prism + lens

.gitignore

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

README.md

@@ -13,14 +13,21 @@ use bad_optics::{
 fn main() {
     let a = ((1, 2), 3);
 
-    // combine lenses
+    // 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);
 
-    // call the lens as a function
+    // you can also call the lens as a function
     let res = lens(a);
     assert_eq!(res, 2);
 
@@ -32,7 +39,7 @@ fn main() {
     let a = set(lens, a, 5);
     assert_eq!(a, ((1, 5), 3));
 
-    // call the lens as a function to modify the value
+    // you can also call the lens as a function to modify the value
     let res = lens(a, |v| v + 1);
     assert_eq!(res, ((1, 3), 3));
 }

examples/main.rs

@@ -1,19 +1,23 @@
-use bad_optics::{
-    lenses::{_0, _1},
-    *,
-};
+use bad_optics::lenses::*;
 
 fn main() {
     let a = ((1, 2), 3);
 
-    // combine lenses
+    // 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);
 
-    // call the lens as a function
+    // you can also call the lens as a function
     let res = lens(a);
     assert_eq!(res, 2);
 
@@ -25,7 +29,7 @@ fn main() {
     let a = set(lens, a, 5);
     assert_eq!(a, ((1, 5), 3));
 
-    // call the lens as a function to modify the value
+    // you can also call the lens as a function to modify the value
     let res = lens(a, |v| v + 1);
-    assert_eq!(res, ((1, 3), 3));
+    assert_eq!(res, ((1, 6), 3));
 }

examples/struct_fields.rs

@@ -0,0 +1,27 @@
+use bad_optics::lenses::*;
+
+#[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

@@ -1,42 +1,324 @@
-use crate::{Lens, LensOver, LensTrait, LensView};
+use crate::{
+    lenses::{Lens, LensOver, LensView},
+    prisms::{Prism, PrismPreview},
+    traversals::{Traversal, TraversalOver, TraversalTraverse},
+};
 
 #[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> std::ops::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> std::ops::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> std::ops::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> std::ops::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> std::ops::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> std::ops::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> std::ops::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> std::ops::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> std::ops::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()
+            .map(|v| B::traverse(&self.1 .0, v))
+            .flatten()
+            .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));
     }
 }

src/fns.rs

@@ -1,5 +1,10 @@
-use crate::{Lens, LensOver, LensView};
+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>,
@@ -7,7 +12,7 @@ where
     type Output = L::Field;
 
     extern "rust-call" fn call_once(self, args: (A,)) -> Self::Output {
-        L::view(args.0)
+        L::view(&self.0, args.0)
     }
 }
 impl<L, A> std::ops::FnMut<(A,)> for Lens<L>
@@ -15,7 +20,7 @@ where
     L: LensView<A>,
 {
     extern "rust-call" fn call_mut(&mut self, args: (A,)) -> Self::Output {
-        L::view(args.0)
+        L::view(&self.0, args.0)
     }
 }
 impl<L, A> std::ops::Fn<(A,)> for Lens<L>
@@ -23,10 +28,11 @@ where
     L: LensView<A>,
 {
     extern "rust-call" fn call(&self, args: (A,)) -> Self::Output {
-        L::view(args.0)
+        L::view(&self.0, args.0)
     }
 }
 
+// lens over
 impl<L, A, F> std::ops::FnOnce<(A, F)> for Lens<L>
 where
     L: LensOver<A>,
@@ -35,7 +41,7 @@ where
     type Output = A;
 
     extern "rust-call" fn call_once(self, args: (A, F)) -> Self::Output {
-        L::over(args.0, args.1)
+        L::over(&self.0, args.0, args.1)
     }
 }
 impl<L, A, F> std::ops::FnMut<(A, F)> for Lens<L>
@@ -44,7 +50,7 @@ where
     F: FnOnce(L::Field) -> L::Field,
 {
     extern "rust-call" fn call_mut(&mut self, args: (A, F)) -> Self::Output {
-        L::over(args.0, args.1)
+        L::over(&self.0, args.0, args.1)
     }
 }
 impl<L, A, F> std::ops::Fn<(A, F)> for Lens<L>
@@ -53,6 +59,127 @@ where
     F: FnOnce(L::Field) -> L::Field,
 {
     extern "rust-call" fn call(&self, args: (A, F)) -> Self::Output {
-        L::over(args.0, args.1)
+        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/first.rs

