
Data.Maybe  Portability  portable  Stability  stable  Maintainer  libraries@haskell.org 



Description 
The Maybe type, and associated operations.


Synopsis 



Documentation 

data Maybe a 
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, where all errors are represented by Nothing. A richer
error monad can be built using the Either type.
 Constructors   Instances  


maybe :: b > (a > b) > Maybe a > b 
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.


isJust :: Maybe a > Bool 
The isJust function returns True iff its argument is of the
form Just _.


isNothing :: Maybe a > Bool 
The isNothing function returns True iff its argument is Nothing.


fromJust :: Maybe a > a 
The fromJust function extracts the element out of a Just and
throws an error if its argument is Nothing.


fromMaybe :: a > Maybe a > a 
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.


listToMaybe :: [a] > Maybe a 
The listToMaybe function returns Nothing on an empty list
or Just a where a is the first element of the list.


maybeToList :: Maybe a > [a] 
The maybeToList function returns an empty list when given
Nothing or a singleton list when not given Nothing.


catMaybes :: [Maybe a] > [a] 
The catMaybes function takes a list of Maybes and returns
a list of all the Just values.


mapMaybe :: (a > Maybe b) > [a] > [b] 
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.


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