path: root/src/lib.rs

//! A macro that uses the traits from the [higher] crate and generates a Free [`Monad`][higher::Monad] type for a given [`Functor`][higher::Functor].
//! 
//! This is a port of the Control.Monad.Free part of the ["free" Haskell package](https://hackage.haskell.org/package/free) by Edward Kmett.
//! 
//! # What is a Free Monad?
//! A Free Monad is the left-adjoint to the Forget-Functor from the category of Monads into the category of Endofunctors.
//! 
//! From a programmer's perspective, however, it is a nifty way to create a [`Monad`][higher::Monad], that is "based" on a given [`Functor`][higher::Functor]
//! and does not impose any additional structure beyond the [Monad Laws](https://wiki.haskell.org/Monad_laws).
//! 
//! The structure of the Free [`Monad`][higher::Monad] is defined by the underlying [`Functor`][higher::Functor].
//! For instance, if the underlying [`Functor`][higher::Functor] is a [`Vec`], the corresponding Free [`Monad`][higher::Monad] will be a linked tree.
//! If the underlying [`Functor`][higher::Functor] is an [`Option`], the corresponding Free [`Monad`][higher::Monad] is a linked list.
//! And so on, and so forth.
//! 
//! There are many use cases for such a data structure, the most well known one is the creation of embedded
//! [Domain Specific Languages](https://en.wikipedia.org/wiki/Domain-specific_language) (eDSLs).
//! Going into detail would go beyond the scope of this documentation, however. Please check out Nikolay Yakimov's
//! [Introduction to Free Monads](https://serokell.io/blog/introduction-to-free-monads) for that.
//! 
//! There is also a [blog post about the development of this macro](https://www.grois.info/posts/2023-03/2023-03-11-adventures-with-free-monads-and-higher.xhtml),
//! that presents a simple (but inexact) mental picture
//! (by means of actual [pictures](https://www.grois.info/posts/2023-03/2023-03-11-adventures-with-free-monads-and-higher.xhtml#ugly_drawings))
//! of how the different [`Monad`][higher::Monad] operations (bind, fmap, pure, apply) work on Free Monads.
//! 
//! # How to use the macro?
//! 
//! For details, please see the [free] macro directly. In short, the syntax is either `free!(FreeMonadTypeName<'a, A>, FunctorItsBasedOn<FreeMonadTypeName<'a, A>>)`,
//! or, if the lifetime of the Free Monad depends on the lifetime of the function passed to the Functor's fmap function,
//! `free!(<'a>, FreeMonadTypeName<'a,A>, FunctorItsBasedOn<'a,FreeMonadTypeName<'a,A>>)`, where `'a` is the affected lifetime.
//! 
//! # Examples
//! Please check the "tests" folder in the project's repo. While it currently just contains some trivial test cases, it will be extended over time to
//! contain more involved examples.
//! 
//! # 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.
//! 
//! # A word of warning:
//! This crate should be considered a proof-of-concept. Its memory complexity is horrendous, and the performance of the Free Monad's [`Apply`][higher::Apply]
//! implementation can only be described as abysmal due to its reliance on deep copies.

pub extern crate higher;

