GHC/Exts.hs

    {-# LANGUAGE Unsafe #-}
    {-# LANGUAGE MagicHash, UnboxedTuples, AutoDeriveTypeable, TypeFamilies,
                 MultiParamTypeClasses, FlexibleInstances, NoImplicitPrelude #-}

    -----------------------------------------------------------------------------
    -- |
    -- Module      :  GHC.Exts
    -- Copyright   :  (c) The University of Glasgow 2002
    -- License     :  see libraries/base/LICENSE
    --
    -- Maintainer  :  cvs-ghc@haskell.org
    -- Stability   :  internal
    -- Portability :  non-portable (GHC Extensions)
    --
    -- GHC Extensions: this is the Approved Way to get at GHC-specific extensions.
    --
    -- Note: no other base module should import this module.
    -----------------------------------------------------------------------------

    module GHC.Exts
           (
            -- * Representations of some basic types
            Int(..),Word(..),Float(..),Double(..),
            Char(..),
            Ptr(..), FunPtr(..),

            -- * The maximum tuple size
            maxTupleSize,

            -- * Primitive operations
            module GHC.Prim,
            shiftL#, shiftRL#, iShiftL#, iShiftRA#, iShiftRL#,
            uncheckedShiftL64#, uncheckedShiftRL64#,
            uncheckedIShiftL64#, uncheckedIShiftRA64#,
            isTrue#,

            -- * Fusion
            build, augment,

            -- * Overloaded string literals
            IsString(..),

            -- * Debugging
            breakpoint, breakpointCond,

            -- * Ids with special behaviour
            lazy, inline,

            -- * Safe coercions
            --
            -- | These are available from the /Trustworthy/ module "Data.Coerce" as well
            --
            -- @since 4.7.0.0
            Data.Coerce.coerce, Data.Coerce.Coercible,

            -- * Transform comprehensions
            Down(..), groupWith, sortWith, the,

            -- * Event logging
            traceEvent,

            -- * SpecConstr annotations
            SpecConstrAnnotation(..),

            -- * The call stack
            currentCallStack,

            -- * The Constraint kind
            Constraint,

            -- * Overloaded lists
            IsList(..)
           ) where

    import GHC.Prim hiding (coerce, Constraint)
    import GHC.Base hiding (coerce) -- implicitly comes from GHC.Prim
    import GHC.Word
    import GHC.Int
    import GHC.Ptr
    import GHC.Stack

    import qualified Data.Coerce
    import Data.String
    import Data.OldList
    import Data.Data
    import Data.Ord
    import Data.Version ( Version(..), makeVersion )
    import qualified Debug.Trace

    -- XXX This should really be in Data.Tuple, where the definitions are
    maxTupleSize :: Int
    maxTupleSize = 62

    -- | 'the' ensures that all the elements of the list are identical
    -- and then returns that unique element
    the :: Eq a => [a] -> a
    the (x:xs)
      | all (x ==) xs = x
      | otherwise     = error "GHC.Exts.the: non-identical elements"
    the []            = error "GHC.Exts.the: empty list"

    -- | The 'sortWith' function sorts a list of elements using the
    -- user supplied function to project something out of each element
    sortWith :: Ord b => (a -> b) -> [a] -> [a]
    sortWith f = sortBy (\x y -> compare (f x) (f y))

    -- | The 'groupWith' function uses the user supplied function which
    -- projects an element out of every list element in order to first sort the
    -- input list and then to form groups by equality on these projected elements
    {-# INLINE groupWith #-}
    groupWith :: Ord b => (a -> b) -> [a] -> [[a]]
    groupWith f xs = build (\c n -> groupByFB c n (\x y -> f x == f y) (sortWith f xs))

    groupByFB :: ([a] -> lst -> lst) -> lst -> (a -> a -> Bool) -> [a] -> lst
    groupByFB c n eq xs0 = groupByFBCore xs0
      where groupByFBCore [] = n
            groupByFBCore (x:xs) = c (x:ys) (groupByFBCore zs)
                where (ys, zs) = span (eq x) xs

    -- -----------------------------------------------------------------------------
    -- tracing

    traceEvent :: String -> IO ()
    traceEvent = Debug.Trace.traceEventIO
    {-# DEPRECATED traceEvent "Use 'Debug.Trace.traceEvent' or 'Debug.Trace.traceEventIO'" #-} -- deprecated in 7.4

    {- **********************************************************************
    *                                                                       *
    *              SpecConstr annotation                                    *
    *                                                                       *
    ********************************************************************** -}

    -- Annotating a type with NoSpecConstr will make SpecConstr
    -- not specialise for arguments of that type.

    -- This data type is defined here, rather than in the SpecConstr module
    -- itself, so that importing it doesn't force stupidly linking the
    -- entire ghc package at runtime

    data SpecConstrAnnotation = NoSpecConstr | ForceSpecConstr
                    deriving( Data, Typeable, Eq )

    {- **********************************************************************
    *                                                                       *
    *              The IsList class                                         *
    *                                                                       *
    ********************************************************************** -}

    -- | The 'IsList' class and its methods are intended to be used in
    --   conjunction with the OverloadedLists extension.
    --
    -- @since 4.7.0.0
    class IsList l where
      -- | The 'Item' type function returns the type of items of the structure
      --   @l@.
      type Item l

      -- | The 'fromList' function constructs the structure @l@ from the given
      --   list of @Item l@
      fromList  :: [Item l] -> l

      -- | The 'fromListN' function takes the input list's length as a hint. Its
      --   behaviour should be equivalent to 'fromList'. The hint can be used to
      --   construct the structure @l@ more efficiently compared to 'fromList'. If
      --   the given hint does not equal to the input list's length the behaviour of
      --   'fromListN' is not specified.
      fromListN :: Int -> [Item l] -> l
      fromListN _ = fromList

      -- | The 'toList' function extracts a list of @Item l@ from the structure @l@.
      --   It should satisfy fromList . toList = id.
      toList :: l -> [Item l]

    instance IsList [a] where
      type (Item [a]) = a
      fromList = id
      toList = id

    -- | @since 4.8.0.0
    instance IsList Version where
      type (Item Version) = Int
      fromList = makeVersion
      toList = versionBranch