List.iter and Array.iter

These are two very classical functions, from the modules List [2] and Array [1].

These functions are iterators of the following type:

val List.iter : ('a -> unit) -> 'a list -> unit
val Array.iter : ('a -> unit) -> 'a array -> unit

They allow to apply a certain function f : 'a -> unit which acts by side effects (e.g., printing) to each element of a list or an array.

Very convenient!

A better version?

But... quite often, we need more than just the value of x of each element, we might need their index i (for instance to print something like "the ith value contains x[i]").

So, we would like to have two other iterator functions with the following signature:

val List.iteri : (int -> 'a -> unit) -> 'a list -> unit
val Array.iteri : (int -> 'a -> unit) -> 'a array -> unit

Now, f accepts as first argument the index i corresponding to its second argument the value xi. That will be very convenient to avoid for loops!

for i in [2*i for i in range(8)]:
    print i

Two functions that took their time to appear

We can check that Array.iteri has been implemented back in 1997 [3], but its straightforward equivalent for lists List.iteri has only been added in... 2010 with OCaml v4.00.1 ! (cf [4]).

So what's the issue?

These two functions are very useful, and so they are used a lot.

During some programming projects and exercises that were asked to me during some courses at ENS Cachan, I have been ask to make my OCaml code work on at least a machine in room 411, machines which were old at this time!

And that's why in December 2013, we lost a few minutes (during an oral exam) trying to find the source of a bug, and I included a tiny "patch" v3.12 → v4.01.0 which adds manually the code for List.iteri in my project [5].

In particular, this link [6] shows the different modifications on my code, after I saw this bug.

Anyway, we would have liked to get them before!

That's basically what this (useless) article was about.

References