OCaml: Pattern matching vs If/else statements

Question

So, I'm totally new to OCaml and am moving pretty slowly in getting my first functions implemented. One thing I'm having trouble understanding is when to use pattern matching abilities like

let foo = 
[] -> true
| _  -> false;;

vs using the if else structure like

let foo a = 
if a = [] then true else false;;

When should I use each?

Answer 1

I don't think there's a clear cut answer to that question. First, the obvious case of pattern matching is when you need destructing, e.g.:

let rec sum = function
    | [] -> 0
    | head :: tail -> head + sum tail;;

Another obvious case is when you're defining a recursive function, pattern matching make the edge condition clearer, e.g.:

let rec factorial = function
    | 0 -> 1
    | n -> n * factorial(n - 1);;

instead of:

let rec factorial = function n -> 
  if n = 0 then 
    1 
  else
    n * factorial(n-1);;

That might not be a great example, just use your imagination to figure out more complex edge conditions! ;-)

In term of regular (say C like) languages, I could say that you should use pattern matching instead of switch/case and if in place of the ternary operator. For everything else it's kind of a grey zone but pattern matching is usually preferred in the ML family of languages.