Control/DeepSeq.hs

    {-# LANGUAGE CPP #-}
    #if __GLASGOW_HASKELL__ >= 702 && MIN_VERSION_array(0,4,0)
    {-# LANGUAGE Safe #-}
    #endif
    -----------------------------------------------------------------------------
    -- |
    -- Module      :  Control.DeepSeq
    -- Copyright   :  (c) The University of Glasgow 2001-2009
    -- License     :  BSD-style (see the file LICENSE)
    --
    -- Maintainer  :  libraries@haskell.org
    -- Stability   :  stable
    -- Portability :  portable
    --
    -- This module provides an overloaded function, 'deepseq', for fully
    -- evaluating data structures (that is, evaluating to \"Normal Form\").
    --
    -- A typical use is to prevent resource leaks in lazy IO programs, by
    -- forcing all characters from a file to be read. For example:
    --
    -- > import System.IO
    -- > import Control.DeepSeq
    -- >
    -- > main = do
    -- >     h <- openFile "f" ReadMode
    -- >     s <- hGetContents h
    -- >     s `deepseq` hClose h
    -- >     return s
    --
    -- 'deepseq' differs from 'seq' as it traverses data structures deeply,
    -- for example, 'seq' will evaluate only to the first constructor in
    -- the list:
    --
    -- > > [1,2,undefined] `seq` 3
    -- > 3
    --
    -- While 'deepseq' will force evaluation of all the list elements:
    --
    -- > > [1,2,undefined] `deepseq` 3
    -- > *** Exception: Prelude.undefined
    --
    -- Another common use is to ensure any exceptions hidden within lazy
    -- fields of a data structure do not leak outside the scope of the
    -- exception handler, or to force evaluation of a data structure in one
    -- thread, before passing to another thread (preventing work moving to
    -- the wrong threads).
    --
    -- /Since: 1.1.0.0/
    module Control.DeepSeq (
         deepseq, ($!!), force,
         NFData(..),
      ) where

    import Data.Int
    import Data.Word
    import Data.Ratio
    import Data.Complex
    import Data.Array
    import Data.Fixed
    import Data.Version

    infixr 0 $!!

    -- | 'deepseq': fully evaluates the first argument, before returning the
    -- second.
    --
    -- The name 'deepseq' is used to illustrate the relationship to 'seq':
    -- where 'seq' is shallow in the sense that it only evaluates the top
    -- level of its argument, 'deepseq' traverses the entire data structure
    -- evaluating it completely.
    --
    -- 'deepseq' can be useful for forcing pending exceptions,
    -- eradicating space leaks, or forcing lazy I/O to happen.  It is
    -- also useful in conjunction with parallel Strategies (see the
    -- @parallel@ package).
    --
    -- There is no guarantee about the ordering of evaluation.  The
    -- implementation may evaluate the components of the structure in
    -- any order or in parallel.  To impose an actual order on
    -- evaluation, use 'pseq' from "Control.Parallel" in the
    -- @parallel@ package.
    --
    -- /Since: 1.1.0.0/
    deepseq :: NFData a => a -> b -> b
    deepseq a b = rnf a `seq` b

    -- | the deep analogue of '$!'.  In the expression @f $!! x@, @x@ is
    -- fully evaluated before the function @f@ is applied to it.
    --
    -- /Since: 1.2.0.0/
    ($!!) :: (NFData a) => (a -> b) -> a -> b
    f $!! x = x `deepseq` f x

    -- | a variant of 'deepseq' that is useful in some circumstances:
    --
    -- > force x = x `deepseq` x
    --
    -- @force x@ fully evaluates @x@, and then returns it.  Note that
    -- @force x@ only performs evaluation when the value of @force x@
    -- itself is demanded, so essentially it turns shallow evaluation into
    -- deep evaluation.
    --
    -- /Since: 1.2.0.0/
    force :: (NFData a) => a -> a
    force x = x `deepseq` x

    -- | A class of types that can be fully evaluated.
    --
    -- /Since: 1.1.0.0/
    class NFData a where
        -- | rnf should reduce its argument to normal form (that is, fully
        -- evaluate all sub-components), and then return '()'.
        --
        -- The default implementation of 'rnf' is
        --
        -- > rnf a = a `seq` ()
        --
        -- which may be convenient when defining instances for data types with
        -- no unevaluated fields (e.g. enumerations).
        rnf :: a -> ()
        rnf a = a `seq` ()

    instance NFData Int
    instance NFData Word
    instance NFData Integer
    instance NFData Float
    instance NFData Double

    instance NFData Char
    instance NFData Bool
    instance NFData ()

    instance NFData Int8
    instance NFData Int16
    instance NFData Int32
    instance NFData Int64

    instance NFData Word8
    instance NFData Word16
    instance NFData Word32
    instance NFData Word64

