The Maybe type encapsulates an optional value. A value of type Maybe a either contains a value of type a (represented as Just a), or it is empty (represented as Nothing). Using Maybe is a good way to deal with errors or exceptional cases without resorting to drastic measures such as error.
The Maybe type is also a monad. It is a simple kind of error monad, error monad can be built using the Data.Either.Either type.

The Maybe type, and associated operations.

The MaybeT monad transformer adds the ability to fail to a monad.
A sequence of actions succeeds, producing a value, only if all the actions in the sequence are successful. If one fails, the rest of the sequence is skipped and the composite action fails.
For a variant allowing a range of error values, see Control.Monad.Trans.Error.

The parameterizable maybe monad, obtained by composing an arbitrary monad with the Maybe monad.
Computations are actions that may produce a value or fail.
The return function yields a successful computation, while >>= sequences two subcomputations, failing on the first error.

Support for computations with failures.
Version 0.1.2

Support for computations with failures. This is a fork of the MaybeT package by Eric Kidd, but compatible with the type-family based monad classes of the monads-tf package.
This package is deprecated: the MaybeT transformer exists in the transformers package nowadays, and the only advantage this package provides over that one is the presence of a MonadFix instance, but it's incorrect anyway.
Version 0.2.0.1

Support for computations with failures. This package is a fork from the MaybeT package by Eric Kidd, changed to depend on transformers instead of mtl. It also adds a few more utility functions.
Version 0.2

The maybe function takes a default value, a function, and a Maybe value. If the Maybe value is Nothing, the function returns the default value. Otherwise, it applies the function to the value inside the Just and returns the result.

Allocate storage and marshal a storable value wrapped into a Maybe
* the nullPtr is used to represent Nothing

The maybeToList function returns an empty list when given Nothing or a singleton list when not given Nothing.

Converts a withXXX combinator into one marshalling a value wrapped into a Maybe, using nullPtr to represent Nothing.

Maybench is a tool for comparing the performance between two versions of the same program, on a series of benchmarks that you design.
Maybench aims to be easy to use, almost as easy as running "time your-program arg1..arg2". Ideally, it should be a simple matter for outsiders to write timing tests for your programming project and contribute them as part of your performance testing suite.
The Darcs repository is available at http://code.haskell.org/maybench.
Version 0.2.4.1

The fromMaybe function takes a default value and and Maybe value. If the Maybe is Nothing, it returns the default values; otherwise, it returns the value contained in the Maybe.

The listToMaybe function returns Nothing on an empty list or Just a of the list.

The mapMaybe function is a version of map which can throw out elements. In particular, the functional argument returns something of type Maybe b. If this is Nothing, no element is added on to the result list. If it just Just b, then b is included in the result list.

optionMaybe p tries to apply parser p. If p fails without consuming input, it return Nothing, otherwise it returns Just the value returned by p.

Tries to generate a value that satisfies a predicate.