
+    Lens(ToRefInner(f))
+}
+/// Makes a lens that implements `LensView<T>` with the provided function
+pub fn to_ref<T, U>(f: impl Fn(&T) -> U + 'static) -> Lens<ToRefInner<T, U>> {
+    Lens(ToRefInner(Arc::new(f)))
+}

src/lib.rs

@@ -1,153 +1,47 @@
-/// Base trait
-pub trait LensTrait {}
-
-/// For lenses that allow viewing
-pub trait LensView<T>: LensTrait {
-    type Field;
-
-    fn view(thing: T) -> Self::Field;
-}
-
-/// For lenses that allow setting
-pub trait LensOver<T>: LensView<T> {
-    fn over<F>(thing: T, f: F) -> T
-    where
-        F: FnOnce(Self::Field) -> Self::Field;
-
-    fn set(thing: T, v: Self::Field) -> T {
-        Self::over(thing, |_| v)
-    }
-}
-
-/// Wrapper type
-pub struct Lens<T: LensTrait>(T);
-
-impl<L: LensTrait> LensTrait for Lens<L> {}
-impl<L, T> LensView<T> for Lens<L>
-where
-    L: LensView<T>,
-{
-    type Field = L::Field;
-
-    fn view(thing: T) -> Self::Field {
-        L::view(thing)
-    }
-}
-impl<L, T> LensOver<T> for Lens<L>
-where
-    L: LensOver<T>,
-{
-    fn over<F>(thing: T, f: F) -> T
-    where
-        F: FnOnce(Self::Field) -> Self::Field,
-    {
-        L::over(thing, f)
-    }
-}
-
-pub fn view<T, L: LensView<T>>(_lens: L, thing: T) -> L::Field {
-    L::view(thing)
-}
-pub fn set<T, L: LensOver<T>>(_lens: L, thing: T, v: L::Field) -> T {
-    L::set(thing, v)
-}
-pub fn over<T, L: LensOver<T>>(_lens: L, thing: T, f: impl FnOnce(L::Field) -> L::Field) -> T {
-    L::over(thing, f)
-}
-
-mod combinations;
-
-// TODO add fn impls
-
-// TODO add third_from_tuple, etc
-
-// TODO array traversals
-
-// TODO make over work with changing types
+#![feature(unboxed_closures, fn_traits, const_trait_impl)]
+#![allow(clippy::type_complexity)]
 
+pub mod combinations;
+mod fns;
 pub mod lenses;
+pub mod prisms;
+pub mod traversals;
 
-#[cfg(test)]
-mod tests {
-    use super::{
-        lenses::{_0, _1},
-        *,
-    };
+pub mod prelude {
+    pub use bad_optics_derive::Optics;
 
-    #[test]
-    fn view_first_from_tuple() {
-        let a = (1, 2);
-        assert_eq!(view(_0, a), 1);
+    pub use crate::combinations::*;
 
-        let a = (1, 2);
-        assert_eq!(view(_0, &a), &1);
+    pub use crate::lenses::*;
+    pub use crate::prisms::*;
+    pub use crate::traversals::*;
 
-        let mut a = (1, 2);
-        assert_eq!(view(_0, &mut a), &mut 1);
+    pub use crate::has_lens::*;
+}
 
-        let a = (1, 2, 3);
-        assert_eq!(view(_0, a), 1);
+pub mod has_lens {
+    use crate::prelude::{lens::FuncLens, lens_with_ref::LensWithRef, to::ToRefInner};
 
-        let a = (1, 2, 3);
-        assert_eq!(view(_0, &a), &1);
+    use super::prelude::*;
+
+    pub trait HasLens {
+        type Lenses;
     }
 
-    #[test]
-    fn view_first_from_tuple_mut_works() {
-        let mut a = (1, 2);
-        *view(_0, &mut a) += 1;
-
-        assert_eq!(a, (2, 2));
+    pub trait HasLensOf<T>: Sized {
+        fn get() -> Vec<Lens<LensWithRef<Lens<FuncLens<Self, T>>, Lens<ToRefInner<Self, T>>, Self>>>;
     }
 
-    #[test]
-    fn set_first_from_tuple() {
-        let a = (1, 2);
-        let a = set(_0, a, 3);
-        assert_eq!(a, (3, 2));
+    #[macro_export]
+    macro_rules! lens {
+        ($name:ident :: $func:ident) => {
+            <$name as HasLens>::Lenses::$func()
+        };
     }
-
-    #[test]
-    fn over_first_from_tuple() {
-        let a = (1, 2);
-        let a = over(_0, a, |v| v + 1);
-        assert_eq!(a, (2, 2));
-    }
-
-    #[test]
-    fn over_first_from_array() {
-        let a = [1, 2, 3, 4];
-
-        let a = over(_0, a, |v| v + 1);
-        assert_eq!(a, [2, 2, 3, 4]);
-    }
-
-    #[test]
-    fn second() {
-        let a = (1, 2);
-        let a = over(_1, a, |v| v + 1);
-        assert_eq!(a, (1, 3));
-
-        let a = [1, 2, 3, 4];
-        let a = over(_1, a, |v| v + 1);
-        assert_eq!(a, [1, 3, 3, 4]);
-    }
-
-    #[test]
-    fn view_combination() {
-        let a = ((1, 2), 3);
-
-        let lens = _0 + _1;
-        let a = view(lens, a);
-        assert_eq!(a, 2);
-    }
-
-    #[test]
-    fn over_combination() {
-        let a = ((1, 2), 3);
-
-        let lens = _0 + _1;
-        let a = over(lens, a, |v| v + 1);
-        assert_eq!(a, ((1, 3), 3));
+    #[macro_export]
+    macro_rules! lenses {
+        ($name:ident :: $ty:ident) => {
+            <$name as HasLensOf<$ty>>::get()
+        };
     }
 }

