Simple, ready-to-use typed realization of SmallF*ck esoteric language. Highly inspired by this article.

use typed_sf::*;
type SetTrue<Next = EOF> = Cycle<Flip, Flip<Next>>;
// [*<<[*]*>>>] // Move any-sized chunk of True's 2 cells left

#[allow(non_camel_case_types)]
type prog = Cycle<Flip<Left<Left<SetTrue<Right<Right<Right>>>>>>>;

#[allow(non_camel_case_types)]
type result = Run<
    prog,
    State<Nil, True, Cons<True, Nil>>
>;

assert_eq!(
    <result as StateTrait>::val(),
    (vec![true, true, false, false], false, Vec::new())
);

Links:

Github repo: https://github.com/Zote-the-Mighty-4o2/typed-sf.rs
Documentation: https://docs.rs/typed-sf/

lib.rs:

Type-level implementation of SF. Proving that type-system turing-complete. Highly inspired by this article.

Features

typed-sf supports "no_std" feature with limitations. Runtime representation of List's and [State][StateTrait]'s unavailable (they are using [Vec] to represent themselves).

Full list of unsupported features:

• [List::val()] (That includes [Nil] and Cons).
• RuntimeState.
• [StateTrait::val()] (That includes State and DefaultState).

Also where is "require_docs" feature which forbids undocumented pub items. If You want to contrubute, feel free to turn off this default feature.

How it works

First, there's some traits that i'm calling type-level enums ([Bit], List). Have a look at it:

enum Bit {
True,
False
}

impl Bit {
fn neg(self) -> Bit {
match self {
Bit::True => Bit::False,
Bit::False => Bit::True
}
}
}

assert_eq!(Bit::True, Bit::False.neg());
assert_eq!(Bit::False, Bit::True.neg());

trait Bit {
type Neg: Bit;
fn real() -> bool;
}
struct True;
struct False;

impl Bit for True {
type Neg = False;
fn real() -> bool { true }
}

impl Bit for False {
type Neg = True;
fn real() -> bool { false }
}

assert_eq!(True::real(), <False as Bit>::Neg::real());
assert_eq!(False::real(), <True as Bit>::Neg::real());

Of course, what's more bolierplate, but also instead of matching in runtime, compiler solves type-equasions in compile-time for us.

To type-encode functions i'm using something like this:

#
// A and B are arguments to 'function'.
trait Call<A, B> { type Return; }

struct Sum;
struct Diff;

impl<A, B> Call<A, B> for Sum
where
A: Add<B>
{
type Return = <A as Add<B>>::Output;
}

impl<A, B> Call<A, B> for Diff
where
A: Sub<B>
{
type Return = <A as Sub<B>>::Output;
}

type Apply<A, Fn, B> = <Fn as Call<A, B>>::Return;

What we can use later as:

#
struct X;
struct Y;
struct XaddY;
struct XsubY;

impl Add<Y> for X {
type Output = XaddY;
}
impl Sub<Y> for X {
type Output = XsubY;
}

/* Add `::val() -> Self` function to all types...