/// The macro that generates a Free [`Monad`][higher::Monad] type for a given [`Functor`][higher::Functor].
/// 
/// To declare a Free [`Monad`][higher::Monad] over a [`Functor`][higher::Functor] named `Funky<A>`, the syntax would be `free!(FreeFunky<A>, Funky<FreeFunky<A>>)`.
/// This declares an enum named `FreeFunky<A>`, and implements all traits needed for it to be a [`Monad`][higher::Monad].
/// 
/// # Restrictions
/// It is currently not supported to create a Free Monad for a Functor that does not implement [`Clone`]. This is because it is in general not
/// possible to implement [`Apply`][higher::Apply] for a non-cloneable Free Monad, and because of how Rust resolves trait bounds in recursive types.
/// 
/// In addition, for the result to actually be a [`Monad`][higher::Monad], the `Pure` type (the type the Free Monad is generic over) needs to support [`Clone`]
/// too. This is again because of the requirement of [`Apply`][higher::Apply], which in turn is a requirement of [`Monad`][higher::Monad]. However,
/// it is typically not necessary to have a fully fledged [`Monad`][higher::Monad]. In most use cases, it's enough to have 
/// [`Functor`][higher::Functor] + [`Bind`][higher::Bind] + [`Pure`][higher::Pure].
/// 
/// The Free Monad type is implemented recursively. It is therefore akin to a linked tree, with all the respective performance implications.
/// 
/// Furthermore, the implementation of [`Apply`][higher::Apply] creates a potentially high number of deep copies of the `self` parameter.
/// It should therefore be avoided, unless one really needs its 
/// [tree-merging behaviour](https://www.grois.info/posts/2023-03/2023-03-11-adventures-with-free-monads-and-higher.xhtml#ugly_apply_drawing).
/// 
/// # Usage
/// As stated above, the syntax to create a Free Monad is usually to call the macro with the desired Free Monad type as first,
/// and the [`Functor`][higher::Functor] it should be based on as second parameter.
/// 
/// For example, a Free Monad based on [`Option`] could simply be created like this:
/// ```
/// # #[macro_use] extern crate higher_free_macro;
/// # use higher_free_macro::higher::*;
/// free!(FreeOption<A>, Option<FreeOption<A>>);
/// ```
/// 
/// The type created by this is indeed a Monad, as long as the wrapped type is [`Clone`]:
/// ```
/// # #[macro_use] extern crate higher_free_macro;
/// # use higher_free_macro::higher::*;
/// free!(FreeOption<A>, Option<FreeOption<A>>);
/// 
/// fn returns_a_monad<'a, A>(a : A) -> impl Monad<'a,A> where A : Clone + 'a {
///     FreeOption::Pure(a)
/// }
/// ```
/// Since, strictly speaking, [`Apply`][higher::Apply] is not required to express the properties of a Monad (the mathematical structure, not the trait),
/// one might want to skip the requirement of [`Clone`]. The result is still [`Bind`][higher::Bind], [`Functor`][higher::Functor] and [`Pure`][higher::Pure],
/// so in the mathematical sense a Monad:
/// ```
/// # #[macro_use] extern crate higher_free_macro;
/// # use higher_free_macro::higher::*;
/// free!(FreeOption<A>, Option<FreeOption<A>>);
///
/// fn returns_a_bind_pure_functor<'a, A>(a : A) -> impl Bind<'a,A> + Pure<A> + Functor<'a,A>
///     where A : 'a
/// {
///     FreeOption::Pure(a)
/// }
/// ```
/// 
/// That said, the macro also supports multiple generic parameters. The parameter for which the traits will be implemented is the first generic parameter
/// of the to-be-created Free Monad type. For instance, a Free Monad based on [`Result`] would be:
/// ```
/// # #[macro_use] extern crate higher_free_macro;
/// # use higher_free_macro::higher::*;
/// free!(FreeResult<A,E>, Result<FreeResult<A,E>,E>);
/// 
/// fn returns_a_monad<'a, A, E>(r : Result<A,E>) -> impl Monad<'a,A> 
///     where A : Clone + 'a, E : Clone
