fail package:Cabal-syntax

Lazily reconstruct a value previously written to a file.

Compatibility layer for Control.Monad.Fail

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

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).

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

Add an error, but not fail the parser yet. For fatal failure use parseFatalFailure

Add an fatal error.

A mzero.

Lazily reconstruct a value previously written to a file.

A temporary extension to help library authors check if their code will compile with the new planned desugaring of fail.