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//! 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 $(,$other_generics:ident)*>, $f:ty) => {
#[derive(Clone)]
$v enum $name<$($other_lifetimes,)* $generic $(,$other_generics)*> {
Pure($generic),
Free(Box<$f>)
}
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))))
}
$v fn retract<'free_macro_reserved_lifetime>(self) -> <$f as $crate::higher::Bind<'free_macro_reserved_lifetime,Self>>::Target<$generic> where $f : $crate::higher::Monad<'free_macro_reserved_lifetime,Self>, <$f as $crate::higher::Bind<'free_macro_reserved_lifetime,Self>>::Target<$generic> : $crate::higher::Pure<$generic> {
use $crate::higher::{Bind, Pure};
match self {
$name::Pure(a) => {<$f as $crate::higher::Bind<'free_macro_reserved_lifetime,Self>>::Target::<$generic>::pure(a)},
$name::Free(m) => {m.bind(|a| a.retract())}
}
}
}
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))},
$name::Free(fa) => {$name::Free(Box::new(fa.fmap(|x| __fmap_impl(x, f))))},
}
}
__fmap_impl(self, &f)
}
}
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,)* $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, $generic>>::Target<$crate::higher::apply::ApplyFn<'free_macro_reserved_lifetime, $generic, FreeMacroReservedType>>,
) -> <Self as $crate::higher::Apply<'free_macro_reserved_lifetime, $generic>>::Target<FreeMacroReservedType>
where
FreeMacroReservedType: 'free_macro_reserved_lifetime,
{
$crate::higher::apply::ap(f,self)
}
}
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
FreeMacroReservedType2: Fn($generic) -> Self::Target<FreeMacroReservedType>,
{
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)},
$name::Free(fa) => {$name::Free(Box::new(fa.fmap(|x| __bind_impl(x, f))))},
}
}
__bind_impl(self, &f)
}
}
};
(<$a:lifetime>, $v:vis $name:ident<$($other_lifetimes:lifetime,)+ $generic:ident>, $f:ty) =>{
#[derive(Clone)]
$v enum $name<$($other_lifetimes,)* $generic> {
Pure($generic),
Free(Box<$f>)
}
impl<$($other_lifetimes : $a,)* $generic> $name<$($other_lifetimes,)* $generic> where $generic : $a {
$v fn lift_f(functor : <$f as $crate::higher::Functor<$a, Self>>::Target<$generic>) -> Self{
use $crate::higher::Functor;
Self::Free(Box::new(functor.fmap(|a| Self::Pure(a))))
}
$v fn retract(self) -> <$f as $crate::higher::Bind<$a,Self>>::Target<$generic> where $f : $crate::higher::Monad<$a,Self>, <$f as $crate::higher::Bind<$a,Self>>::Target<$generic> : $crate::higher::Pure<$generic> {
use $crate::higher::{Bind, Pure};
match self {
$name::Pure(a) => {<$f as $crate::higher::Bind<$a,Self>>::Target::<$generic>::pure(a)},
$name::Free(m) => {m.bind(|a| a.retract())}
}
}
}
impl<$($other_lifetimes : $a,)* A> $crate::higher::Functor<$a,A> for $name<$($other_lifetimes,)*A> where A : $a {
type Target<T> = $name<$($other_lifetimes,)* T>;
fn fmap<B,F>(self, f: F) -> Self::Target<B>
where F: Fn(A) -> B + $a
{
fn __fmap_impl<$($other_lifetimes : $a,)* A, B, F>(s : $name<$($other_lifetimes,)* A>, f : std::rc::Rc<F>) -> $name<$($other_lifetimes,)* B> where A : $a, F: Fn(A) -> B + $a{
use $crate::higher::Functor;
match s {
$name::Pure(a) => {$name::Pure(f(a))},
$name::Free(fa) => {$name::Free(Box::new(fa.fmap(move |x : $name<$($other_lifetimes,)*A>| __fmap_impl(x, f.clone()))))},
}
}
let r = std::rc::Rc::new(f);
__fmap_impl(self, r)
}
}
impl<$($other_lifetimes,)* A> $crate::higher::Pure<A> for $name<$($other_lifetimes,)* A> {
fn pure(value : A) -> Self {
Self::Pure(value)
}
}
impl<$($other_lifetimes : $a,)* A> $crate::higher::Apply<$a, A> for $name<$($other_lifetimes,)* A> where A: $a + Clone, Self : Clone{
type Target<T> = $name<$($other_lifetimes,)* T> where T:$a;
fn apply<B>(
self,
f: <Self as $crate::higher::Apply<$a, A>>::Target<$crate::higher::apply::ApplyFn<$a, A, B>>,
) -> <Self as $crate::higher::Apply<$a, A>>::Target<B>
where
B: $a,
{
$crate::higher::apply::ap(f,self)
}
}
impl<$($other_lifetimes : $a,)* A> $crate::higher::Bind<$a,A> for $name<$($other_lifetimes,)*A> where A : $a{
type Target<T> = $name<$($other_lifetimes,)* T>;
fn bind<B, F>(self, f: F) -> Self::Target<B>
where
F: Fn(A) -> Self::Target<B> + $a,
{
fn __bind_impl<$($other_lifetimes : $a,)* A, B, F>(s : $name<$($other_lifetimes,)*A>, f : std::rc::Rc<F>) -> $name<$($other_lifetimes,)* B> where A : $a, F: Fn(A) -> $name<$($other_lifetimes,)* B> + $a{
use $crate::higher::Functor;
match s {
$name::Pure(a) => {f(a)},
$name::Free(fa) => {$name::Free(Box::new(fa.fmap(move |x : $name<$($other_lifetimes,)*A>| __bind_impl(x, f.clone()))))},
}
}
let r = std::rc::Rc::new(f);
__bind_impl(self, r)
}
}
};
}
|