/// {
///     FreeResult::lift_f(r)
/// }
/// ```
/// 
/// Furthermore, the use case that the lifetime of the Free Monad depends on the lifetime of the mapping functions is supported too.
/// This is particularly useful, because it enables the usage of (non-constant) continuation functions, what is a requirement for 
/// using the Free Monad for an embedded Domain Specific Language (eDSL).
/// 
/// Such a [`Functor`][higher::Functor] could for instance look like this:
/// ```
/// # #[macro_use] extern crate higher_free_macro;
/// # use higher_free_macro::higher::*;
/// #[derive(Clone)]
/// struct FunctorWithCont<'a, A>(std::rc::Rc<dyn Fn(i32)->A + 'a>);
/// impl<'a,A : 'a> Functor<'a,A> for FunctorWithCont<'a, A>{
///     type Target<T> = FunctorWithCont<'a, T>;
///     fn fmap<B,F>(self, f :F) -> Self::Target<B> where F : Fn(A)->B + 'a{
///         FunctorWithCont(std::rc::Rc::new(move |x| f((self.0)(x))))
///     }
/// }
/// ```
/// 
/// Sadly, the macro syntax is a bit more convoluted in this case. The relevant lifetime has to be stated explicitly as the first parameter, like this:
/// ```
/// # #[macro_use] extern crate higher_free_macro;
/// # use higher_free_macro::higher::*;
/// free!(<'a>, FreeFunctorWithCont<'a,A>, FunctorWithCont<'a,FreeFunctorWithCont<'a,A>>);
/// # #[derive(Clone)]
/// # struct FunctorWithCont<'a, A>(std::rc::Rc<dyn Fn(i32)->A + 'a>);
/// # impl<'a,A : 'a> Functor<'a,A> for FunctorWithCont<'a, A>{
/// #     type Target<T> = FunctorWithCont<'a, T>;
/// #     fn fmap<B,F>(self, f :F) -> Self::Target<B> where F : Fn(A)->B + 'a{
/// #         FunctorWithCont(std::rc::Rc::new(move |x| f((self.0)(x))))
/// #     }
/// # }
/// ```
/// 
/// # Generated Functions
/// In addition to the trait implementations for [`Bind`][higher::Bind], [`Functor`][higher::Functor], [`Apply`][higher::Apply] and [`Pure`][higher::Pure],
/// the macro also generates associated functions for the Free Monad type. These functions are:  
/// `fn lift_f(functor : F) -> Self`  
/// `fn retract(self)-> F where F : Bind + Pure`  
/// where `F` is the [`Functor`][higher::Functor] the Free Monad is based on, specialized for the `Pure` type.
/// A concrete example will make this more clear. Let's take our `FreeOption<A>` example from above. In this case, the signatures are  
/// `fn lift_f(functor : Option<A>) -> FreeOption<A>` and  
/// `fn retract(self : FreeOption<A>) -> Option<A>`
/// 
/// `lift_f()` converts a base Functor into the corresponding Free Monad, meaning that the Functor gets wrapped in `Free`, and the value it holds gets
/// mapped into a `Pure`. The (simplified for readability) formula is:  
/// `Self::Free(functor.fmap(|a| Self::Pure(a)))`
/// 
/// `retract()` is the left-inverse of `lift_f()`. `|x| retract(lift_f(x))` is (ignoring type coercion) equivalent to [`identity`][std::convert::identity]:
/// ```
/// # #[macro_use] extern crate higher_free_macro;
/// # use higher_free_macro::higher::*;
/// free!(FreeOption<A>, Option<FreeOption<A>>);
/// fn main() {
///     let free_monad = FreeOption::lift_f(Some(12345u32));
///     match &free_monad {
///         FreeOption::Free(o) => {
///             match &**o {
///                 Some(p) => {
///                     match p {
///                         FreeOption::Pure(v) => assert_eq!(v, &12345u32),
///                         FreeOption::Free(_) => unreachable!()
///                     }
///                 },
///                 None => unreachable!()
///             }
///         },
///         FreeOption::Pure(_) => unreachable!()
///     }
///     let and_back = free_monad.retract();
///     assert_eq!(and_back, Some(12345u32));
/// }
/// ```
#[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(Self::Pure)))
            }

