Hoogle Search

Within LTS Haskell 24.51 (ghc-9.10.3)

Note that Stackage only displays results for the latest LTS and Nightly snapshot. Learn more.

  1. module Control.Monad.MaybeK

    A continuation-passing variant of Maybe for short-circuiting at failure. This is based largely on code from the Haskell Wiki (http://www.haskell.org/haskellwiki/Performance/Monads) which was released under a simple permissive license (http://www.haskell.org/haskellwiki/HaskellWiki:Copyrights). However, various changes and extensions have been made, which are subject to the BSD license of this package.

  2. data MaybeK a

    unification-fd Control.Monad.MaybeK

    A continuation-passing encoding of Maybe; also known as Codensity Maybe, if you're familiar with that terminology. N.B., this is not the 2-continuation implementation based on the Church encoding of Maybe. The latter tends to have worse performance than non-continuation based implementations. This is generally more efficient than using Maybe for two reasons. First is that it right associates all binds, ensuring that bad associativity doesn't artificially introduce midpoints in short-circuiting to the nearest handler. Second is that it removes the need for intermediate case expressions. N.B., the Alternative and MonadPlus instances are left-biased in a. Thus, they are not commutative.

  3. data MaybeKT (m :: Type -> Type) a

    unification-fd Control.Monad.MaybeK

    A monad transformer version of MaybeK.

  4. type MaybeMaybeManageHook = Query Maybe Maybe Endo WindowSet

    xmonad-contrib XMonad.Hooks.FloatConfigureReq

    A variant of MaybeManageHook that additionally may or may not make changes to the WindowSet.

  5. type MaybeManageHook = Query Maybe Endo WindowSet

    xmonad-contrib XMonad.Hooks.ManageHelpers

    A ManageHook that may or may not have been executed; the outcome is embedded in the Maybe

  6. catMaybes :: [Maybe a] -> [a]

    base Data.Maybe

    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]

  7. fromMaybe :: a -> Maybe a -> a

    base Data.Maybe

    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

  8. listToMaybe :: [a] -> Maybe a

    base Data.Maybe

    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]

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

    base Data.Maybe

    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]

  10. bitSizeMaybe :: Bits a => a -> Maybe Int

    base Data.Bits

    Return the number of bits in the type of the argument. The actual value of the argument is ignored. Returns Nothing for types that do not have a fixed bitsize, like Integer.

Page 103 of many | Previous | Next