GHC.Data.Maybe

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.

The fromMaybe function takes a default value and a Maybe value. If the Maybe is Nothing, it returns the default value; otherwise, it returns the value contained in the Maybe.

Examples

Basic usage:

>>> fromMaybe "" (Just "Hello, World!")"Hello, World!"

>>> fromMaybe "" Nothing""

Read an integer from a string using readMaybe. If we fail to parse an integer, we want to return 0 by default:

>>> import GHC.Internal.Text.Read ( readMaybe )>>> fromMaybe 0 (readMaybe "5")5>>> fromMaybe 0 (readMaybe "")0

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.

Examples

Basic usage:

>>> maybe False odd (Just 3)True

>>> maybe False odd NothingFalse

Read an integer from a string using readMaybe. If we succeed, return twice the integer; that is, apply (*2) to it. If instead we fail to parse an integer, return 0 by default:

>>> import GHC.Internal.Text.Read ( readMaybe )>>> maybe 0 (*2) (readMaybe "5")10>>> maybe 0 (*2) (readMaybe "")0

Apply show to a Maybe Int. If we have Just n, we want to show the underlying Int n. But if we have Nothing, we return the empty string instead of (for example) "Nothing":

>>> maybe "" show (Just 5)"5">>> maybe "" show Nothing""

The catMaybes function takes a list of Maybes and returns a list of all the Just values.

Examples

Basic usage:

>>> catMaybes [Just 1, Nothing, Just 3][1,3]

When constructing a list of Maybe values, catMaybes can be used to return all of the "success" results (if the list is the result of a map, then mapMaybe would be more appropriate):

>>> import GHC.Internal.Text.Read ( readMaybe )>>> [readMaybe x :: Maybe Int | x <- ["1", "Foo", "3"] ][Just 1,Nothing,Just 3]>>> catMaybes $ [readMaybe x :: Maybe Int | x <- ["1", "Foo", "3"] ][1,3]

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

Examples

Basic usage:

>>> fromJust (Just 1)1

>>> 2 * (fromJust (Just 10))20

>>> 2 * (fromJust Nothing)*** Exception: Maybe.fromJust: Nothing...

WARNING: This function is partial. You can use case-matching instead.

The isJust function returns True iff its argument is of the form Just _.

Examples

Basic usage:

>>> isJust (Just 3)True

>>> isJust (Just ())True

>>> isJust NothingFalse

Only the outer constructor is taken into consideration:

>>> isJust (Just Nothing)True

The isNothing function returns True iff its argument is Nothing.

Examples

Basic usage:

>>> isNothing (Just 3)False

>>> isNothing (Just ())False

>>> isNothing NothingTrue

Only the outer constructor is taken into consideration:

>>> isNothing (Just Nothing)False

The listToMaybe function returns Nothing on an empty list or Just a where a is the first element of the list.

Examples

Basic usage:

>>> listToMaybe []Nothing

>>> listToMaybe [9]Just 9

>>> listToMaybe [1,2,3]Just 1

Composing maybeToList with listToMaybe should be the identity on singleton/empty lists:

>>> maybeToList $ listToMaybe [5][5]>>> maybeToList $ listToMaybe [][]

But not on lists with more than one element:

>>> maybeToList $ listToMaybe [1,2,3][1]

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 is Just b, then b is included in the result list.

Examples

Using mapMaybe f x is a shortcut for catMaybes $ map f x in most cases:

>>> import GHC.Internal.Text.Read ( readMaybe )>>> let readMaybeInt = readMaybe :: String -> Maybe Int>>> mapMaybe readMaybeInt ["1", "Foo", "3"][1,3]>>> catMaybes $ map readMaybeInt ["1", "Foo", "3"][1,3]

If we map the Just constructor, the entire list should be returned:

>>> mapMaybe Just [1,2,3][1,2,3]

The maybeToList function returns an empty list when given Nothing or a singleton list when given Just.

Examples

Basic usage:

>>> maybeToList (Just 7)[7]

>>> maybeToList Nothing[]

One can use maybeToList to avoid pattern matching when combined with a function that (safely) works on lists:

>>> import GHC.Internal.Text.Read ( readMaybe )>>> sum $ maybeToList (readMaybe "3")3>>> sum $ maybeToList (readMaybe "")0

Flipped version of fromMaybe, useful for chaining.

Takes a list of Maybes and returns the first Just if there is one, or Nothing otherwise.

Takes computations returnings Maybes; tries each one in order. The first one to return a Just wins. Returns Nothing if all computations return Nothing.

The parameterizable maybe monad, obtained by composing an arbitrary monad with the Maybe monad.

Computations are actions that may produce a value or exit.

The return function yields a computation that produces that value, while >>= sequences two subcomputations, exiting if either computation does.

Try performing an IO action, failing on error.