            $v fn retract<'free_macro_reserved_lifetime>(self) -> <$f as $crate::higher::Bind<'free_macro_reserved_lifetime,Self>>::Target<$generic> where $f : $crate::higher::Bind<'free_macro_reserved_lifetime,Self>, <$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{
                    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 $(,$other_generics:ident)*>, $f:ty) =>{
        #[derive(Clone)]
        $v enum $name<$($other_lifetimes,)* $generic $(,$other_generics)*> {
            Pure($generic),
            Free(Box<$f>)
        }
        impl<$($other_lifetimes : $a,)* $generic $(,$other_generics)*> $name<$($other_lifetimes,)* $generic $(,$other_generics)*> 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(Self::Pure)))
            }

            $v fn retract(self) -> <$f as $crate::higher::Bind<$a,Self>>::Target<$generic> where $f : $crate::higher::Bind<$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(Self::retract)}
                }
            }
        }
    
        impl<$($other_lifetimes : $a,)* $generic $(,$other_generics)*> $crate::higher::Functor<$a,$generic> for $name<$($other_lifetimes,)* $generic $(,$other_generics)*> where $generic : $a {
            type Target<FreeMacroReservedType> = $name<$($other_lifetimes,)* FreeMacroReservedType $(,$other_generics)*>;
            fn fmap<FreeMacroReservedType,F>(self, f: F) -> Self::Target<FreeMacroReservedType> 
                where F: Fn($generic) -> FreeMacroReservedType + $a
            {
                fn __fmap_impl<$($other_lifetimes : $a,)* $generic $(,$other_generics)*, FreeMacroReservedType, F>(s : $name<$($other_lifetimes,)* $generic $(,$other_generics)*>, f : std::rc::Rc<F>) -> $name<$($other_lifetimes,)* FreeMacroReservedType $(,$other_generics)*> where $generic : $a, F: Fn($generic) -> FreeMacroReservedType + $a{
                    match s {
                        $name::Pure(a) => {$name::Pure(f(a))},
                        $name::Free(fa) => {$name::Free(Box::new(fa.fmap(move |x : $name<$($other_lifetimes,)* $generic $(,$other_generics)*>| __fmap_impl(x, f.clone()))))},
                    }
                }

                let r = std::rc::Rc::new(f);
                __fmap_impl(self, r)
            }
        }

        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<$($other_lifetimes : $a,)* $generic $(,$other_generics)*> $crate::higher::Apply<$a, $generic> for $name<$($other_lifetimes,)* $generic $(,$other_generics)*> where $generic: $a + Clone, Self : Clone{
            type Target<FreeMacroReservedType> = $name<$($other_lifetimes,)* FreeMacroReservedType $(,$other_generics)*> where FreeMacroReservedType:$a;
            fn apply<FreeMacroReservedType>(
                self,
                f: <Self as $crate::higher::Apply<$a, $generic>>::Target<$crate::higher::apply::ApplyFn<$a, $generic, FreeMacroReservedType>>,
            ) -> <Self as $crate::higher::Apply<$a, $generic>>::Target<FreeMacroReservedType>
            where
            FreeMacroReservedType: $a,
            {
                $crate::higher::apply::ap(f,self)
            }
        }

        impl<$($other_lifetimes : $a,)* $generic $(,$other_generics)*> $crate::higher::Bind<$a,$generic> for $name<$($other_lifetimes,)* $generic $(,$other_generics)*> where $generic : $a{
            type Target<FreeMacroReservedType> = $name<$($other_lifetimes,)* FreeMacroReservedType $(,$other_generics)*>;
            fn bind<FreeMacroReservedType, F>(self, f: F) -> Self::Target<FreeMacroReservedType>
            where
                F: Fn($generic) -> Self::Target<FreeMacroReservedType> + $a,
            {
                fn __bind_impl<$($other_lifetimes : $a,)* $generic $(,$other_generics)*, FreeMacroReservedType, F>(s : $name<$($other_lifetimes,)* $generic $(,$other_generics)*>, f : std::rc::Rc<F>) -> $name<$($other_lifetimes,)* FreeMacroReservedType $(,$other_generics)*> where $generic : $a, F: Fn($generic) -> $name<$($other_lifetimes,)* FreeMacroReservedType $(,$other_generics)*> + $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,)* $generic $(,$other_generics)*>| __bind_impl(x, f.clone()))))},
                    }
                }
                let r = std::rc::Rc::new(f);
                __bind_impl(self, r)
            }
        }
    };
}

#[cfg(test)]
mod free_monad_tests{
    use higher::{Pure, Functor, Bind, Apply};

    use super::free;
    
    free!(FreeVec<A>, Vec<FreeVec<A>>);

