Comparing a string character with another character somehow implicitly casts one of them to int?

Learning
1

I encountered a weird error while comparing a string character with another character. I wasn’t able to find an explanation of this on the internet, so I would appreciate if anyone could explain what’s happening here! :pray:

I’m using base. Let’s start with a simple utop session:

utop # open Base ;;

utop # let p = "abc" ;;
val p : string = "abc"

utop # p.[String.length p - 1] ;;
- : char = 'c'

So far so good. I’m able to get the last character of the string. But when trying to compare this character with another, I see a weird error (attaching a screenshot to highlight a relevant part)

utop
utop796×48 9.35 KB

In the text, for a11y:

utop # if p.[String.length p - 1] = 'c' then print_endline "WORKS!!" ;;
Error: This expression has type char but an expression was expected of type int

My question: what actually happens here and why OCaml suddently complains that one of the types is int?

  • I tried using == but base outputs a deprecation warning.
  • I’m able to use Caml.(==) p.[String.length p - 1] 'c' but this looks weird. Is there a better way?

OCaml version:

$ ocaml --version
The OCaml toplevel, version 5.0.0

For context, I’m trying to implement a function append_path that appends two file paths and strips extra path separators, so append_path "foo/" "/bar" would be foo/bar and not foo//bar.

let append_path (p1 : string) (p2 : string) =
  match () with
  | () when String.is_empty p1 -> p2
  | () when String.is_empty p2 -> p1
  | () when Caml.(==) p1.[String.length p1 - 1] '/' && Caml.(==) p2.[0] '/' ->
    p1 ^ String.drop_prefix p2 1
  | _ -> p1 ^ p2

If anyone has any suggestions on making it more idiomatic or reusing some of the existing functions, I would appreciate that a lot as well! :hugs:

2

open Base makes (=) work on int only by shadowing the polymorphic comparison operator.

You can replace it by Char.equal:

if Char.equal p.[String.length p - 1] 'c'

You can also locally open the Char module to make (=) work on char.

6 Likes
3

There will probably be many opinions on what is an idiomatic implementation of the function you talk about, but in any case I’ve never seen this construct being used before:

match () with
| () when cond₁ -> ...
| () when cond₂ -> ...
| _ -> ...

why not just:

if cond₁ then ... else
if cond₂ then ... else
  ...
2 Likes
4

You already got a narrowly targeted answer, here’s a suggestion.

Arguably this is more idiomatic (Oca)ml:

match p.[String.length p - 1] with
| 'c' -> print_endline "WORKS!!"
| _  -> ()
3 Likes
5

I just wasn’t sure how parsing in OCaml works exactly, and I like having guards with | aka in Haskell, so I came up with this syntax on the spot as an alternative to nested if-then-else expressions.