Ocaml syntax error: why are parentheses enclosing (if …) in function application necessary?

Question

Entering this code to ocaml toplevel results in syntax error.

(fun n -> n + 1) if true then 1 else 2

But this code is OK.

(fun n -> n + 1) (if true then 1 else 2)

Why are parentheses necessary?

Answer 1

Because an expression like

f if a then b else x y

would be ambiguous (besides being hard to read).

Answer 2

This is a consequence of the priority table of ocaml operators priority . A function has left associativity, whereas if does not have associativity.