\DOC assert_exn

\TYPE {assert_exn : ('a -> bool) -> 'a -> exn -> 'a}

\SYNOPSIS
Checks that a value satisfies a predicate.

\DESCRIBE
{assert_exn p x e} returns {x} if the application {p x} evaluates to
{true}. Otherwise, {assert_exn p x e} raises {e}

\FAILURE
{assert_exn p x e} fails with exception {e} if the predicate {p} yields
{false} when applied to the value {x}. If the application {p x} raises
an exception {ex}, then {assert_exn p x e} raises {ex}.



\EXAMPLE
{
- null [];
> val it = true : bool

- assert_exn null ([]:int list) (Fail "non-empty list");
> val it = [] : int list

- null [1];
> false : bool

- assert_exn null [1] (Fail "non-empty list");;
! Uncaught exception:
! Fail  "non-empty list"
}

\SEEALSO
Lib.can, Lib.assert.

\ENDDOC