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Diffstat (limited to 'libraries/base/Data/Maybe.hs')
| -rw-r--r-- | libraries/base/Data/Maybe.hs | 189 |
1 files changed, 189 insertions, 0 deletions
diff --git a/libraries/base/Data/Maybe.hs b/libraries/base/Data/Maybe.hs index 23d393da02..3d9a5a9f36 100644 --- a/libraries/base/Data/Maybe.hs +++ b/libraries/base/Data/Maybe.hs @@ -33,6 +33,10 @@ module Data.Maybe import GHC.Base +-- $setup +-- Allow the use of some Prelude functions in doctests. +-- >>> import Prelude ( (*), odd, show, sum ) + -- --------------------------------------------------------------------------- -- Functions over Maybe @@ -40,23 +44,105 @@ import GHC.Base -- 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 Nothing +-- False +-- +-- 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 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 +-- "" +-- maybe :: b -> (a -> b) -> Maybe a -> b maybe n _ Nothing = n maybe _ f (Just x) = f x -- | The 'isJust' function returns 'True' iff its argument is of the -- form @Just _@. +-- +-- ==== __Examples__ +-- +-- Basic usage: +-- +-- >>> isJust (Just 3) +-- True +-- +-- >>> isJust (Just ()) +-- True +-- +-- >>> isJust Nothing +-- False +-- +-- Only the outer constructor is taken into consideration: +-- +-- >>> isJust (Just Nothing) +-- True +-- isJust :: Maybe a -> Bool isJust Nothing = False isJust _ = True -- | The 'isNothing' function returns 'True' iff its argument is 'Nothing'. +-- +-- ==== __Examples__ +-- +-- Basic usage: +-- +-- >>> isNothing (Just 3) +-- False +-- +-- >>> isNothing (Just ()) +-- False +-- +-- >>> isNothing Nothing +-- True +-- +-- Only the outer constructor is taken into consideration: +-- +-- >>> isNothing (Just Nothing) +-- False +-- isNothing :: Maybe a -> Bool isNothing Nothing = True isNothing _ = False -- | 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 +-- fromJust :: Maybe a -> a fromJust Nothing = error "Maybe.fromJust: Nothing" -- yuck fromJust (Just x) = x @@ -64,23 +150,108 @@ fromJust (Just x) = x -- | 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'. +-- +-- ==== __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 Text.Read ( readMaybe ) +-- >>> fromMaybe 0 (readMaybe "5") +-- 5 +-- >>> fromMaybe 0 (readMaybe "") +-- 0 +-- fromMaybe :: a -> Maybe a -> a fromMaybe d x = case x of {Nothing -> d;Just v -> v} -- | The 'maybeToList' function returns an empty list when given -- 'Nothing' or a singleton list when not given 'Nothing'. +-- +-- ==== __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 Text.Read ( readMaybe ) +-- >>> sum $ maybeToList (readMaybe "3") +-- 3 +-- >>> sum $ maybeToList (readMaybe "") +-- 0 +-- maybeToList :: Maybe a -> [a] maybeToList Nothing = [] maybeToList (Just x) = [x] -- | 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] +-- listToMaybe :: [a] -> Maybe a listToMaybe [] = Nothing listToMaybe (a:_) = Just a -- | The 'catMaybes' function takes a list of 'Maybe's 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 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] +-- catMaybes :: [Maybe a] -> [a] catMaybes ls = [x | Just x <- ls] @@ -89,6 +260,24 @@ catMaybes ls = [x | Just x <- ls] -- 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 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] +-- mapMaybe :: (a -> Maybe b) -> [a] -> [b] mapMaybe _ [] = [] mapMaybe f (x:xs) = |