[FIXED] How to append end of file to a string

Issue

I just hit that "problem" : is there a smart way to insert the end of file (ASCII 0) character in a string?

By "smart", I mean something better than

let s = "foo" ^ (String.make 1 (Char.chr 0))
let s = "foo\000"

that is, something which would reflect that we are adding an EOF, not a "mystery char which ascii value is 0".

---

EDIT: Mmh... indeed I was messing with eof being a char. But anyway, in C you can have

#include <stdio.h>

int main(void)
{
    char a = getchar();
    if (a = EOF)
        printf("eof");
    else 
        printf("not eof");
    return 0;
}

Where you can test whether a char is an EOF (and (int) EOF is -1, not 0 as I was thinking). And similarly, you can set a char to be EOF, etc..

My question is: Is it possible to have something similar in ocaml ?

Solution

Your C has two errors. First, you assign EOF to a instead of comparing a with EOF. Second, getchar() returns an int. It returns an int expressly so that it can return EOF, a value not representable by a char. Your code (with the first error corrected), which assigns getchar()s value to a char before testing it, will fail to process a file with a char of value 255 in it:

$ gcc -Wall getchar.c -o getchar
$ echo -e "\xFF" > fake-eof
$ echo " " > space
$ ./getchar < fake-eof
eof
$ ./getchar < space
not eof

The trick with getchar returning int, of returning a larger type so that your return can include the smaller type and alternately other kinds of information, is a trick that's wholly unnecessary in OCaml due to its more advanced type system. OCaml could have

(* using hypothetical c_getchar, a wrapper for the getchar() in C that returns an int *)

let getchar_opt () =
  match c_getchar () with
  | -1 -> None
  | c -> Some (char_of_int c)

let getchar_exn () =
  match c_getchar () with
  | -1 -> raise End_of_file
  | c -> char_of_int c

type `a ior = EOF | Value of 'a

let getchar_ior () =
  match c_getchar_ior () with
  | -1 -> EOF
  | c -> Value (char_of_int c)

Of course Pervasives.input_char in OCaml raises an exception on EOF rather than doing one of these other things. If you want a non-exceptional interface, you could wrap input_char with your own version that catches the exception, or you could - depending on your program - use Unix.read instead, which returns the number of bytes it was able to read, which is 0 on EOF.

Answered By - Julian Fondren

Answer Checked By - Terry (PHPFixing Volunteer)