During my exploration of functionnal programming, I encountered the notion of Parser Combinator when working on a small compiler for LLVM's Kaleidoscope example. Later on I saw a few articles and repository for Parser Combinator in Zig (mecha, hexop's blog)

This is my approach to implement the concept following closely what can be found in the Parsec library for the Haskell language.

All parsers are function, with the same base signature:

fn parser(Stream, std.mem.Allocator, *BaseState) anyerror!Result(T);

fn letterA(stream: Stream, _: std.mem.Allocator, _: *BaseState) anyerror!Result(u8)
{
    if (stream.isEOF()) return Result(u8).unexpected(stream);

    const c = stream.peek();
    if (c == 'A') {
        return Result(u8).success('A', stream.eat(1));
    }
    return Result(u8).unexpected(stream);
}

Stream is a simple object that represent a string with some information on the position of the cursor.

BaseState is a object used to define user state variable and state extension (for specialized parser).