fail package:hledger

indicates program failure with an exit code. The exact interpretation of the code is operating-system dependent. In particular, some values may be prohibited (e.g. 0 on a POSIX-compliant system).

Exception thrown by assertFailure etc.

When a value is bound in do-notation, the pattern on the left hand side of <- might not match. In this case, this class provides a function to recover. A Monad without a MonadFail instance may only be used in conjunction with pattern that always match, such as newtypes, tuples, data types with only a single data constructor, and irrefutable patterns (~pat). Instances of MonadFail should satisfy the following law: fail s should be a left zero for >>=,

fail s >>= f  =  fail s

If your Monad is also MonadPlus, a popular definition is

fail _ = mzero

Unconditionally signals that a failure has occured. All other assertions can be expressed with the form:

if conditionIsMet
then return ()
else assertFailure msg

The computation exitFailure is equivalent to exitWith (ExitFailure exitfail), where exitfail is implementation-dependent.