[commit: ghc] master: Add doctest examples for Data.Maybe (8688f6a)
git at git.haskell.org
git at git.haskell.org
Mon Dec 8 17:28:03 UTC 2014
Repository : ssh://git@git.haskell.org/ghc
On branch : master
Link : http://ghc.haskell.org/trac/ghc/changeset/8688f6a5f652e8f41a19faf1a935073d85efffa0/ghc
>---------------------------------------------------------------
commit 8688f6a5f652e8f41a19faf1a935073d85efffa0
Author: Michael Orlitzky <michael at orlitzky.com>
Date: Mon Dec 8 18:27:16 2014 +0100
Add doctest examples for Data.Maybe
To actually *run* the doctests, you need to do a little
hacking. Somewhere after the `GHC.Base` import, you'll need to reimport
`Maybe(..)` from `Prelude`, clobbering the `GHC.Base` definition. After
that, doctest should be runnable from the `Data/` directory.
Reviewed By: austin, hvr
Differential Revision: https://phabricator.haskell.org/D561
>---------------------------------------------------------------
8688f6a5f652e8f41a19faf1a935073d85efffa0
libraries/base/Data/Maybe.hs | 189 +++++++++++++++++++++++++++++++++++++++++++
1 file changed, 189 insertions(+)
diff --git a/libraries/base/Data/Maybe.hs b/libraries/base/Data/Maybe.hs
index 23d393d..3d9a5a9 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 at . If we have @Just n@, we want to show
+-- the underlying 'Int' @n at . 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 _ at .
+--
+-- ==== __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 at . 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) =
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