src/prisms/mod.rs

@@ -0,0 +1,117 @@
+pub mod result;
+pub use result::{_Err, _Ok};
+
+pub mod option;
+pub use option::{_None, _Some};
+
+/// Wrapper type
+#[derive(Clone, Copy)]
+#[repr(transparent)]
+pub struct Prism<P>(pub(crate) P);
+
+pub trait PrismPreview<T> {
+    type Field;
+
+    fn preview(&self, thing: T) -> Option<Self::Field>;
+    fn review(&self, thing: Self::Field) -> T;
+
+    // TODO id like for this to not need clone
+    fn over<F>(&self, thing: T, f: F) -> T
+    where
+        F: FnOnce(Self::Field) -> Self::Field,
+        T: Clone,
+    {
+        Self::preview(self, thing.clone()).map_or(thing, |a| Self::review(self, f(a)))
+    }
+
+    fn set(&self, thing: T, v: Self::Field) -> T
+    where
+        T: Clone,
+        Self::Field: Clone,
+    {
+        Self::over(self, thing, move |_| v)
+    }
+}
+
+impl<P, T> PrismPreview<T> for Prism<P>
+where
+    P: PrismPreview<T>,
+{
+    type Field = P::Field;
+
+    fn preview(&self, thing: T) -> Option<Self::Field> {
+        P::preview(&self.0, thing)
+    }
+
+    fn review(&self, thing: Self::Field) -> T {
+        P::review(&self.0, thing)
+    }
+}
+
+pub fn preview<T, P: PrismPreview<T>>(prism: P, thing: T) -> Option<P::Field> {
+    P::preview(&prism, thing)
+}
+pub fn review<T, P: PrismPreview<T>>(prism: P, thing: P::Field) -> T {
+    P::review(&prism, thing)
+}
+pub fn over<T: Clone, P: PrismPreview<T>>(
+    prism: P,
+    thing: T,
+    f: impl FnOnce(P::Field) -> P::Field,
+) -> T {
+    P::over(&prism, thing, f)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn preview_result() {
+        let a: Result<i32, i32> = Ok(3);
+        assert_eq!(_Ok(a), Some(3));
+
+        let a: Result<i32, i32> = Err(3);
+        assert_eq!(preview(_Ok, a), None);
+
+        let a: Result<i32, i32> = Ok(3);
+        assert_eq!(_Err(a), None);
+
+        let a: Result<i32, i32> = Err(3);
+        assert_eq!(preview(_Err, a), Some(3));
+    }
+
+    #[test]
+    fn preview_option() {
+        let a = Some(3);
+        assert_eq!(_Some(a), Some(3));
+
+        let a = Some(3);
+        assert_eq!(preview(_None, a), Some(()));
+
+        let a: Option<i32> = None;
+        assert_eq!(preview(_Some, a), None);
+
+        let a: Option<i32> = None;
+        assert_eq!(preview(_None, a), Some(()));
+    }
+
+    #[test]
+    fn review_result() {
+        assert_eq!(review(_Ok, 3), Ok::<i32, i32>(3));
+        assert_eq!(review(_Err, 3), Err::<i32, i32>(3));
+    }
+
+    #[test]
+    fn review_option() {
+        assert_eq!(review(_Some, 3), Some(3));
+        assert_eq!(review(_None, ()), None::<()>);
+    }
+
+    #[test]
+    fn over_option() {
+        assert_eq!(over(_Some, Some(3), |v| v + 1), Some(4));
+        assert_eq!(_Some(Some(3), |v| v + 1), Some(4));
+        assert_eq!(over(_None, None, |_v: ()| ()), None::<()>);
+    }
+}

src/prisms/option.rs

@@ -0,0 +1,34 @@
+use super::*;
+
+#[derive(Clone, Copy)]
+pub struct SomeInner;
+#[allow(non_upper_case_globals)]
+pub const _Some: Prism<SomeInner> = Prism(SomeInner);
+
+impl<T> PrismPreview<Option<T>> for SomeInner {
+    type Field = T;
+
+    fn preview(&self, thing: Option<T>) -> Option<Self::Field> {
+        thing
+    }
+
+    fn review(&self, thing: Self::Field) -> Option<T> {
+        Some(thing)
+    }
+}
+
+#[derive(Clone, Copy)]
+pub struct NoneInner;
+#[allow(non_upper_case_globals)]
+pub const _None: Prism<NoneInner> = Prism(NoneInner);
+impl<T> PrismPreview<Option<T>> for NoneInner {
+    type Field = ();
+
+    fn preview(&self, _thing: Option<T>) -> Option<Self::Field> {
+        Some(())
+    }
+
+    fn review(&self, _thing: Self::Field) -> Option<T> {
+        None
+    }
+}

