Copyright	(c) The University of Glasgow 2001-2009
License	BSD-style (see the file LICENSE)
Maintainer	libraries@haskell.org
Stability	stable
Portability	portable
Safe Haskell	Safe
Language	Haskell2010

Control.DeepSeq

Description

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

Synopsis

Documentation

deepseq :: NFData a => a -> b -> b Source #

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

($!!) :: NFData a => (a -> b) -> a -> b infixr 0 Source #

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

force :: NFData a => a -> a Source #

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.

force can be conveniently used in combination with ViewPatterns:

{-# LANGUAGE BangPatterns, ViewPatterns #-}
import Control.DeepSeq

someFun :: ComplexData -> SomeResult
someFun (force -> !arg) = {- 'arg' will be fully evaluated -}

Another useful application is to combine force with evaluate in order to force deep evaluation relative to other IO operations:

import Control.Exception (evaluate)
import Control.DeepSeq

main = do
  result <- evaluate $ force $ pureComputation
  {- 'result' will be fully evaluated at this point -}
  return ()

Since: 1.2.0.0

class NFData a where Source #

A class of types that can be fully evaluated.

Since: 1.1.0.0

Methods

rnf :: a -> () Source #

rnf should reduce its argument to normal form (that is, fully evaluate all sub-components), and then return '()'.

`Generic` `NFData` deriving

Starting with GHC 7.2, you can automatically derive instances for types possessing a Generic instance.

{-# LANGUAGE DeriveGeneric #-}

import GHC.Generics (Generic)
import Control.DeepSeq

data Foo a = Foo a String
             deriving (Eq, Generic)

instance NFData a => NFData (Foo a)

data Colour = Red | Green | Blue
              deriving Generic

instance NFData Colour

Starting with GHC 7.10, the example above can be written more concisely by enabling the new DeriveAnyClass extension:

{-# LANGUAGE DeriveGeneric, DeriveAnyClass #-}

import GHC.Generics (Generic)
import Control.DeepSeq

data Foo a = Foo a String
             deriving (Eq, Generic, NFData)

data Colour = Red | Green | Blue
              deriving (Generic, NFData)

Compatibility with previous `deepseq` versions

Prior to version 1.4.0.0, the default implementation of the rnf method was defined as

rnf a = seq a ()

However, starting with deepseq-1.4.0.0, the default implementation is based on DefaultSignatures allowing for more accurate auto-derived NFData instances. If you need the previously used exact default rnf method implementation semantics, use

instance NFData Colour where rnf x = seq x ()

or alternatively

{-# LANGUAGE BangPatterns #-}
instance NFData Colour where rnf !_ = ()

rnf :: (Generic a, GNFData (Rep a)) => a -> () Source #

rnf should reduce its argument to normal form (that is, fully evaluate all sub-components), and then return '()'.

`Generic` `NFData` deriving

Starting with GHC 7.2, you can automatically derive instances for types possessing a Generic instance.

{-# LANGUAGE DeriveGeneric #-}

import GHC.Generics (Generic)
import Control.DeepSeq

data Foo a = Foo a String
             deriving (Eq, Generic)

instance NFData a => NFData (Foo a)

data Colour = Red | Green | Blue
              deriving Generic

instance NFData Colour

Starting with GHC 7.10, the example above can be written more concisely by enabling the new DeriveAnyClass extension:

{-# LANGUAGE DeriveGeneric, DeriveAnyClass #-}

import GHC.Generics (Generic)
import Control.DeepSeq

data Foo a = Foo a String
             deriving (Eq, Generic, NFData)

data Colour = Red | Green | Blue
              deriving (Generic, NFData)

Compatibility with previous `deepseq` versions

Prior to version 1.4.0.0, the default implementation of the rnf method was defined as

rnf a = seq a ()

However, starting with deepseq-1.4.0.0, the default implementation is based on DefaultSignatures allowing for more accurate auto-derived NFData instances. If you need the previously used exact default rnf method implementation semantics, use

instance NFData Colour where rnf x = seq x ()

or alternatively

{-# LANGUAGE BangPatterns #-}
instance NFData Colour where rnf !_ = ()

