[Git][ghc/ghc][wip/marge_bot_batch_merge_job] 2 commits: Enhance Documentation of functions exported by Data.Function
Marge Bot (@marge-bot)
gitlab at gitlab.haskell.org
Mon Jan 22 16:02:12 UTC 2024
Marge Bot pushed to branch wip/marge_bot_batch_merge_job at Glasgow Haskell Compiler / GHC
Commits:
b8201ad1 by Jade at 2024-01-22T11:02:05-05:00
Enhance Documentation of functions exported by Data.Function
This patch aims to improve the documentation of functions exported
in Data.Function
Tracking: #17929
Fixes: #10065
- - - - -
6035c358 by Jade at 2024-01-22T11:02:06-05:00
Improve documentation of hGetLine.
- Add explanation for whether a newline is returned
- Add examples
Fixes #14804
- - - - -
3 changed files:
- libraries/base/src/Data/Function.hs
- libraries/base/src/GHC/Base.hs
- libraries/base/src/GHC/IO/Handle/Text.hs
Changes:
=====================================
libraries/base/src/Data/Function.hs
=====================================
@@ -39,7 +39,12 @@ infixl 1 &
-- | @'fix' f@ is the least fixed point of the function @f@,
-- i.e. the least defined @x@ such that @f x = x at .
--
--- For example, we can write the factorial function using direct recursion as
+-- When @f@ is strict, this means that because, by the definition of strictness,
+-- @f ⊥ = ⊥@ and such the least defined fixed point of any strict function is @⊥@.
+--
+-- ==== __Examples__
+--
+-- We can write the factorial function using direct recursion as
--
-- >>> let fac n = if n <= 1 then 1 else n * fac (n-1) in fac 5
-- 120
@@ -47,12 +52,31 @@ infixl 1 &
-- This uses the fact that Haskell’s @let@ introduces recursive bindings. We can
-- rewrite this definition using 'fix',
--
--- >>> fix (\rec n -> if n <= 1 then 1 else n * rec (n-1)) 5
--- 120
---
-- Instead of making a recursive call, we introduce a dummy parameter @rec@;
-- when used within 'fix', this parameter then refers to 'fix'’s argument, hence
-- the recursion is reintroduced.
+--
+-- >>> fix (\rec n -> if n <= 1 then 1 else n * rec (n-1)) 5
+-- 120
+--
+-- Using 'fix', we can implement versions of 'Data.List.repeat' as @'fix' '.' '(:)'@
+-- and 'Data.List.cycle' as @'fix' '.' '(++)'@
+--
+-- >>> take 10 $ fix (0:)
+-- [0,0,0,0,0,0,0,0,0,0]
+--
+-- >>> map (fix (\rec n -> if n < 2 then n else rec (n - 1) + rec (n - 2))) [1..10]
+-- [1,1,2,3,5,8,13,21,34,55]
+--
+-- ==== __Implementation Details__
+--
+-- The current implementation of 'fix' uses structural sharing
+--
+-- @'fix' f = let x = f x in x@
+--
+-- A more straightforward but non-sharing version would look like
+--
+-- @'fix' f = f ('fix' f)@
fix :: (a -> a) -> a
fix f = let x = f x in x
@@ -60,11 +84,20 @@ fix f = let x = f x in x
-- unary function @u@ to two arguments @x@ and @y at . From the opposite
-- perspective, it transforms two inputs and combines the outputs.
--
--- @((+) \``on`\` f) x y = f x + f y@
+-- @(op \``on`\` f) x y = f x \``op`\` f y@
+--
+-- ==== __Examples__
--
--- Typical usage: @'Data.List.sortBy' ('Prelude.compare' \`on\` 'Prelude.fst')@.