src/prisms/result.rs

@@ -0,0 +1,32 @@
+use super::*;
+
+#[derive(Clone, Copy)]
+pub struct OkInner;
+#[allow(non_upper_case_globals)]
+pub const _Ok: Prism<OkInner> = Prism(OkInner);
+impl<T, E> PrismPreview<Result<T, E>> for OkInner {
+    type Field = T;
+
+    fn preview(&self, thing: Result<T, E>) -> Option<Self::Field> {
+        thing.ok()
+    }
+    fn review(&self, thing: Self::Field) -> Result<T, E> {
+        Ok(thing)
+    }
+}
+
+#[derive(Clone, Copy)]
+pub struct ErrInner;
+#[allow(non_upper_case_globals)]
+pub const _Err: Prism<ErrInner> = Prism(ErrInner);
+
+impl<T, E> PrismPreview<Result<T, E>> for ErrInner {
+    type Field = E;
+
+    fn preview(&self, thing: Result<T, E>) -> Option<Self::Field> {
+        thing.err()
+    }
+    fn review(&self, thing: Self::Field) -> Result<T, E> {
+        Err(thing)
+    }
+}

src/traversals/both.rs

@@ -0,0 +1,23 @@
+use crate::traversals::{Traversal, TraversalOver, TraversalTraverse};
+
+#[derive(Clone, Copy)]
+pub struct BothInner;
+#[allow(non_upper_case_globals)]
+pub const both: Traversal<BothInner> = Traversal(BothInner);
+
+impl<T> TraversalTraverse<(T, T)> for BothInner {
+    type Field = T;
+
+    fn traverse(&self, (a, b): (T, T)) -> Vec<Self::Field> {
+        vec![a, b]
+    }
+}
+
+impl<T> TraversalOver<(T, T)> for BothInner {
+    fn over<F>(&self, (a, b): (T, T), mut f: F) -> (T, T)
+    where
+        F: FnMut(Self::Field) -> Self::Field,
+    {
+        (f(a), f(b))
+    }
+}

