OCaml: Pattern matching on strings of variable length

Question

I'm trying to write a program that accepts user input (one line at a time) and evaluates this user input in some ways.

I have a function getline : unit -> string that reads a string from stdin and returns it. The commands have the following format:

#command1 arg1 arg2 arg3
#cmd2 arg1 arg2
#nextcommand arg1 arg2 arg3 arg4
#quit
#exit

Exemplary calls to the commands could then look as follows:

#command1 2 6 3
#cmd2 6 3
#nextcommand a b3 2 3

The main problem is the function evalcommand : string -> unit that takes a whole line obtained by getline. I started

let evalcommand command = match command with
    | "#quit" | "#exit" -> () (* no problem; both length 4 and no arguments *)
    | (* how to capture arguments properly? *)

How can I cope with commands whose lengths are differing (such as e.g. command1 and cmd2) and that have additional arguments with them?

Not-so-nice alternatives already considered:

Using a parser (possibly generated by a parser generator): This was the first thing that came to my mind, but since I only want to support five or six commands, it seemed like an overkill to me.
Checking prefixes of different lengths: Of course, I could just check prefixes of different length of the command string, but that seems to involve lots of boilerplate code.

List representation of strings: I know that I could just transform each String into a char list and then do something like

let evalcommand command = match String.to_list with
        | ['#';'e';'x';'i';'t'] -> () (* cumbersome for many commands *)
        | '#'::'c'::'m'::'d'::'2'::args -> consume_args args (* cumbersome *)
        | (* and so on *)

Mikmatch: I also had a look at Mikmatch, but I'd prefer doing it with OCaml's intrinsic features if possible.
Hashtable approach: Of course I can use a hashtable that maps Strings to String list -> unit (i.e. functions taking a certain amount of strings as argument, returning ()).

So here's the final question: Is there any elegant way that solves this poblem in a straightforward, easily maintainable way (preferrably with little amount of code)?

If things get more complicated you can define a grammar and parse them with the Camlp4.Struct.Grammar module. Or through Lex/Yacc. — nlucaroni

Marth Marth · Accepted Answer · 2014-03-01T11:02:43

Not the most elegant solution, but why not simply split on spaces ?

open Str;;

let split_on_spaces s = split (regexp " ") s;;

let evalcmd cmd = match (split_on_spaces cmd) with
  | ["#quit"] | ["#exit"] -> ()
  (* match on a known number on arguments *)
  | ["#command";arg1;arg2;arg3] -> ()
  (* unknown number *)
  | "#cmd2" :: arg_list -> ()

OCaml: Pattern matching on strings of variable length

Not-so-nice alternatives already considered:

2 Answers