+-- >>> sortBy (compare `on` length) [[0, 1, 2], [0, 1], [], [0]]
+-- [[],[0],[0,1],[0,1,2]]
--
--- Algebraic properties:
+-- >>> ((+) `on` length) [1, 2, 3] [-1]
+-- 4
+--
+-- >>> ((,) `on` (*2)) 2 3
+-- (4,6)
+--
+-- ==== __Algebraic properties__
--
-- * @(*) \`on\` 'id' = (*) -- (if (*) ∉ {⊥, 'const' ⊥})@
--
@@ -118,9 +151,19 @@ on :: (b -> b -> c) -> (a -> b) -> a -> a -> c
-- convenience. Its precedence is one higher than that of the forward
-- application operator '$', which allows '&' to be nested in '$'.
--
+--
+-- This is a version of @'flip' 'id'@, where 'id' is specialized from @a -> a@ to @(a -> b) -> (a -> b)@
+-- which by the associativity of @(->)@ is @(a -> b) -> a -> b at .
+-- flipping this yields @a -> (a -> b) -> b@ which is the type signature of '&'
+--
+-- ==== __Examples__
+--
-- >>> 5 & (+1) & show
-- "6"
--
+-- >>> sqrt $ [1 / n^2 | n <- [1..1000]] & sum & (*6)
+-- 3.1406380562059946
+--
-- @since 4.8.0.0
(&) :: forall r a (b :: TYPE r). a -> (a -> b) -> b
x & f = f x
@@ -130,7 +173,15 @@ x & f = f x
--
-- It is equivalent to @'flip' ('Data.Bool.bool' 'id')@.
--
--- Algebraic properties:
+-- ==== __Examples__
+--
+-- >>> map (\x -> applyWhen (odd x) (*2) x) [1..10]
+-- [2,2,6,4,10,6,14,8,18,10]
+--
+-- >>> map (\x -> applyWhen (length x > 6) ((++ "...") . take 3) x) ["Hi!", "This is amazing", "Hope you're doing well today!", ":D"]
+-- ["Hi!","Thi...","Hop...",":D"]
+--
+-- ==== __Algebraic properties__
--
-- * @applyWhen 'True' = 'id'@
--
=====================================
libraries/base/src/GHC/Base.hs
=====================================
@@ -1703,6 +1703,20 @@ maxInt = I# 0x7FFFFFFFFFFFFFFF#
-- | Identity function.
--
-- > id x = x
+--
+-- This function might seem useless at first glance, but it can be very useful
+-- in a higher order context.
+--
+-- ==== __Examples__
+--
+-- >>> length $ filter id [True, True, False, True]
+-- 3
+--
+-- >>> Just (Just 3) >>= id
+-- Just 3
+--
+-- >>> foldr id 0 [(^3), (*5), (+2)]
+-- 1000
id :: a -> a
id x = x
@@ -1736,6 +1750,13 @@ breakpointCond _ r = r
data Opaque = forall a. O a
-- | @const x y@ always evaluates to @x@, ignoring its second argument.
--
+-- > const x = \_ -> x
+--
+-- This function might seem useless at first glance, but it can be very useful
+-- in a higher order context.
+--
+-- ==== __Examples__
+--
-- >>> const 42 "hello"
-- 42
--
@@ -1744,7 +1765,22 @@ data Opaque = forall a. O a
const :: a -> b -> a
const x _ = x
--- | Function composition.
+-- | Right to left function composition.
+--
+-- prop> (f . g) x = f (g x)
+--
+-- prop> f . id = f = id . f
+--
+-- ==== __Examples__
+--
+-- >>> map ((*2) . length) [[], [0, 1, 2], [0]]
+-- [0,6,2]
+--
+-- >>> foldr (.) id [(+1), (*3), (^3)] 2
+-- 25
+--
+-- >>> let (...) = (.).(.) in ((*2)...(+)) 5 10
+-- 30
{-# INLINE (.) #-}
-- Make sure it has TWO args only on the left, so that it inlines
-- when applied to two functions, even if there is no final argument
@@ -1753,8 +1789,17 @@ const x _ = x
-- | @'flip' f@ takes its (first) two arguments in the reverse order of @f at .
