1 -----------------------------------------------------------------------------
4 -- Copyright : (c) The University of Glasgow 2002
5 -- License : see libraries/base/LICENSE
7 -- Maintainer : cvs-ghc@haskell.org
8 -- Stability : internal
9 -- Portability : non-portable (GHC Extensions)
11 -- GHC Extensions: this is the Approved Way to get at GHC-specific extensions.
13 -----------------------------------------------------------------------------
17 -- * Representations of some basic types
18 Int(..),Word(..),Float(..),Double(..),Integer(..),Char(..),
21 -- * Primitive operations
23 shiftL#, shiftRL#, iShiftL#, iShiftRA#, iShiftRL#,
24 uncheckedShiftL64#, uncheckedShiftRL64#,
25 uncheckedIShiftL64#, uncheckedIShiftRA64#,
30 -- * Overloaded string literals
34 breakpoint, breakpointCond,
36 -- * Ids with special behaviour
39 -- * Transform comprehensions
40 Down(..), groupWith, sortWith, the
56 -- | The 'Down' type allows you to reverse sort order conveniently. A value of type
57 -- @'Down' a@ contains a value of type @a@ (represented as @'Down' a@).
58 -- If @a@ has an @'Ord'@ instance associated with it then comparing two
59 -- values thus wrapped will give you the opposite of their normal sort order.
60 -- This is particularly useful when sorting in generalised list comprehensions,
61 -- as in: @then sortWith by 'Down' x@
62 newtype Down a = Down a deriving (Eq)
64 instance Ord a => Ord (Down a) where
65 compare (Down x) (Down y) = y `compare` x
67 -- | 'the' ensures that all the elements of the list are identical
68 -- and then returns that unique element
69 the :: Eq a => [a] -> a
72 | otherwise = error "GHC.Exts.the: non-identical elements"
73 the [] = error "GHC.Exts.the: empty list"
75 -- | The 'sortWith' function sorts a list of elements using the
76 -- user supplied function to project something out of each element
77 sortWith :: Ord b => (a -> b) -> [a] -> [a]
78 sortWith f = sortBy (\x y -> compare (f x) (f y))
80 -- | The 'groupWith' function uses the user supplied function which
81 -- projects an element out of every list element in order to to first sort the
82 -- input list and then to form groups by equality on these projected elements
83 {-# INLINE groupWith #-}
84 groupWith :: Ord b => (a -> b) -> [a] -> [[a]]
85 groupWith f xs = build (\c n -> groupByFB c n (\x y -> f x == f y) (sortWith f xs))
87 groupByFB :: ([a] -> lst -> lst) -> lst -> (a -> a -> Bool) -> [a] -> lst
88 groupByFB c n eq xs = groupByFBCore xs
89 where groupByFBCore [] = n
90 groupByFBCore (x:xs) = c (x:ys) (groupByFBCore zs)
91 where (ys, zs) = span (eq x) xs