Why is 'assert' not considered as a function of type bool->unit, but a keyword?

Question

I do not follow why assert is not considered as a function of type bool -> unit, but a keyword.

# assert (2=2) ;;
- : unit = ()
# assert;;
Line 1, characters 6-8:
Error: Syntax error

Answer 1

This is required by the two-words keyword:

assert false

which has type 'a rather than unit.

This is especially useful when using assert false to mock function

let not_yet_implemented _ = assert false

or when stating that a branch of a pattern matching is unreachable due to an invariant:

let f = function
| A | B as x ->
  begin match x with
  | A -> 1
  | B -> 2
  | C -> assert false
  end
| C -> 2