make adding lenses be const

README.md

@@ -1,13 +1,21 @@
 # bad optics
 
-ergonomic no-macro lenses for rust
+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::{_0, _1},
-*,
+    lenses::{over, set},
+    prelude::*,
 };
 
 fn main() {
@@ -41,6 +49,12 @@ fn main() {
 
     // 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));
 }
 ```
+
+## 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

examples/convoluted.rs

@@ -1,4 +1,4 @@
-use bad_optics::{lenses::*, prisms::*, traversals::*};
+use bad_optics::prelude::*;
 
 #[derive(Debug, PartialEq, Clone)]
 struct MyStruct {

examples/main.rs

@@ -1,4 +1,7 @@
-use bad_optics::lenses::*;
+use bad_optics::{
+    lenses::{over, set},
+    prelude::*,
+};
 
 fn main() {
     let a = ((1, 2), 3);

examples/struct_fields.rs

@@ -1,4 +1,4 @@
-use bad_optics::lenses::*;
+use bad_optics::prelude::*;
 
 #[derive(Debug, PartialEq, Clone)]
 struct MyStruct {

src/combinations.rs

@@ -3,6 +3,7 @@ use crate::{
     prisms::{Prism, PrismPreview},
     traversals::{Traversal, TraversalOver, TraversalTraverse},
 };
+use std::ops::Add;
 
 #[derive(Clone, Copy)]
 pub struct Combination<A, B>(A, B);
@@ -10,7 +11,7 @@ pub struct Combination<A, B>(A, B);
 // additions
 
 // lens + lens
-impl<A, B> std::ops::Add<Lens<B>> for Lens<A> {
+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 {
@@ -18,7 +19,7 @@ impl<A, B> std::ops::Add<Lens<B>> for Lens<A> {
     }
 }
 // prism + prism
-impl<A, B> std::ops::Add<Prism<B>> for Prism<A> {
+impl<A, B> const Add<Prism<B>> for Prism<A> {
     type Output = Prism<Combination<Prism<A>, Prism<B>>>;
 
     fn add(self, rhs: Prism<B>) -> Self::Output {
@@ -26,7 +27,7 @@ impl<A, B> std::ops::Add<Prism<B>> for Prism<A> {
     }
 }
 // traversal + traversal
-impl<A, B> std::ops::Add<Traversal<B>> for Traversal<A> {
+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 {
@@ -34,7 +35,7 @@ impl<A, B> std::ops::Add<Traversal<B>> for Traversal<A> {
     }
 }
 // traversal + lens
-impl<A, B> std::ops::Add<Lens<B>> for Traversal<A> {
+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 {
@@ -42,7 +43,7 @@ impl<A, B> std::ops::Add<Lens<B>> for Traversal<A> {
     }
 }
 // lens + traversal
-impl<A, B> std::ops::Add<Traversal<B>> for Lens<A> {
+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 {
@@ -50,7 +51,7 @@ impl<A, B> std::ops::Add<Traversal<B>> for Lens<A> {
     }
 }
 // traversal + prism
-impl<A, B> std::ops::Add<Prism<B>> for Traversal<A> {
+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 {
@@ -58,7 +59,7 @@ impl<A, B> std::ops::Add<Prism<B>> for Traversal<A> {
     }
 }
 // prism + traversal
-impl<A, B> std::ops::Add<Traversal<B>> for Prism<A> {
+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 {
@@ -66,7 +67,7 @@ impl<A, B> std::ops::Add<Traversal<B>> for Prism<A> {
     }
 }
 // lens + prism
-impl<A, B> std::ops::Add<Prism<B>> for Lens<A> {
+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 {
@@ -74,7 +75,7 @@ impl<A, B> std::ops::Add<Prism<B>> for Lens<A> {
     }
 }
 // prism + traversal
-impl<A, B> std::ops::Add<Lens<B>> for Prism<A> {
+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 {
@@ -321,4 +322,17 @@ mod tests {
         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/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/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/mod.rs

@@ -1,16 +1,22 @@
-mod fields;
+pub mod fields;
 
-mod identity;
+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;
 
-mod to;
+pub mod to;
 pub use to::{to, to_from_boxed};
-mod lens;
+pub mod lens;
 pub use lens::{lens, lens_from_boxed};
 
 /// Wrapper type

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/lib.rs

@@ -1,7 +1,15 @@
-#![feature(unboxed_closures, fn_traits)]
+#![feature(unboxed_closures, fn_traits, const_trait_impl)]
 
-mod combinations;
+pub mod combinations;
 mod fns;
 pub mod lenses;
 pub mod prisms;
 pub mod traversals;
+
+pub mod prelude {
+    pub use crate::combinations::*;
+
+    pub use crate::lenses::*;
+    pub use crate::prisms::*;
+    pub use crate::traversals::*;
+}

src/prisms/mod.rs

@@ -1,7 +1,7 @@
-mod result;
+pub mod result;
 pub use result::{_Err, _Ok};
 
-mod option;
+pub mod option;
 pub use option::{_None, _Some};
 
 /// Wrapper type

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/mod.rs

@@ -1,9 +1,14 @@
-mod both;
+pub mod both;
 pub use both::both;
 
-mod each;
+pub mod each;
 pub use each::each;
 
+pub mod head;
+pub use head::_head;
+pub mod tail;
+pub use tail::_tail;
+
 use crate::{
     lenses::{Lens, LensOver, LensView},
     prisms::{Prism, PrismPreview},
@@ -57,7 +62,7 @@ where
 // 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>> {
+    pub const fn to_traversal(self) -> Traversal<Lens<L>> {
         Traversal(self)
     }
 }
@@ -93,7 +98,7 @@ where
 // 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>> {
+    pub const fn to_traversal(self) -> Traversal<Prism<L>> {
         Traversal(self)
     }
 }
@@ -154,4 +159,26 @@ mod tests {
         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]);
+    }
 }

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]);