    #[test]
    fn test_lift_f_no_lifetime(){
        let f = FreeVec::lift_f(vec![1,2,3]);
        match f {
            FreeVec::Free(v) => {
                match &**v {
                    [FreeVec::Pure(a),FreeVec::Pure(b),FreeVec::Pure(c)] => {
                        assert_eq!(vec![*a,*b,*c], vec![1,2,3]);
                    },
                    _ => unreachable!()
                }
            },
            _ => unreachable!()
        }
    }

    #[test]
    fn test_retract_no_lifetime(){
        let f = FreeVec::lift_f(vec![1,2,3]);
        let v = f.retract();
        assert_eq!(v, vec![1,2,3]);
    }

    #[test]
    fn test_pure_no_lifetime(){
        let f = FreeVec::pure(3);
        match f {
            FreeVec::Pure(v) => assert_eq!(v,3),
            FreeVec::Free(_) => unreachable!(),
        }
    }

    #[test]
    fn test_fmap_no_lifetime(){
        let f = FreeVec::lift_f(vec![1,2,3]);
        let f = f.fmap(|x| (x as f32)/2.0);
        match f {
            FreeVec::Free(f) => {
                match &**f{
                    [FreeVec::Pure(a), FreeVec::Pure(b), FreeVec::Pure(c)] => {
                        assert_eq!(vec![0.5f32, 1f32, 1.5f32], vec![*a,*b,*c]);
                    },
                    _ => unreachable!()
                }
            },
            _ => unreachable!()
        }
    }

    #[test]
    fn test_bind_no_lifetime(){
        let f = FreeVec::lift_f(vec![1,2]);
        let f = f.bind(|x| if x % 2 == 0 { FreeVec::lift_f(vec![x as f32,x as f32 + 1.0f32])} else { FreeVec::Pure(x as f32)});
        match f {
            FreeVec::Free(f) => {
                match &**f {
                    [FreeVec::Pure(a),FreeVec::Free(b)] => {
                        assert_eq!(*a, 1.0f32);
                        match &***b {
                            [FreeVec::Pure(a), FreeVec::Pure(b)] => {
                                assert_eq!(*a, 2.0f32);
                                assert_eq!(*b, 3.0f32);
                            },
                            _ => unreachable!()
                        }
                    },
                    _ => unreachable!()
                }
            },
            _ => unreachable!()
        }
    }

    #[test]
    fn test_apply_no_lifetime(){
        let f = FreeVec::Free(Box::new(vec![FreeVec::Free(Box::new(vec![FreeVec::Pure((|x| (x as i32)*2) as fn(u32) -> i32), FreeVec::Pure((|x| (x as i32)+2) as fn(u32)->i32)])), FreeVec::Pure((|x| (x as i32)-5) as fn(u32)->i32)]));
        let m = FreeVec::Free(Box::new(vec![FreeVec::Pure(5u32), FreeVec::Free(Box::new(vec![FreeVec::Pure(6u32), FreeVec::Pure(7u32)]))]));
        let m = m.apply(f.fmap(Into::into));
        //what have I gotten myself into...
        //at least the mapped sub-trees are all identical in shape to m, so, they can be tested by the same test function...
        let check_mlike_structure = |m : &FreeVec<_>, p1,p2,p3| {
            match m {
                FreeVec::Free(m) => {
                    match &***m{
                        [FreeVec::Pure(l),FreeVec::Free(r)] => {
                            assert_eq!(*l,p1);
                            match &***r{
                                [FreeVec::Pure(l), FreeVec::Pure(r)] => {
                                    assert_eq!(*l, p2);
                                    assert_eq!(*r, p3);
                                },
                                _ => unreachable!()
                            }
                        },
                        _ => unreachable!()
                    }
                },
                _ => unreachable!()
            }
        };
        //now, where are those sub-trees exactly, in this monstrosity?
        match m {
            FreeVec::Free(m) => {
                match &**m{
                    [FreeVec::Free(l), r] => {
                        match &***l {
                            [a,b] => {
                                check_mlike_structure(a, 10,12,14);
                                check_mlike_structure(b, 7,8,9);
                            },
                            _ => unreachable!()
                        }
                        check_mlike_structure(r, 0,1,2)
                    },
                    _ => unreachable!()
                }
            },
            _ => unreachable!()
        }
    }

}