fortraith

Compile-time compiler that compiles Forth to compile-time trait expressions.

What?

Rust's trait system is Turing complete. This crate uses the principles from trait-eval to implement necessary traits for forth evaluation and provides a forth! macro that transpiles forth's syntax to trait expressions.

Show me!

Here's a simple factorial implementation, the only non-standard word here is pred which is a decrement operator, equivalent to 1 -:

forth!(
    : factorial (n -- n) 1 swap fact0 ;
    : fact0 (n n -- n) dup 1 = if drop else dup rot * swap pred fact0 then ;
    5 factorial .
);

This prints 120. As you can see not only can you define functions easily, but even conditional recursion is possible! Now check out how it looks compiled to trait expressions (courtesy of cargo expand):

pub trait factorial {
   type Result;
}
impl<V, N> factorial for Node<V, N>
where
    Self: one,
    <Self as one>::Result: swap,
    <<Self as one>::Result as swap>::Result: fact0,
{
    type Result = <<<Self as one>::Result as swap>::Result as fact0>::Result;
}
pub trait fact0 {
    type Result;
}
impl <V ,N> fact0 for Node <V ,N>
where
    Self: dup,
    <Self as dup>::Result: one,
    <<Self as dup>::Result as one>::Result: eq,
    <<<Self as dup>::Result as one>::Result as eq>::Result: iff,
    <<<<Self as dup>::Result as one>::Result as eq>::Result as iff>::Result: drop,
    <<<<<Self as dup>::Result as one>::Result as eq>::Result as iff>::Result as drop>::Result: elsef,
    <<<<<<Self as dup>::Result as one>::Result as eq>::Result as iff>::Result as drop>::Result as elsef>::Result: dup,
    <<<<<<<Self as dup>::Result as one>::Result as eq>::Result as iff>::Result as drop>::Result as elsef>::Result as dup>::Result: rot,
    <<<<<<<<Self as dup>::Result as one>::Result as eq>::Result as iff>::Result as drop>::Result as elsef>::Result as dup>::Result as rot>::Result: mult,
    <<<<<<<<<Self as dup>::Result as one>::Result as eq>::Result as iff>::Result as drop>::Result as elsef>::Result as dup>::Result as rot>::Result as mult>::Result: swap,
    <<<<<<<<<<Self as dup>::Result as one>::Result as eq>::Result as iff>::Result as drop>::Result as elsef>::Result as dup>::Result as rot>::Result as mult>::Result as swap>::Result: pred,
    <<<<<<<<<<<Self as dup>::Result as one>::Result as eq>::Result as iff>::Result as drop>::Result as elsef>::Result as dup>::Result as rot>::Result as mult>::Result as swap>::Result as pred>::Result: fact0,
    <<<<<<<<<<<<Self as dup>::Result as one>::Result as eq>::Result as iff>::Result as drop>::Result as elsef>::Result as dup>::Result as rot>::Result as mult>::Result as swap>::Result as pred>::Result as fact0>::Result: then
{
    type Result = <<<<<<<<<<<<<Self as dup>::Result as one>::Result as eq>::Result as iff>::Result as drop>::Result as elsef>::Result as dup>::Result as rot>::Result as mult>::Result as swap>::Result as pred>::Result as fact0>::Result as then>::Result;
}
println!("{}", <<<Empty as five>::Result as factorial>::Result as top>::Result::eval());

Yeah, writing that manually would be no fun.

What can I do with it?

Quite a bit is actually supported as you can see above. Every operation from trait-eval is re-exported to work on the stack (except if which is done differently), and a few additional stack operations are provided. See docs for the details.