--
+-- prop> flip f x y = f y x
+--
+-- prop> flip . flip = id
+--
+-- ==== __Examples__
+--
-- >>> flip (++) "hello" "world"
-- "worldhello"
+--
+-- >>> let (.>) = flip (.) in (+1) .> show $ 5
+-- "6"
flip :: (a -> b -> c) -> b -> a -> c
flip f x y = f y x
@@ -1766,15 +1811,18 @@ flip f x y = f y x
-- (\x -> undefined x) `seq` () and thus would just evaluate to (), but now
-- it is equivalent to undefined `seq` () which diverges.
-{- | @($)@ is the __function application__ operator.
+{- | @'($)'@ is the __function application__ operator.
-Applying @($)@ to a function @f@ and an argument @x@ gives the same result as applying @f@ to @x@ directly. The definition is akin to this:
+Applying @'($)'@ to a function @f@ and an argument @x@ gives the same result as applying @f@ to @x@ directly. The definition is akin to this:
@
($) :: (a -> b) -> a -> b
($) f x = f x
@
+This is @'id'@ specialized from @a -> a@ to @(a -> b) -> (a -> b)@ which by the associativity of @(->)@
+is the same as @(a -> b) -> a -> b at .
+
On the face of it, this may appear pointless! But it's actually one of the most useful and important operators in Haskell.
The order of operations is very different between @($)@ and normal function application. Normal function application has precedence 10 - higher than any operator - and associates to the left. So these two definitions are equivalent:
@@ -1791,7 +1839,7 @@ expr = min 5 $ 1 + 5
expr = (min 5) (1 + 5)
@
-=== Uses
+==== __Examples__
A common use cases of @($)@ is to avoid parentheses in complex expressions.
@@ -1820,7 +1868,7 @@ applyFive = map ($ 5) [(+1), (2^)]
>>> [6, 32]
@
-=== Technical Remark (Representation Polymorphism)
+==== __Technical Remark (Representation Polymorphism)__
@($)@ is fully representation-polymorphic. This allows it to also be used with arguments of unlifted and even unboxed kinds, such as unboxed integers:
=====================================
libraries/base/src/GHC/IO/Handle/Text.hs
=====================================
@@ -179,16 +179,28 @@ hGetChar handle =
-- | Computation 'hGetLine' @hdl@ reads a line from the file or
-- channel managed by @hdl at .
+-- 'hGetLine' does not return the newline as part of the result.
+--
+-- A line is separated by the newline
+-- set with 'System.IO.hSetNewlineMode' or 'nativeNewline' by default.
+-- The read newline character(s) are not returned as part of the result.
+--
+-- If 'hGetLine' encounters end-of-file at any point while reading
+-- in the middle of a line, it is treated as a line terminator and the (partial)
+-- line is returned.
--
-- This operation may fail with:
--
-- * 'isEOFError' if the end of file is encountered when reading
-- the /first/ character of the line.
--
--- If 'hGetLine' encounters end-of-file at any other point while reading
--- in a line, it is treated as a line terminator and the (partial)
--- line is returned.
-
+-- ==== __Examples__
+--
+-- >>> withFile "/home/user/foo" ReadMode hGetLine >>= putStrLn
+-- this is the first line of the file :O
+--
+-- >>> withFile "/home/user/bar" ReadMode (replicateM 3 . hGetLine)
+-- ["this is the first line","this is the second line","this is the third line"]
hGetLine :: Handle -> IO String
hGetLine h =
wantReadableHandle_ "hGetLine" h $ \ handle_ ->
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/compare/b9342014ac11ba3b44146be3838e66ea959e2efd...6035c358fcba4d940394abe5aac26502c9ab508f
--
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/compare/b9342014ac11ba3b44146be3838e66ea959e2efd...6035c358fcba4d940394abe5aac26502c9ab508f
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