diff options
Diffstat (limited to 'src/lib.rs')
| -rw-r--r-- | src/lib.rs | 57 |
1 files changed, 37 insertions, 20 deletions
@@ -1,14 +1,31 @@ +//! A macro that uses the traits from the [higher] crate and generates a Free [`Monad`][higher::Monad] type for a given [`Functor`][higher::Functor]. +//! +//! # Free Monad? What is that? +//! A Free Monad is the left-adjoint to the Forget-Functor from the category of Monads into the category of Endofunctors. +//! +//! This of course doesn't explain what one can do with a Free Monad data type. There are plenty of blog posts and websites that +//! explore different use cases. A very good explanation is given by Nikolay Yakimov's +//! [Introduction to Free Monads](https://serokell.io/blog/introduction-to-free-monads). +//! +//! # Why a Macro? +//! Until [non-lifetime binders](https://github.com/rust-lang/rust/issues/108185) become stable, this seems to be the easiest way. +//! In generic code, the type signature would be `enum Free<A,F> where F : Functor<Free<A,F>>`. If one now wants to implement the [`Functor`][higher::Functor] +//! trait for this, it is not really possible to express the `Target<T> = Free<A,F::Target<Free<A,F::Target<...>>>>` generic associated type. +//! +//! See the [blog post about this crate](https://www.grois.info/posts/2023-03/2023-03-11-adventures-with-free-monads-and-higher.xhtml) +//! for a more detailed explanation. + pub extern crate higher; #[macro_export] macro_rules! free { - ($v:vis $name:ident<$($other_lifetimes:lifetime,)* $generic:ident>, $f:ty) => { + ($v:vis $name:ident<$($other_lifetimes:lifetime,)* $generic:ident $(,$other_generics:ident)*>, $f:ty) => { #[derive(Clone)] - $v enum $name<$($other_lifetimes,)* $generic> { + $v enum $name<$($other_lifetimes,)* $generic $(,$other_generics)*> { Pure($generic), Free(Box<$f>) } - impl<$($other_lifetimes,)* $generic> $name<$($other_lifetimes,)* $generic>{ + impl<$($other_lifetimes,)* $generic $(,$other_generics)*> $name<$($other_lifetimes,)* $generic $(,$other_generics)*>{ $v fn lift_f(functor : <$f as $crate::higher::Functor<Self>>::Target<$generic>) -> Self{ use $crate::higher::Functor; Self::Free(Box::new(functor.fmap(|a| Self::Pure(a)))) @@ -23,10 +40,10 @@ macro_rules! free { } } - impl<'free_macro_reserved_lifetime, $($other_lifetimes,)* A> $crate::higher::Functor<'free_macro_reserved_lifetime,A> for $name<$($other_lifetimes,)* A> { - type Target<T> = $name<$($other_lifetimes,)* T>; - fn fmap<B,F>(self, f: F) -> Self::Target<B> where F: Fn(A) -> B + 'free_macro_reserved_lifetime{ - fn __fmap_impl<'free_macro_reserved_lifetime, $($other_lifetimes,)* A, B, F>(s : $name<$($other_lifetimes,)* A>, f: &F) -> $name<$($other_lifetimes,)* B> where F: Fn(A) -> B + 'free_macro_reserved_lifetime{ + impl<'free_macro_reserved_lifetime, $($other_lifetimes,)* $generic $(,$other_generics)*> $crate::higher::Functor<'free_macro_reserved_lifetime,$generic> for $name<$($other_lifetimes,)* $generic $(,$other_generics)*> { + type Target<FreeMacroReservedType> = $name<$($other_lifetimes,)* FreeMacroReservedType $(,$other_generics)*>; + fn fmap<FreeMacroReservedType,FreeMacroReservedType2>(self, f: FreeMacroReservedType2) -> Self::Target<FreeMacroReservedType> where FreeMacroReservedType2: Fn($generic) -> FreeMacroReservedType + 'free_macro_reserved_lifetime{ + fn __fmap_impl<'free_macro_reserved_lifetime, $($other_lifetimes,)* $generic $(,$other_generics)*, FreeMacroReservedType, FreeMacroReservedType2>(s : $name<$($other_lifetimes,)* $generic $(,$other_generics)*>, f: &FreeMacroReservedType2) -> $name<$($other_lifetimes,)* FreeMacroReservedType $(,$other_generics)*> where