OCaml: a function that returns function

Question

When a function returns a function as result, let's say:

let func = function
| pat -> (function | pat -> x
                   | b -> y)

the two pat are not the same thing, the second pat can match to anything, so the last line is never reached. Any hint on how to solve this?

Answer 1

If you want to express the second pat should be equal to the matched value against the first pat in your code, your code is wrong. It must be:

let func = function pat ->
  function 
    | pat' when pat = pat' -> x
    | b -> y

Pattern variables introduce new fresh variables and cannot refer to values bound to variables with the same name.

Answer 2

You should not use pattern matching just to test equality. You should use if then else. Your function is equivalent to:

let func a b if a = b then x else y

Using guards (the when part of camlspotter's answer) will work but is bad style (the caml compiler used to complain about it).