Regarding OCaml Pattern Matching Syntax

Question

I am following this OCaml tutorial.

They provided the two functions below and said that they are equivalent.

let string_of_int x = match x with
   | 0 -> "zero"
   | 1 -> "one"
   | 2 -> "two"
   | _ -> "many"

let string_of_int2 = function
    | 0 -> "zero"
    | 1 -> "one"
    | 2 -> "two"
    | _ -> "many"

My query is regarding the syntax of the above functions.

I wrote the same function but instead of | 0 -> I simply did 0 -> and the function still works in the same way. Is there any particular reason that the tutorial added the extra | in their function?
In the second function what is the use of the function keyword and why was this keyword absent in the first function?

antron antron · Accepted Answer · 2016-04-01T17:34:25

Some people think it looks nicer and more organized, and it allows you to change the order of cases using cut & paste without having to worry about which one didn't have the |.
The function syntax is an abbreviation: function [CASES] is the same as fun x -> match x with [CASES]. There is a subtle difference, which is with function it is not possible to shadow another variable by the name of the parameter.

let string_of_int x = [EXP] is itself an abbreviation for let string_of_int = fun x -> [EXP].

So, to a close approximation, the "canonical" syntax uses fun and match, everything else is sugar. If you apply these two expansions to the two versions of the function, you will see that the same code results (modulo alpha-equivalence, of course :) ).

Regarding OCaml Pattern Matching Syntax

1 Answers