src/traversals/each.rs

@@ -0,0 +1,146 @@
+use crate::traversals::{Traversal, TraversalOver, TraversalTraverse};
+
+#[derive(Clone, Copy)]
+pub struct EachInner;
+#[allow(non_upper_case_globals)]
+pub const each: Traversal<EachInner> = Traversal(EachInner);
+
+impl<T> TraversalTraverse<Vec<T>> for EachInner {
+    type Field = T;
+
+    fn traverse(&self, thing: Vec<T>) -> Vec<Self::Field> {
+        thing
+    }
+}
+impl<T> TraversalOver<Vec<T>> for EachInner {
+    fn over<F>(&self, thing: Vec<T>, f: F) -> Vec<T>
+    where
+        F: FnMut(Self::Field) -> Self::Field,
+    {
+        thing.into_iter().map(f).collect()
+    }
+}
+
+// TODO i'd like to have this so we get it for free on any iterable
+// problem is, arrays/tuples don't implement FromIter
+// and having both the blanket implementation and the one below complains cause
+// other crates could add the trait in the future
+
+// impl<I, T> TraversalTraverse<I> for EachInner
+// where
+//     I: IntoIterator<Item = T>,
+// {
+//     type Field = T;
+
+//     fn traverse(&self, thing: I) -> Vec<Self::Field> {
+//         thing.into_iter().collect()
+//     }
+// }
+
+// impl<I, T> TraversalOver<I> for EachInner
+// where
+//     I: IntoIterator<Item = T> + FromIterator<T>,
+// {
+//     fn over<F>(&self, thing: I, f: F) -> I
+//     where
+//         F: FnMut(Self::Field) -> Self::Field,
+//     {
+//         thing.into_iter().map(f).collect()
+//     }
+// }
+
+macro_rules! make_tuples {
+    ($f:ident, ( $( $v:ident ),* ), ( $( $t:ident ),* ) ) => {
+        impl<T> TraversalTraverse<( $($t,)* )> for EachInner {
+            type Field = T;
+
+            fn traverse(&self, ( $($v,)* ): ($($t,)*)) -> Vec<Self::Field> {
+                vec![ $($v,)* ]
+            }
+        }
+        impl<T> TraversalOver<( $($t,)* )> for EachInner {
+            fn over<F>(
+                &self,
+                ($($v,)*): ($($t,)*),
+                mut f: F
+            ) -> ( $($t,)* )
+            where
+              F: FnMut(Self::Field) -> Self::Field
+            {
+
+              ( $( f($v), )* )
+            }
+        }
+
+        impl<'a, T> TraversalTraverse<&'a ( $($t,)* )> for EachInner {
+            type Field = &'a T;
+
+            fn traverse(&self, ( $($v,)* ): &'a ($($t,)*)) -> Vec<Self::Field> {
+                vec![ $($v,)* ]
+            }
+        }
+        impl<'a, T> TraversalTraverse<&'a mut ( $($t,)* )> for EachInner {
+            type Field = &'a mut T;
+
+            fn traverse(&self, ( $($v,)* ): &'a mut ($($t,)*)) -> Vec<Self::Field> {
+              vec![ $($v,)* ]
+            }
+        }
+    };
+}
+
+make_tuples!(t, (t), (T));
+make_tuples!(t, (t, u), (T, T));
+make_tuples!(t, (t, u, v), (T, T, T));
+make_tuples!(t, (t, u, v, w), (T, T, T, T));
+make_tuples!(t, (t, u, v, w, x), (T, T, T, T, T));
+make_tuples!(t, (t, u, v, w, x, y), (T, T, T, T, T, T));
+make_tuples!(t, (t, u, v, w, x, y, z), (T, T, T, T, T, T, T));
+// not doing more cause i'm lazy, open a pr if you need more :)
+
+macro_rules! make_arrays {
+    ($f:ident, $n:expr, [$( $v:ident ),*]) => {
+      impl<T> TraversalTraverse<[T; $n]> for EachInner {
+            type Field = T;
+
+            fn traverse(&self, [ $($v,)* ]: [T; $n]) -> Vec<Self::Field> {
+                vec![ $($v,)* ]
+            }
+        }
+      impl<T> TraversalOver<[T; $n]> for EachInner {
+            fn over<F>(
+              &self,
+              [ $($v,)* ]: [T; $n],
+              mut fun: F
+            ) -> [T; $n]
+            where
+              F: FnMut(Self::Field) -> Self::Field
+            {
+              [$(fun($v),)*]
+            }
+        }
+
+      impl<'a, T> TraversalTraverse<&'a [T; $n]> for EachInner {
+            type Field = &'a T;
+
+            fn traverse(&self, [ $($v,)* ]: &'a [T; $n]) -> Vec<Self::Field> {
+                vec![ $($v,)* ]
+            }
+        }
+      impl<'a, T> TraversalTraverse<&'a mut [T; $n]> for EachInner {
+            type Field = &'a mut T;
+
+            fn traverse(&self, [ $($v,)* ]: &'a mut [T; $n]) -> Vec<Self::Field> {
+                vec![ $($v,)* ]
+            }
+        }
+    };
+}
+
+make_arrays!(t, 1, [t]);
+make_arrays!(t, 2, [t, _a]);
+make_arrays!(t, 3, [t, _a, _b]);
+make_arrays!(t, 4, [t, _a, _b, _c]);
+make_arrays!(t, 5, [t, _a, _b, _c, _d]);
+make_arrays!(t, 6, [t, _a, _b, _c, _d, _e]);
+make_arrays!(t, 7, [t, _a, _b, _c, _d, _e, _g]);

