Example code from "Real World Ocaml" doesn't run as intended.

Question

I'm learning ocaml so it's maybe trivial.

When I try to build executable of this code:

open Core.Std

let build_counts () =
  In_channel.fold_lines stdin ~init:[] ~f:(fun counts line ->
    let count =
      match List.Assoc.find counts line with
      | None -> 0
      | Some x -> x
    in
    List.Assoc.add counts line (count + 1)
  )

let () =
  build_counts ()
  |> List.sort ~cmp:(fun (_,x) (_,y) -> Int.descending x y)
  |> (fun l -> List.take l 10)
  |> List.iter ~f:(fun (line,count) -> printf "%3d: %s\n" count line)

I get this error:

Error: This pattern matches values of type 'a option but a pattern was expected which matches values of type equal:(Stdio__.Import.string -> Stdio__.Import.string -> bool) -> 'b option

Where is the problem?

Link: https://realworldocaml.org/v1/en/html/files-modules-and-programs.html

Richard-Degenne Richard-Degenne · Accepted Answer · 2017-07-11T09:01:45

Below is the type signature for List.Assoc.find:

('a, 'b) Base__List.Assoc.t -> equal:('a -> 'a -> bool) -> 'a -> 'b option

The first argument is the associative list (counts in the example). The last argument (of type 'a) is the key you're looking for (this is line in your example). There is another argument, however, of type 'a -> 'a -> bool which labeled equal. It's pretty straight-forward that it is a comparison function used by List.Assoc.find to see whether two keys are equals.

In the case where 'a is string, a simple (=) is enough. You can fix your code by replacing your match line with the following:

match List.Assoc.find counts ~equal:(=) line with

The List.Assoc.add function follows the same pattern. You should replace the last line of the build_counts function with the following:

List.Assoc.add counts ~equal:(=) line (count + 1)

As a side-note, Real World OCaml is getting quite old (this is why some examples are outdated), and the authors are working on a second edition.

Example code from "Real World Ocaml" doesn't run as intended.

1 Answers