by Vanessa McHale | 2017-12-26 02:29

Functional programming often gets criticized as being slow, so I wanted to present an example where it is in fact several times faster than the equivalent imperative code1.

The problem is to count the lengths of Collatz sequences, which you may remember from here. In Rust, this is straightforward:

fn modular(a: i64, b: i64) -> i64 { ((a % b) + b) % b } pub fn collatz_length(mut i: i64) -> i64 { let mut l = 1; while i != 1 { i = match modular(i, 2) { 0 => i / 2, _ => 3 * i + 1, }; l += 1; } l }

This is a thoroughly imperative style - we use a mutable variable to track state, and we use a while loop rather than recursion. Our C code will be similarly simple:

int collatz_c(int n) { int l = 0; while (n != 1) { if (n % 2 == 0) n = n / 2; else n = 3 * n + 1; l++; } return l; }

The implementation in ATS is notably less straightforward (you may find this section of the manual to be helpful):

#include "share/atspre_staload.hats" fun collatz_length {n : nat} (n : int(n)) : int = let fnx loop {n : nat}{l : addr} (pf : !int @ l | n : int(n), res : ptr(l)) : void = if n > 1 then let val () = !res := 1 + !res in if n mod 2 = 0 then loop(pf | n / 2, res) else loop(pf | 3 * n + 1, res) end var res: int with pf = 1 val () = loop(pf | n, addr@res) in res end

Here, we do something that is not possible to do in C - we safely stack-allocate a function argument. In addition, we use recursion, allowing us to model the actual problem2.

In practice, this approach leads to dramatically faster code - our ATS code is 1.5 to 2.8 times faster than the equivalent Rust.

Input Language Time 2223 ATS 132.6 ns 2223 C 201.7 ns 2223 Rust 202 ns 10971 ATS 193.1 ns 10971 C 298.7 ns 10971 Rust 400 ns 106239 ATS 253.5 ns 106239 C 399.2 ns 106239 Rust 738 ns

While this particular example may be simple-minded, I think it is nonetheless a nice demonstration that functional programming can indeed be faster than imperative.

2: In fact, had we write a more conventional implementation using pattern matching and recursion, we'd get about the same performance as Rust without the painful compromise of imperative programming.