src/traversals/head.rs

@@ -0,0 +1,101 @@
+use crate::traversals::{Traversal, TraversalOver, TraversalTraverse};
+
+#[derive(Clone, Copy)]
+pub struct HeadInner;
+#[allow(non_upper_case_globals)]
+pub const _head: Traversal<HeadInner> = Traversal(HeadInner);
+
+macro_rules! make_tuples {
+    (( $( $v:ident ),* ), ( $( $t:ident ),* ) ) => {
+        impl< $($t,)* > TraversalTraverse<( $($t,)* )> for HeadInner {
+            type Field = T;
+
+            fn traverse(&self, ( head, $($v,)* ): ($($t,)*)) -> Vec<Self::Field> {
+                vec![head]
+            }
+        }
+        impl< $($t,)* > TraversalOver<( $($t,)* )> for HeadInner {
+            fn over<F>(
+                &self,
+                (head, $($v,)*): ($($t,)*),
+                mut f: F
+            ) -> ( $($t,)* )
+            where
+              F: FnMut(Self::Field) -> Self::Field
+            {
+
+                ( f(head), $($v,)* )
+            }
+        }
+
+        impl<'a, $($t,)* > TraversalTraverse<&'a ( $($t,)* )> for HeadInner {
+            type Field = &'a T;
+
+            fn traverse(&self, (head, $($v,)* ): &'a ($($t,)*)) -> Vec<Self::Field> {
+                vec![ head ]
+            }
+        }
+        impl<'a, $($t,)* > TraversalTraverse<&'a mut ( $($t,)* )> for HeadInner {
+            type Field = &'a mut T;
+
+            fn traverse(&self, (head, $($v,)* ): &'a mut ($($t,)*)) -> Vec<Self::Field> {
+              vec![ head ]
+            }
+        }
+    };
+}
+
+make_tuples!((), (T));
+make_tuples!((_u), (T, U));
+make_tuples!((_u, _v), (T, U, V));
+make_tuples!((_u, _v, _w), (T, U, V, W));
+make_tuples!((_u, _v, _w, _x), (T, U, V, W, X));
+make_tuples!((_u, _v, _w, _x, _y), (T, U, V, W, X, Y));
+make_tuples!((_u, _v, _w, _x, _y, _z), (T, U, V, W, X, Y, Z));
+
+macro_rules! make_arrays {
+    ($n:expr, [$( $v:ident ),*]) => {
+      impl<T> TraversalTraverse<[T; $n]> for HeadInner {
+            type Field = T;
+
+            fn traverse(&self, [ head,  $($v,)* ]: [T; $n]) -> Vec<Self::Field> {
+                vec![head]
+            }
+        }
+      impl<T> TraversalOver<[T; $n]> for HeadInner {
+            fn over<F>(
+              &self,
+              [ head, $($v,)* ]: [T; $n],
+              mut fun: F
+            ) -> [T; $n]
+            where
+              F: FnMut(Self::Field) -> Self::Field
+            {
+              [fun(head), $(($v),)*]
+            }
+        }
+
+      impl<'a, T> TraversalTraverse<&'a [T; $n]> for HeadInner {
+            type Field = &'a T;
+
+            fn traverse(&self, [head, $($v,)* ]: &'a [T; $n]) -> Vec<Self::Field> {
+                vec![head]
+            }
+        }
+      impl<'a, T> TraversalTraverse<&'a mut [T; $n]> for HeadInner {
+            type Field = &'a mut T;
+
+            fn traverse(&self, [head, $($v,)* ]: &'a mut [T; $n]) -> Vec<Self::Field> {
+                vec![head]
+            }
+        }
+    };
+}
+
+make_arrays!(1, []);
+make_arrays!(2, [_a]);
+make_arrays!(3, [_a, _b]);
+make_arrays!(4, [_a, _b, _c]);
+make_arrays!(5, [_a, _b, _c, _d]);
+make_arrays!(6, [_a, _b, _c, _d, _e]);
+make_arrays!(7, [_a, _b, _c, _d, _e, _g]);

src/traversals/init.rs

@@ -0,0 +1,98 @@
+use crate::traversals::{Traversal, TraversalOver, TraversalTraverse};
+
+#[derive(Clone, Copy)]
+pub struct InitInner;
+#[allow(non_upper_case_globals)]
+pub const _init: Traversal<InitInner> = Traversal(InitInner);
+
+macro_rules! make_tuples {
+    (( $( $v:ident ),* ), ( $( $t:ident ),* ) ) => {
+        impl<T> TraversalTraverse<( $($t,)* )> for InitInner {
+            type Field = T;
+
+            fn traverse(&self, ( $($v,)* _last ): ($($t,)*)) -> Vec<Self::Field> {
+                vec![ $($v,)* ]
+            }
+        }
+        impl<T> TraversalOver<( $($t,)* )> for InitInner {
+            fn over<F>(
+                &self,
+                ($($v,)* last): ($($t,)*),
+                mut f: F
+            ) -> ( $($t,)* )
+            where
+              F: FnMut(Self::Field) -> Self::Field
+            {
+                ( $(f($v),)* last )
+            }
+        }
+
+        impl<'a, T> TraversalTraverse<&'a ( $($t,)* )> for InitInner {
+            type Field = &'a T;
+
+            fn traverse(&self, ($($v,)* _last): &'a ($($t,)*)) -> Vec<Self::Field> {
+                vec![ $($v,)* ]
+            }
+        }
+        impl<'a, T> TraversalTraverse<&'a mut ( $($t,)* )> for InitInner {
+            type Field = &'a mut T;
+
+            fn traverse(&self, ($($v,)* _last): &'a mut ($($t,)*)) -> Vec<Self::Field> {
+              vec![ $($v,)* ]
+            }
+        }
+    };
+}
+
+make_tuples!((_u), (T, T));
+make_tuples!((_u, _v), (T, T, T));
+make_tuples!((_u, _v, _w), (T, T, T, T));
+make_tuples!((_u, _v, _w, _x), (T, T, T, T, T));
+make_tuples!((_u, _v, _w, _x, _y), (T, T, T, T, T, T));
+make_tuples!((_u, _v, _w, _x, _y, _z), (T, T, T, T, T, T, T));
+
+macro_rules! make_arrays {
+    ($n:expr, [$( $v:ident ),*]) => {
+        impl<T> TraversalTraverse<[T; $n]> for InitInner {
+            type Field = T;
+
+            fn traverse(&self, [ $($v,)* _last]: [T; $n]) -> Vec<Self::Field> {
+                vec![ $($v,)* ]
+            }
+        }
+        impl<T> TraversalOver<[T; $n]> for InitInner {
+            fn over<F>(
+              &self,
+              [ $($v,)* last]: [T; $n],
+              mut fun: F
+            ) -> [T; $n]
+            where
+              F: FnMut(Self::Field) -> Self::Field
+            {
+                [$(fun($v),)* last]
+            }
+        }
+
+        impl<'a, T> TraversalTraverse<&'a [T; $n]> for InitInner {
+            type Field = &'a T;
+
+            fn traverse(&self, [$($v,)* _last]: &'a [T; $n]) -> Vec<Self::Field> {
+                vec![ $($v,)* ]
+            }
+        }
+        impl<'a, T> TraversalTraverse<&'a mut [T; $n]> for InitInner {
+            type Field = &'a mut T;
+
+            fn traverse(&self, [$($v,)* _last ]: &'a mut [T; $n]) -> Vec<Self::Field> {
+                vec![ $($v,)* ]
+            }
+        }
+    };
+}
+
+make_arrays!(2, [_a]);
+make_arrays!(3, [_a, _b]);
+make_arrays!(4, [_a, _b, _c]);
+make_arrays!(5, [_a, _b, _c, _d]);
+make_arrays!(6, [_a, _b, _c, _d, _e]);
+make_arrays!(7, [_a, _b, _c, _d, _e, _g]);

