Maybe -package -containers
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.
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
Convert a peek combinator into a one returning Nothing if applied to a nullPtr
Converts a withXXX combinator into one marshalling a value wrapped into a Maybe, using nullPtr to represent Nothing.
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 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.
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