FreeMacroReservedType2: Fn($generic) -> FreeMacroReservedType + 'free_macro_reserved_lifetime{ use $crate::higher::Functor; match s { $name::Pure(a) => {$name::Pure(f(a))}, @@ -37,32 +54,32 @@ macro_rules! free { } } - impl<$($other_lifetimes,)* A> $crate::higher::Pure<A> for $name<$($other_lifetimes,)* A> { - fn pure(value : A) -> Self { + impl<$($other_lifetimes,)* $generic $(,$other_generics)*> $crate::higher::Pure<$generic> for $name<$($other_lifetimes,)* $generic $(,$other_generics)*> { + fn pure(value : $generic) -> Self { Self::Pure(value) } } - impl<'free_macro_reserved_lifetime, $($other_lifetimes,)* A> $crate::higher::Apply<'free_macro_reserved_lifetime, A> for $name<$($other_lifetimes,)* A> where A: 'free_macro_reserved_lifetime + Clone, Self : Clone { - type Target<T> = $name<$($other_lifetimes,)* T> where T:'free_macro_reserved_lifetime; - fn apply<B>( + impl<'free_macro_reserved_lifetime, $($other_lifetimes,)* $generic $(,$other_generics)*> $crate::higher::Apply<'free_macro_reserved_lifetime, $generic> for $name<$($other_lifetimes,)* $generic $(,$other_generics)*> where $generic: 'free_macro_reserved_lifetime + Clone, Self : Clone { + type Target<FreeMacroReservedType> = $name<$($other_lifetimes,)* FreeMacroReservedType $(,$other_generics)*> where FreeMacroReservedType:'free_macro_reserved_lifetime; + fn apply<FreeMacroReservedType>( self, - f: <Self as $crate::higher::Apply<'free_macro_reserved_lifetime, A>>::Target<$crate::higher::apply::ApplyFn<'free_macro_reserved_lifetime, A, B>>, - ) -> <Self as $crate::higher::Apply<'free_macro_reserved_lifetime, A>>::Target<B> + f: <Self as $crate::higher::Apply<'free_macro_reserved_lifetime, $generic>>::Target<$crate::higher::apply::ApplyFn<'free_macro_reserved_lifetime, $generic, FreeMacroReservedType>>, + ) -> <Self as $crate::higher::Apply<'free_macro_reserved_lifetime, $generic>>::Target<FreeMacroReservedType> where - B: 'free_macro_reserved_lifetime, + FreeMacroReservedType: 'free_macro_reserved_lifetime, { $crate::higher::apply::ap(f,self) } } - impl<'free_macro_reserved_lifetime, $($other_lifetimes,)* A> $crate::higher::Bind<'free_macro_reserved_lifetime,A> for $name<$($other_lifetimes,)* A>{ - type Target<T> = $name<$($other_lifetimes,)* T>; - fn bind<B, F>(self, f: F) -> Self::Target<B> + impl<'free_macro_reserved_lifetime, $($other_lifetimes,)* $generic $(,$other_generics)*> $crate::higher::Bind<'free_macro_reserved_lifetime,$generic> for $name<$($other_lifetimes,)* $generic $(,$other_generics)*>{ + type Target<FreeMacroReservedType> = $name<$($other_lifetimes,)* FreeMacroReservedType $(,$other_generics)*>; + fn bind<FreeMacroReservedType, FreeMacroReservedType2>(self, f: FreeMacroReservedType2) -> Self::Target<FreeMacroReservedType> where - F: Fn(A) -> Self::Target<B>, + FreeMacroReservedType2: Fn($generic) -> Self::Target<FreeMacroReservedType>, { - fn __bind_impl<'free_macro_reserved_lifetime, $($other_lifetimes,)* A, B, F>(s : $name<$($other_lifetimes,)* A>, f: &F) -> $name<$($other_lifetimes,)* B> where F: Fn(A) -> $name<$($other_lifetimes,)* B> + 'free_macro_reserved_lifetime{ + fn __bind_impl<'free_macro_reserved_lifetime, $($other_lifetimes,)* $generic $(,$other_generics)*, FreeMacroReservedType, FreeMacroReservedType2>(s : $name<$($other_lifetimes,)* $generic $(,$other_generics)*>, f: &FreeMacroReservedType2) -> $name<$($other_lifetimes,)* FreeMacroReservedType $(,$other_generics)*> where FreeMacroReservedType2: Fn($generic) -> $name<$($other_lifetimes,)* FreeMacroReservedType $(,$other_generics)*> + 'free_macro_reserved_lifetime{ use $crate::higher::Functor; match s { $name::Pure(a) => {f(a)}, |