src/traversals/last.rs

@@ -0,0 +1,99 @@
+use crate::traversals::{Traversal, TraversalOver, TraversalTraverse};
+
+#[derive(Clone, Copy)]
+pub struct LastInner;
+#[allow(non_upper_case_globals)]
+pub const _last: Traversal<LastInner> = Traversal(LastInner);
+
+macro_rules! make_tuples {
+    (( $( $v:ident ),* ), ( $( $t:ident ),* ) ) => {
+        impl<T> TraversalTraverse<( $($t,)* )> for LastInner {
+            type Field = T;
+
+            fn traverse(&self, ( $($v,)* last ): ($($t,)*)) -> Vec<Self::Field> {
+                vec![ last ]
+            }
+        }
+        impl<T> TraversalOver<( $($t,)* )> for LastInner {
+            fn over<F>(
+                &self,
+                ($($v,)* last): ($($t,)*),
+                mut f: F
+            ) -> ( $($t,)* )
+            where
+              F: FnMut(Self::Field) -> Self::Field
+            {
+                ( $($v,)* f(last), )
+            }
+        }
+
+        impl<'a, T> TraversalTraverse<&'a ( $($t,)* )> for LastInner {
+            type Field = &'a T;
+
+            fn traverse(&self, ($($v,)* last): &'a ($($t,)*)) -> Vec<Self::Field> {
+                vec![ last ]
+            }
+        }
+        impl<'a, T> TraversalTraverse<&'a mut ( $($t,)* )> for LastInner {
+            type Field = &'a mut T;
+
+            fn traverse(&self, ($($v,)* last): &'a mut ($($t,)*)) -> Vec<Self::Field> {
+              vec![ last ]
+            }
+        }
+    };
+}
+
+make_tuples!((_u), (T, T));
+make_tuples!((_u, _v), (T, T, T));
+make_tuples!((_u, _v, _w), (T, T, T, T));
+make_tuples!((_u, _v, _w, _x), (T, T, T, T, T));
+make_tuples!((_u, _v, _w, _x, _y), (T, T, T, T, T, T));
+make_tuples!((_u, _v, _w, _x, _y, _z), (T, T, T, T, T, T, T));
+
+macro_rules! make_arrays {
+    ($n:expr, [$( $v:ident ),*]) => {
+        impl<T> TraversalTraverse<[T; $n]> for LastInner {
+            type Field = T;
+
+            fn traverse(&self, [ $($v,)* last]: [T; $n]) -> Vec<Self::Field> {
+                vec![ last ]
+            }
+        }
+        impl<T> TraversalOver<[T; $n]> for LastInner {
+            fn over<F>(
+              &self,
+              [ $($v,)* last]: [T; $n],
+              mut fun: F
+            ) -> [T; $n]
+            where
+              F: FnMut(Self::Field) -> Self::Field
+            {
+                [$(($v),)* fun(last)]
+            }
+        }
+
+        impl<'a, T> TraversalTraverse<&'a [T; $n]> for LastInner {
+            type Field = &'a T;
+
+            fn traverse(&self, [$($v,)* last]: &'a [T; $n]) -> Vec<Self::Field> {
+                vec![ last ]
+            }
+        }
+        impl<'a, T> TraversalTraverse<&'a mut [T; $n]> for LastInner {
+            type Field = &'a mut T;
+
+            fn traverse(&self, [ $($v,)* last ]: &'a mut [T; $n]) -> Vec<Self::Field> {
+                vec![ last ]
+            }
+        }
+    };
+}
+
+make_arrays!(1, []);
+make_arrays!(2, [_a]);
+make_arrays!(3, [_a, _b]);
+make_arrays!(4, [_a, _b, _c]);
+make_arrays!(5, [_a, _b, _c, _d]);
+make_arrays!(6, [_a, _b, _c, _d, _e]);
+make_arrays!(7, [_a, _b, _c, _d, _e, _g]);