@@ -1,21 +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 {
             fn over<F>(
+                &self,
                 mut tup: ($($t,)*),
                 f: F
             ) -> ( $($t,)* )
@@ -30,14 +30,14 @@ macro_rules! make_tuples {
         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 {
             type Field = &'a mut T;
 
-            fn view(( $($v,)* ): &'a mut ($($t,)*)) -> Self::Field {
+            fn view(&self, ( $($v,)* ): &'a mut ($($t,)*)) -> Self::Field {
                 $f
             }
         }
@@ -57,12 +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]
@@ -78,14 +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/identity.rs

@@ -1,19 +1,19 @@
-use crate::{LensOver, LensTrait, LensView};
+use crate::lenses::{Lens, LensOver, LensView};
 
-#[allow(non_camel_case_types)]
 #[derive(Clone, Copy)]
-pub struct id;
+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 +1,42 @@
+use crate::lenses::{Lens, LensOver, LensView};
+
+type Getter<T, U> = dyn Fn(T) -> U;
+type Setter<T, U> = dyn Fn(T, U) -> T;
+
+pub struct FuncLens<T, U>(pub(crate) Box<Getter<T, U>>, pub(crate) Box<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_boxed<T, U>(
+    getter: Box<Getter<T, U>>,
+    setter: Box<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(Box::new(getter), Box::new(setter)))
+}

src/lenses/mod.rs

@@ -5,3 +5,221 @@ mod first;
 pub use first::_0;
 mod second;
 pub use second::_1;
+
+mod to;
+pub use to::{to, to_from_boxed};
+mod lens;
+pub use lens::{lens, lens_from_boxed};
+
+/// 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_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

@@ -1,21 +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 {
             type Field = T;
 
-            fn view(( $($v,)* ): ($($t,)*)) -> Self::Field {
+            fn view(&self, ( $($v,)* ): ($($t,)*)) -> Self::Field {
                 $f
             }
         }
         impl< $($t,)* > LensOver<( $($t,)* )> for _1Inner {
             fn over<F>(
+                &self,
                 mut tup: ($($t,)*),
                 f: F
             ) -> ( $($t,)* )
@@ -30,14 +30,14 @@ macro_rules! make_tuples {
         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 {
             type Field = &'a mut T;
 
-            fn view(( $($v,)* ): &'a mut ($($t,)*)) -> Self::Field {
+            fn view(&self, ( $($v,)* ): &'a mut ($($t,)*)) -> Self::Field {
                 $f
             }
         }
@@ -57,12 +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]
@@ -78,14 +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/to.rs

@@ -0,0 +1,29 @@
+use crate::lenses::{Lens, LensView};
+
+use super::lens::FuncLens;
+
+pub struct ToInner<T, U>(Box<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_boxed<T, U>(f: Box<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(Box::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, Box::new(setter)))
+    }
+}

src/lib.rs

@@ -1,173 +1,7 @@
 #![feature(unboxed_closures, fn_traits)]
 
-/// 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
-#[derive(Clone, Copy)]
-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)
-}
-
-// TODO add fn impls
-
-// TODO add third_from_tuple, etc
-
-// TODO array traversals
-
-// TODO make over work with changing types
-
 mod combinations;
 mod fns;
 pub mod lenses;
-
-#[cfg(test)]
-mod tests {
-    use super::{
-        lenses::{_0, _1},
-        *,
-    };
-
-    #[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 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));
-    }
-
-    #[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 lens = _0 + _1;
-
-        let res = lens(a);
-        assert_eq!(res, 2);
-        let res = lens(a, |v| v + 1);
-        assert_eq!(res, ((1, 3), 3));
-    }
-}
+pub mod prisms;
+pub mod traversals;

src/prisms/mod.rs

@@ -0,0 +1,121 @@
+mod result;
+pub use result::{_Err, _Ok};
+
+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,
+    {
+        if let Some(a) = Self::preview(&self, thing.clone()) {
+            Self::review(&self, f(a))
+        } else {
+            thing
+        }
+    }
+
+    fn set(&self, thing: T, v: Self::Field) -> T
+    where
+        T: Clone,
+        Self::Field: Clone,
+    {
+        Self::over(self, thing, move |_| v.clone())
+    }
+}
+
+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/mod.rs

@@ -0,0 +1,157 @@
+mod both;
+pub use both::both;
+
+mod each;
+pub use each::each;
+
+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 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 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]);
+    }
+}