Question

Edit: Disregard this question! See comments below.

I want an OCaml expression which is passed a file (as an “in_channel”), then reads the file line by line, doing some processing, to the end, then returns the result of the processing.

I wrote this test:

let rec sampler_string file string_so_far =
    try 
        let line = input_line file in
        let first_two_letters = String.sub line 0 2 in
        sampler_string file (string_so_far ^ first_two_letters)
    with End_of_file -> string_so_far;;

let a = sampler_string (open_in Sys.argv.(1)) "";;

(Here the “doing some processing” is adding the first two characters of each line to a running tally, and the idea is that at the end a string containing the first two characters of every line should be returned.)

This doesn’t work: OCaml thinks that “sampler_string” produces something of type unit, rather than of type string. (Difficulties then occur later when I try to use the result as a string.) I think this problem is because the only base case happens in an exception (the End_of_file).

So, a specific question and a general question:

1. Is there a way to fix this code, by explicitly telling OCaml to expect that the result of sampler_string should be a string?
2. Is there some standard, better syntax for a routine which reads a file line by line to the end, and returns the result of line-by-line processing?

Answer 1

As Damien Pollet says, your sampler_string function compiles fine (and runs correctly) on my machine as well, ocaml v3.12.0. However, I’ll answer your questions:

1. You can specify types on your functions/values using the : operator. For example, here’s your function with it’s types annotated. You’ll notice that the return type is put at the very end of the function declaration.

   let rec sampler_string (file : in_channel) (string_so_far : string) : string = ...
   

2. I do not know if there’s a better way of reading a file, line-by-line. It certainly is a pain to be forced to deal with an end-of-file via exception. Here’s a blog post on the subject, though the function presented there is of reading a file into a list of lines. Another mailing list version.

A couple of nitpicks:

1. You don’t need to use ;; to separate function/value definitions, ocamlc can figure it out from whitespace.
2. You should close your file sockets.
3. String.sub will throw an exception if your file has a line with less than 2 characters.