# Module Fun

    module Fun: sig .. end

Function manipulation.

- Since 4.08

## Combinators

    val id : 'a -> 'a

`id` is the identity function. For any argument `x`, `id x` is `x`.

    val const : 'a -> 'b -> 'a

`const c` is a function that always returns the value `c`. For any argument `x`, `(const c) x` is `c`.

    val flip : ('a -> 'b -> 'c) -> 'b -> 'a -> 'c

`flip f` reverses the argument order of the binary function `f`. For any arguments `x` and `y`, `(flip f) x y` is `f y x`.

    val negate : ('a -> bool) -> 'a -> bool

`negate p` is the negation of the predicate function `p`. For any argument `x`, `(negate p) x` is `not (p x)`.

## Exception handling

    val protect : finally:(unit -> unit) -> (unit -> 'a) -> 'a

`protect ~finally work` invokes `work ()` and then `finally ()` before `work ()` returns with its value or an exception. In the latter case the exception is re-raised after `finally ()`. If `finally ()` raises an exception, then the exception `Fun.Finally_raised` is raised instead.

`protect` can be used to enforce local invariants whether `work ()` returns normally or raises an exception. However, it does not protect against unexpected exceptions raised inside `finally ()` such as `Out_of_memory`, `Stack_overflow`, or asynchronous exceptions raised by signal handlers (e.g. `Sys.Break`).

Note: It is a programming error if other kinds of exceptions are raised by `finally`, as any exception raised in `work ()` will be lost in the event of a `Fun.Finally_raised` exception. Therefore, one should make sure to handle those inside the finally.

    exception Finally_raised of exn

`Finally_raised exn` is raised by `protect ~finally work` when `finally` raises an exception `exn`. This exception denotes either an unexpected exception or a programming error. As a general rule, one should not catch a `Finally_raised` exception except as part of a catch-all handler.