For-in loops in OCaml
In my previous article, I gave a preview on implementing context managers in OCaml using binding operators.
Guess what? Let-bindings are back. This time, we will use them to emulate for-in loops in OCaml.
For-in loops
But first, let's see how for-in loops look like in other languages.
Rust
Ahh, Rust. Did you know that the original Rust compiler was written in OCaml before getting self-hosted? Pretty cool, huh?
pub fn doubled_positives(list: Vec<i32>) -> Vec<i32> {
let mut new_items = vec![];
for item in list {
if item > 0 {
new_items.push(item * 2);
}
}
new_items
}
The function doubled_positives takes a list of numbers and returns its positive items twice their value.
Okay, I know, this is far from being the most idiomatic Rust code, nor the most efficient. It's fine, we are just showcasing a specific syntactic feature.
ABC
ABC is a programming language of the 80s that greatly inspired the well-known Python1. Unsurprisingly, it supports the same construct as the snakey language, with almost identical notation if not for the uppercase keywords.
HOW TO RETURN doubled.positives list:
PUT {} in new.items
FOR item IN list:
IF item > 0:
INSERT (item * 2) IN new.items
RETURN new.items
The function doubled.positives does the same as the Rust implementation. You will notice that we make use of the for-in loop both to filter out numbers and to double them.
Hey, that sounds like OCaml's
List.filter_map!
That's exactly what it is here! In fact, that's the function we will use to implement our own for-in loop in OCaml.
OCaml
Now is time for our (well, at least my) favorite bactrian language!
We are aiming for something that looks like this:
let$ item = list in
if cond item
then Some (do_something item)
else None
So, いきます!2
let ( let$ ) list func = List.filter_map func list
And... that's it?!
Let's see it in action:
let doubled_positives list =
let$ item = list in
if item > 0
then Some (item * 2)
else None
;;
Et voilà !
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