    -- |/Since: 1.3.0.0/
    instance NFData (Fixed a)

    -- |This instance is for convenience and consistency with 'seq'.
    -- This assumes that WHNF is equivalent to NF for functions.
    --
    -- /Since: 1.3.0.0/
    instance NFData (a -> b)

    --Rational and complex numbers.

    instance (Integral a, NFData a) => NFData (Ratio a) where
      rnf x = rnf (numerator x, denominator x)

    instance (RealFloat a, NFData a) => NFData (Complex a) where
      rnf (x:+y) = rnf x `seq`
                   rnf y `seq`
                   ()

    instance NFData a => NFData (Maybe a) where
        rnf Nothing  = ()
        rnf (Just x) = rnf x

    instance (NFData a, NFData b) => NFData (Either a b) where
        rnf (Left x)  = rnf x
        rnf (Right y) = rnf y

    -- |/Since: 1.3.0.0/
    instance NFData Data.Version.Version where
        rnf (Data.Version.Version branch tags) = rnf branch `seq` rnf tags

    instance NFData a => NFData [a] where
        rnf [] = ()
        rnf (x:xs) = rnf x `seq` rnf xs

    instance (Ix a, NFData a, NFData b) => NFData (Array a b) where
        rnf x = rnf (bounds x, Data.Array.elems x)

    instance (NFData a, NFData b) => NFData (a,b) where
      rnf (x,y) = rnf x `seq` rnf y

    instance (NFData a, NFData b, NFData c) => NFData (a,b,c) where
      rnf (x,y,z) = rnf x `seq` rnf y `seq` rnf z

    instance (NFData a, NFData b, NFData c, NFData d) => NFData (a,b,c,d) where
      rnf (x1,x2,x3,x4) = rnf x1 `seq`
                          rnf x2 `seq`
                          rnf x3 `seq`
                          rnf x4

    instance (NFData a1, NFData a2, NFData a3, NFData a4, NFData a5) =>
             NFData (a1, a2, a3, a4, a5) where
      rnf (x1, x2, x3, x4, x5) =
                      rnf x1 `seq`
                      rnf x2 `seq`
                      rnf x3 `seq`
                      rnf x4 `seq`
                      rnf x5

    instance (NFData a1, NFData a2, NFData a3, NFData a4, NFData a5, NFData a6) =>
             NFData (a1, a2, a3, a4, a5, a6) where
      rnf (x1, x2, x3, x4, x5, x6) =
                      rnf x1 `seq`
                      rnf x2 `seq`
                      rnf x3 `seq`
                      rnf x4 `seq`
                      rnf x5 `seq`
                      rnf x6

    instance (NFData a1, NFData a2, NFData a3, NFData a4, NFData a5, NFData a6, NFData a7) =>
             NFData (a1, a2, a3, a4, a5, a6, a7) where
      rnf (x1, x2, x3, x4, x5, x6, x7) =
                      rnf x1 `seq`
                      rnf x2 `seq`
                      rnf x3 `seq`
                      rnf x4 `seq`
                      rnf x5 `seq`
                      rnf x6 `seq`
                      rnf x7

    instance (NFData a1, NFData a2, NFData a3, NFData a4, NFData a5, NFData a6, NFData a7, NFData a8) =>
             NFData (a1, a2, a3, a4, a5, a6, a7, a8) where
      rnf (x1, x2, x3, x4, x5, x6, x7, x8) =
                      rnf x1 `seq`
                      rnf x2 `seq`
                      rnf x3 `seq`
                      rnf x4 `seq`
                      rnf x5 `seq`
                      rnf x6 `seq`
                      rnf x7 `seq`
                      rnf x8

    instance (NFData a1, NFData a2, NFData a3, NFData a4, NFData a5, NFData a6, NFData a7, NFData a8, NFData a9) =>
             NFData (a1, a2, a3, a4, a5, a6, a7, a8, a9) where
      rnf (x1, x2, x3, x4, x5, x6, x7, x8, x9) =
                      rnf x1 `seq`
                      rnf x2 `seq`
                      rnf x3 `seq`
                      rnf x4 `seq`
                      rnf x5 `seq`
                      rnf x6 `seq`
                      rnf x7 `seq`
                      rnf x8 `seq`
                      rnf x9