Maybe

(Difference between revisions)
 Revision as of 04:11, 7 March 2006 (edit)m (→Library Functions: Typo)← Previous diff Revision as of 01:38, 9 March 2006 (edit) (undo)m (Haskell formatting)Next diff → Line 1: Line 1: {{Standard type|Maybe|module=Data.Maybe|module-doc=Data-Maybe|package=base}} {{Standard type|Maybe|module=Data.Maybe|module-doc=Data-Maybe|package=base}} The '''Maybe''' type is defined as follows: The '''Maybe''' type is defined as follows: - + data Maybe a = Just a | Nothing data Maybe a = Just a | Nothing deriving (Eq, Ord) deriving (Eq, Ord) - + It allows the programmer to specify something may not be there. It allows the programmer to specify something may not be there. Line 25: Line 25: ===Usage example=== ===Usage example=== Using the [[Monad]] class definition can lead to much more compact code. For example: Using the [[Monad]] class definition can lead to much more compact code. For example: - + f::Int -> Maybe Int f::Int -> Maybe Int f 0 = Nothing f 0 = Nothing Line 42: Line 42: h' x = do n <- f x h' x = do n <- f x g n g n - + The functions h and h' will give the same results. ([itex]h 0 = h' 0 = h 100 = h' 100 = Nothing;\ h x = h' x = Just\, x[/itex]). In this case the savings in code size is quite modest, stringing together multiple functions like f and g will be more noticeable. The functions h and h' will give the same results. ([itex]h 0 = h' 0 = h 100 = h' 100 = Nothing;\ h x = h' x = Just\, x[/itex]). In this case the savings in code size is quite modest, stringing together multiple functions like f and g will be more noticeable.

Revision as of 01:38, 9 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.

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. ($h 0 = h' 0 = h 100 = h' 100 = Nothing;\ h x = h' x = Just\, x$). 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.