12Feb

A Play on Regular Expressions, Part 3: The First Haskell-to-Rust Port

Posted by Elf Sternberg as Rust

In the last post on “A Play on Regular Expressions,” I showed how we go from a boolean regular expression to a “rigged” one; one that uses an arbitrary data structure to extract data from the process of recognizing regular expressions. The data structure must conform to a set of mathematical laws (the semiring laws), but that simple requirement led us to some surprisingly robust results.

Now, the question is: Can we port this to Rust?

Easily.

The first thing to do, however, is to not implement a Semiring. A Semiring is a conceptual item, and in Rust it turns out that you can get away without defining a Semiring as a trait; instead, it’s a collection of traits derived from the num_traits crate: Zero, zero, One, one; the capitalized versions are the traits, and the lower case ones are the implementations we have to provide.

I won’t post the entire code here, but you can check it out in Rigged Kleene Regular Expressions in Rust. Here are a few highlights:

The accept() function for the Haskell version looked like this:

acceptw :: Semiring s => Regw c s -> [c] -> s
acceptw Epsw u     = if null u then one else zero
acceptw (Symw f) u = case u of [c] -> f c;  _ -> zero
acceptw (Altw p q) u = acceptw p u `add` acceptw q u
acceptw (Seqw p q) u = sumr [ acceptw p u1 `mul` acceptw q u2 | (u1, u2) <- split u ]
acceptw (Repw r)   u = sumr [ prodr [ acceptw r ui | ui <- ps ] | ps <- parts u ]

The accept() function in Rust looks almost the same:

pub fn acceptw<S>(r: &Regw<S>, s: &[char]) -> S
    where S: Zero + One
{
    match r {
        Regw::Eps => if s.is_empty() { one() } else { zero() },
        Regw::Sym(c) => if s.len() == 1 { c(s[0]) } else { zero() },
        Regw::Alt(r1, r2) => S::add(acceptw(&r1, s), acceptw(&r2, s)),
        Regw::Seq(r1, r2) => split(s)
            .into_iter()
            .map(|(u1, u2)| acceptw(r1, &u1) * acceptw(r2, &u2))
            .fold(S::zero(), sumr),
        Regw::Rep(r) => parts(s)
            .into_iter()
            .map(|ps| ps.into_iter().map(|u| acceptw(r, &u)).fold(S::one(), prod))
            .fold(S::zero(), sumr)
    }
}

There’s a bit more machinery here to support the sum-over and product-over maps. There’s also the where S: Zero + One clause, which tells us that our Semiring must be something that understands those two notions and have implementations for them.

To restore our boolean version of our engine, we have to build a nominal container that supports the various traits of our semiring. To do that, we need to implement the methods associated with Zero, One, Mul, and Add, and explain what they mean to the datatype of our semiring. The actual work is straightforward.

pub struct Recognizer(bool);

impl Zero for Recognizer {
    fn zero() -> Recognizer { Recognizer(false) }
    fn is_zero(&self) -> bool { !self.0 }
}

impl One for Recognizer {
    fn one() -> Recognizer { Recognizer(true) }
}

impl Mul for Recognizer {
    type Output = Recognizer;
    fn mul(self, rhs: Recognizer) -> Recognizer { Recognizer(self.0 && rhs.0) }
}

impl Add for Recognizer {
    type Output = Recognizer;
    fn add(self, rhs: Recognizer) -> Recognizer { Recognizer(self.0 || rhs.0) }
}

Also, unlike Haskell, Rust must be explicitly told what kind of Semiring will be used before processing, whereas Haskell will see what kind of Semiring you need to produce the processed result and hook up the machinery for you, but that’s not surprising. In Rust, you “lift” a straight expression to a rigged one thusly:

let rigged: Regw<Recognizer>  = rig(&evencs);

All in all, porting the Haskell to Rust was extremely straightforward. The code looks remarkably similar, but for one detail. In the Kleene version of regular expressions we’re emulating as closely as possible the “all possible permutations of our input string” implicit in the set-theoretic language of Kleene’s 1956 paper. That slows us down a lot, but in Haskell the code for doing it was extremely straightforward, which two simple functions to create all possible permutations for both the sequence and repetition options:

split []     = [([], [])]
split (c:cs) = ([], c : cs) : [(c : s1, s2) | (s1, s2) <- split cs]
parts []     = [[]]
parts [c]    = [[[c]]]
parts (c:cs) = concat [[(c : p) : ps, [c] : p : ps] | p:ps <- parts cs]

In Rust, these two functions were 21 and 29 lines long, respectively. Rust’s demands that you pay attention to memory usage and the rules about it require that you also be very explicit about when you want it, so Rust knows exactly when you no longer want it and can release it back to the allocator.

Rust’s syntax and support are amazing, and the way Haskell can be ported to Rust with little to no loss of fidelity makes me happy to work in both.

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