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

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

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

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.

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.