src/traversals/mod.rs

@@ -0,0 +1,211 @@
+pub mod both;
+pub use both::both;
+
+pub mod each;
+pub use each::each;
+
+pub mod head;
+pub use head::_head;
+pub mod tail;
+pub use tail::_tail;
+
+pub mod init;
+pub use init::_init;
+pub mod last;
+pub use last::_last;
+
+use crate::{
+    lenses::{Lens, LensOver, LensView},
+    prisms::{Prism, PrismPreview},
+};
+
+/// Wrapper type
+#[derive(Clone, Copy)]
+#[repr(transparent)]
+pub struct Traversal<T>(pub(crate) T);
+
+pub trait TraversalTraverse<T> {
+    type Field;
+
+    fn traverse(&self, thing: T) -> Vec<Self::Field>;
+}
+pub trait TraversalOver<T>: TraversalTraverse<T> {
+    fn over<F>(&self, thing: T, f: F) -> T
+    where
+        F: FnMut(Self::Field) -> Self::Field;
+
+    fn set(&self, thing: T, v: Self::Field) -> T
+    where
+        Self::Field: Clone,
+    {
+        Self::over(self, thing, move |_| v.clone())
+    }
+}
+
+impl<L, T> TraversalTraverse<T> for Traversal<L>
+where
+    L: TraversalTraverse<T>,
+{
+    type Field = L::Field;
+
+    fn traverse(&self, thing: T) -> Vec<Self::Field> {
+        L::traverse(&self.0, thing)
+    }
+}
+impl<L, T> TraversalOver<T> for Traversal<L>
+where
+    L: TraversalOver<T>,
+{
+    fn over<F>(&self, thing: T, f: F) -> T
+    where
+        F: FnMut(Self::Field) -> Self::Field,
+    {
+        L::over(&self.0, thing, f)
+    }
+}
+
+// all lenses are traversals, so we can freely transform them into a traversal
+impl<L> Lens<L> {
+    /// Returns this lens as a traversal
+    pub const fn to_traversal(self) -> Traversal<Lens<L>> {
+        Traversal(self)
+    }
+}
+// we can go back to a lens from a "traversal-ed" lens
+impl<L> Traversal<Lens<L>> {
+    /// Returns the wrapped lens
+    pub fn to_lens(self) -> Lens<L> {
+        self.0
+    }
+}
+impl<L, T> TraversalTraverse<T> for Lens<L>
+where
+    L: LensView<T>,
+{
+    type Field = L::Field;
+
+    fn traverse(&self, thing: T) -> Vec<Self::Field> {
+        vec![L::view(&self.0, thing)]
+    }
+}
+impl<L, T> TraversalOver<T> for Lens<L>
+where
+    L: LensView<T> + LensOver<T>,
+{
+    fn over<F>(&self, thing: T, f: F) -> T
+    where
+        F: FnMut(Self::Field) -> Self::Field,
+    {
+        L::over(&self.0, thing, f)
+    }
+}
+
+// all prisms are traversals, so we can freely transform them into a traversal
+impl<L> Prism<L> {
+    /// Returns this lens as a traversal
+    pub const fn to_traversal(self) -> Traversal<Prism<L>> {
+        Traversal(self)
+    }
+}
+// we can go back to a lens from a "traversal-ed" lens
+impl<L> Traversal<Prism<L>> {
+    /// Returns the wrapped lens
+    pub fn to_prism(self) -> Prism<L> {
+        self.0
+    }
+}
+impl<L, T> TraversalTraverse<T> for Prism<L>
+where
+    L: PrismPreview<T>,
+{
+    type Field = L::Field;
+
+    fn traverse(&self, thing: T) -> Vec<Self::Field> {
+        L::preview(&self.0, thing).into_iter().collect()
+    }
+}
+impl<L, T> TraversalOver<T> for Prism<L>
+where
+    T: Clone,
+    L: PrismPreview<T>,
+{
+    fn over<F>(&self, thing: T, f: F) -> T
+    where
+        F: FnMut(Self::Field) -> Self::Field,
+    {
+        L::over(&self.0, thing, f)
+    }
+}
+
+pub fn traverse<T, L: TraversalTraverse<T>>(lens: L, thing: T) -> Vec<L::Field> {
+    L::traverse(&lens, thing)
+}
+pub fn set<T, L: TraversalOver<T>>(lens: L, thing: T, v: L::Field) -> T
+where
+    <L as TraversalTraverse<T>>::Field: Clone,
+{
+    L::set(&lens, thing, v)
+}
+pub fn over<T, L: TraversalOver<T>>(lens: L, thing: T, f: impl FnMut(L::Field) -> L::Field) -> T {
+    L::over(&lens, thing, f)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn traverse_each_works_on_arrays() {
+        let array = [1, 2, 3, 4];
+
+        let res = each(array);
+        assert_eq!(res, vec![1, 2, 3, 4,]);
+
+        let res = each(array, |v| v + 1);
+        assert_eq!(res, [2, 3, 4, 5]);
+    }
+
+    #[test]
+    fn traverse_head_works_on_arrays() {
+        let array = [1, 2, 3, 4];
+
+        let res = _head(array);
+        assert_eq!(res, vec![1,]);
+
+        let res = _head(array, |v| v + 1);
+        assert_eq!(res, [2, 2, 3, 4,]);
+    }
+
+    #[test]
+    fn traverse_tail_works_on_arrays() {
+        let array = [1, 2, 3, 4];
+
+        let res = _tail(array);
+        assert_eq!(res, vec![2, 3, 4]);
+
+        let res = _tail(array, |v| v + 1);
+        assert_eq!(res, [1, 3, 4, 5]);
+    }
+
+    #[test]
+    fn traverse_init_works_on_arrays() {
+        let array = [1, 2, 3, 4];
+
+        let res = _init(array);
+        assert_eq!(res, vec![1, 2, 3]);
+
+        let res = _init(array, |v| v + 1);
+        assert_eq!(res, [2, 3, 4, 4,]);
+    }
+
+    #[test]
+    fn traverse_last_works_on_arrays() {
+        let array = [1, 2, 3, 4];
+
+        let res = _last(array);
+        assert_eq!(res, vec![4]);
+
+        let res = _last(array, |v| v + 1);
+        assert_eq!(res, [1, 2, 3, 5]);
+    }
+}

