# Maybe -transformers -package

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 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
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.
The catMaybes function takes a list of Maybes and returns a list of all the Just values.
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.
O(n). Map values and collect the Just results. > let f x = if x == "a" then Just "new a" else Nothing > mapMaybe f (fromList [(5,"a"), (3,"b")]) == singleton 5 "new a"
O(n). Map values and collect the Just results. > let f x = if x == "a" then Just "new a" else Nothing > mapMaybe f (fromList [(5,"a"), (3,"b")]) == singleton 5 "new a"
O(n). Map keys/values and collect the Just results. > let f k _ = if k < 5 then Just ("key (:) " ++ (show k)) else Nothing > mapMaybeWithKey f (fromList [(5,"a"), (3,"b")]) == singleton 3 "key (:) 3"
O(n). Map keys/values and collect the Just results. > let f k _ = if k < 5 then Just ("key (:) " ++ (show k)) else Nothing > mapMaybeWithKey f (fromList [(5,"a"), (3,"b")]) == singleton 3 "key (:) 3"
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.