Maybe
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Revision as of 04:11, 7 March 2006
import Data.Maybe |
The Maybe type is defined as follows:
data Maybe a = Just a | Nothing deriving (Eq, Ord)
It allows the programmer to specify something may not be there.
Contents |
1 Type Equation
Maybe satisfies the type equation FX = 1 + X, where the functor F takes a set to a point plus that set.
2 Comparison to imperative languages
Imperative languages may support this by rewriting as a union or allow one to use / return NULL (defined in some manner) to specify a value might not be there.
3 Classes
As one can see from the type definition, Maybe will be an instance of Eq and Ord when the base type is. As well, instances of Functor and Monad are defined for Maybe.
For Functor, the fmap
function moves inside the Just constructor and is identity on the Nothing constructor.
For Monad, the bind operation passes through Just, while Nothing will force the result to always be Nothing.
3.1 Usage example
Using the Monad class definition can lead to much more compact code. For example:
f::Int -> Maybe Int f 0 = Nothing f x = Just x
g :: Int -> Maybe Int g 100 = Nothing g x = Just x
h ::Int -> Maybe Int h x = case f x of Just n -> g n Nothing -> Nothing h' :: Int -> Maybe Int h' x = do n <- f x g n
The functions h
and h'
will give the same results. (). In this case the savings in code size is quite modest, stringing together multiple functions like f
and g
will be more noticeable.
4 Library Functions
When the module is imported, it supplies a variety of useful functions including:
maybe:: b->(a->b) -> Maybe a -> b
- Applies the second argument to the third, when it is Just x, otherwise returns the first argument.
isJust, isNothing
- Test the argument, returing a Bool based on the constructor.
listToMaybe, maybeToList
- Convert to/from a one element or empty list.
mapMaybe
- A different way to filter a list.
See the documentation for Data.Maybe for more explanation and other functions.