src/traversals/tail.rs

@@ -0,0 +1,98 @@
+use crate::traversals::{Traversal, TraversalOver, TraversalTraverse};
+
+#[derive(Clone, Copy)]
+pub struct TailInner;
+#[allow(non_upper_case_globals)]
+pub const _tail: Traversal<TailInner> = Traversal(TailInner);
+
+macro_rules! make_tuples {
+    (( $( $v:ident ),* ), ( $( $t:ident ),* ) ) => {
+        impl<T> TraversalTraverse<( $($t,)* )> for TailInner {
+            type Field = T;
+
+            fn traverse(&self, ( _, $($v,)* ): ($($t,)*)) -> Vec<Self::Field> {
+                vec![$($v,)*]
+            }
+        }
+        impl<T> TraversalOver<( $($t,)* )> for TailInner {
+            fn over<F>(
+                &self,
+                (head, $($v,)*): ($($t,)*),
+                mut fun: F
+            ) -> ( $($t,)* )
+            where
+              F: FnMut(Self::Field) -> Self::Field
+            {
+                (head, $( fun($v) ,)*)
+            }
+        }
+
+        impl<'a, T> TraversalTraverse<&'a ( $($t,)* )> for TailInner {
+            type Field = &'a T;
+
+            fn traverse(&self, (_, $($v,)* ): &'a ($($t,)*)) -> Vec<Self::Field> {
+                vec![ $($v,)* ]
+            }
+        }
+        impl<'a, T> TraversalTraverse<&'a mut ( $($t,)* )> for TailInner {
+            type Field = &'a mut T;
+
+            fn traverse(&self, (_, $($v,)* ): &'a mut ($($t,)*)) -> Vec<Self::Field> {
+                vec![ $($v,)* ]
+            }
+        }
+    };
+}
+
+make_tuples!((_u), (T, T));
+make_tuples!((_u, _v), (T, T, T));
+make_tuples!((_u, _v, _w), (T, T, T, T));
+make_tuples!((_u, _v, _w, _x), (T, T, T, T, T));
+make_tuples!((_u, _v, _w, _x, _y), (T, T, T, T, T, T));
+make_tuples!((_u, _v, _w, _x, _y, _z), (T, T, T, T, T, T, T));
+
+macro_rules! make_arrays {
+    ($n:expr, [$( $v:ident ),*]) => {
+        impl<T> TraversalTraverse<[T; $n]> for TailInner {
+            type Field = T;
+
+            fn traverse(&self, [ _,  $($v,)* ]: [T; $n]) -> Vec<Self::Field> {
+                vec![$($v,)*]
+            }
+        }
+        impl<T> TraversalOver<[T; $n]> for TailInner {
+            fn over<F>(
+              &self,
+              [ head, $($v,)* ]: [T; $n],
+              mut fun: F
+            ) -> [T; $n]
+            where
+              F: FnMut(Self::Field) -> Self::Field
+            {
+              [head, $(fun($v),)*]
+            }
+        }
+
+        impl<'a, T> TraversalTraverse<&'a [T; $n]> for TailInner {
+            type Field = &'a T;
+
+            fn traverse(&self, [_, $($v,)* ]: &'a [T; $n]) -> Vec<Self::Field> {
+                vec![$($v,)*]
+            }
+        }
+        impl<'a, T> TraversalTraverse<&'a mut [T; $n]> for TailInner {
+            type Field = &'a mut T;
+
+            fn traverse(&self, [_, $($v,)* ]: &'a mut [T; $n]) -> Vec<Self::Field> {
+                vec![$($v,)*]
+            }
+        }
+    };
+}
+
+make_arrays!(2, [_a]);
+make_arrays!(3, [_a, _b]);
+make_arrays!(4, [_a, _b, _c]);
+make_arrays!(5, [_a, _b, _c, _d]);
+make_arrays!(6, [_a, _b, _c, _d, _e]);
+make_arrays!(7, [_a, _b, _c, _d, _e, _g]);