Empty character in OCaml

Question

I am trying to do something fairly simple. I want to take a string such as "1,000" and return the string "1000".

Here was my attempt:

String.map (function x -> if x = ',' then '' else x) "1,000";; 

however I get a compiler error saying there is a syntax error wrt ''

Thanks for the insight!

Answer 1

Unfortunately, there's no character like the one you're looking for. There is a string that's 0 characters long (""), but there's no character that's not there at all. All characters (so to speak) are 1 character.

To solve your problem you need a more general operation than String.map. The essence of a map is that its input and output have the same shape but different contents. For strings this means that the input and output are strings of the same length.

Unless you really want to avoid imperative coding (which is actually a great thing to avoid, especially when starting out with OCaml), you would probably do best using String.iter and a buffer (from the Buffer module).

Update

The string_map_partial function given by Andreas Rossberg is pretty nice. Here's another implementation that uses String.iter and a buffer:

let string_map_partial f s =
    let b = Buffer.create (String.length s) in
    let addperhaps c =
        match f c with
        | None -> ()
        | Some c' -> Buffer.add_char b c'
    in
    String.iter addperhaps s;
    Buffer.contents b

Just an alternate implementation with different stylistic tradeoffs. Not faster, probably not slower either. It's still written imperatively (for the same reason).