Forth implemented in Rust trait system
source link: https://github.com/Ashymad/fortraith
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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 evalutaion 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 you can 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 faw additional stack operations are provided. See docs
for the details.
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