Instances

NFData Bool Source #
Methods rnf :: Bool -> () Source #
NFData Char Source #
Methods rnf :: Char -> () Source #
NFData Double Source #
Methods rnf :: Double -> () Source #
NFData Float Source #
Methods rnf :: Float -> () Source #
NFData Int Source #
Methods rnf :: Int -> () Source #
NFData Int8 Source #
Methods rnf :: Int8 -> () Source #
NFData Int16 Source #
Methods rnf :: Int16 -> () Source #
NFData Int32 Source #
Methods rnf :: Int32 -> () Source #
NFData Int64 Source #
Methods rnf :: Int64 -> () Source #
NFData Integer Source #
Methods rnf :: Integer -> () Source #
NFData Word Source #
Methods rnf :: Word -> () Source #
NFData Word8 Source #
Methods rnf :: Word8 -> () Source #
NFData Word16 Source #
Methods rnf :: Word16 -> () Source #
NFData Word32 Source #
Methods rnf :: Word32 -> () Source #
NFData Word64 Source #
Methods rnf :: Word64 -> () Source #
NFData CallStack Source #	Since: 1.4.2.0
Methods rnf :: CallStack -> () Source #
NFData TypeRep Source #	NOTE: Only defined for `base-4.8.0.0` and later Since: 1.4.0.0
Methods rnf :: TypeRep -> () Source #
NFData () Source #
Methods rnf :: () -> () Source #
NFData TyCon Source #	NOTE: Only defined for `base-4.8.0.0` and later Since: 1.4.0.0
Methods rnf :: TyCon -> () Source #
NFData Natural Source #	Since: 1.4.0.0
Methods rnf :: Natural -> () Source #
NFData Void Source #	Defined as `rnf = absurd`. Since: 1.4.0.0
Methods rnf :: Void -> () Source #
NFData Version Source #	Since: 1.3.0.0
Methods rnf :: Version -> () Source #
NFData Unique Source #	Since: 1.4.0.0
Methods rnf :: Unique -> () Source #
NFData ThreadId Source #	Since: 1.4.0.0
Methods rnf :: ThreadId -> () Source #
NFData ExitCode Source #	Since: 1.4.2.0
Methods rnf :: ExitCode -> () Source #
NFData CChar Source #	Since: 1.4.0.0
Methods rnf :: CChar -> () Source #
NFData CSChar Source #	Since: 1.4.0.0
Methods rnf :: CSChar -> () Source #
NFData CUChar Source #	Since: 1.4.0.0
Methods rnf :: CUChar -> () Source #
NFData CShort Source #	Since: 1.4.0.0
Methods rnf :: CShort -> () Source #
NFData CUShort Source #	Since: 1.4.0.0
Methods rnf :: CUShort -> () Source #
NFData CInt Source #	Since: 1.4.0.0
Methods rnf :: CInt -> () Source #
NFData CUInt Source #	Since: 1.4.0.0
Methods rnf :: CUInt -> () Source #
NFData CLong Source #	Since: 1.4.0.0
Methods rnf :: CLong -> () Source #
NFData CULong Source #	Since: 1.4.0.0
Methods rnf :: CULong -> () Source #
NFData CLLong Source #	Since: 1.4.0.0
Methods rnf :: CLLong -> () Source #
NFData CULLong Source #	Since: 1.4.0.0
Methods rnf :: CULLong -> () Source #
NFData CFloat Source #	Since: 1.4.0.0
Methods rnf :: CFloat -> () Source #
NFData CDouble Source #	Since: 1.4.0.0
Methods rnf :: CDouble -> () Source #
NFData CPtrdiff Source #	Since: 1.4.0.0
Methods rnf :: CPtrdiff -> () Source #
NFData CSize Source #	Since: 1.4.0.0
Methods rnf :: CSize -> () Source #
NFData CWchar Source #	Since: 1.4.0.0
Methods rnf :: CWchar -> () Source #
NFData CSigAtomic Source #	Since: 1.4.0.0
Methods rnf :: CSigAtomic -> () Source #
NFData CClock Source #	Since: 1.4.0.0
Methods rnf :: CClock -> () Source #
NFData CTime Source #	Since: 1.4.0.0
Methods rnf :: CTime -> () Source #
NFData CUSeconds Source #	Since: 1.4.0.0
Methods rnf :: CUSeconds -> () Source #
NFData CSUSeconds Source #	Since: 1.4.0.0
Methods rnf :: CSUSeconds -> () Source #
NFData CFile Source #	Since: 1.4.0.0
Methods rnf :: CFile -> () Source #
NFData CFpos Source #	Since: 1.4.0.0
Methods rnf :: CFpos -> () Source #
NFData CJmpBuf Source #	Since: 1.4.0.0
Methods rnf :: CJmpBuf -> () Source #
NFData CIntPtr Source #	Since: 1.4.0.0
Methods rnf :: CIntPtr -> () Source #
NFData CUIntPtr Source #	Since: 1.4.0.0
Methods rnf :: CUIntPtr -> () Source #
NFData CIntMax Source #	Since: 1.4.0.0
Methods rnf :: CIntMax -> () Source #
NFData CUIntMax Source #	Since: 1.4.0.0
Methods rnf :: CUIntMax -> () Source #
NFData All Source #	Since: 1.4.0.0
Methods rnf :: All -> () Source #
NFData Any Source #	Since: 1.4.0.0
Methods rnf :: Any -> () Source #
NFData Fingerprint Source #	Since: 1.4.0.0
Methods rnf :: Fingerprint -> () Source #
NFData SrcLoc Source #	Since: 1.4.2.0
Methods rnf :: SrcLoc -> () Source #
NFData a => NFData [a] Source #
Methods rnf :: [a] -> () Source #
NFData a => NFData (Maybe a) Source #
Methods rnf :: Maybe a -> () Source #
NFData a => NFData (Ratio a) Source #
Methods rnf :: Ratio a -> () Source #
NFData (Ptr a) Source #	Since: 1.4.2.0
Methods rnf :: Ptr a -> () Source #
NFData (FunPtr a) Source #	Since: 1.4.2.0
Methods rnf :: FunPtr a -> () Source #
NFData a => NFData (Identity a) Source #	Since: 1.4.0.0
Methods rnf :: Identity a -> () Source #
NFData a => NFData (Min a) Source #	Since: 1.4.2.0
Methods rnf :: Min a -> () Source #
NFData a => NFData (Max a) Source #	Since: 1.4.2.0
Methods rnf :: Max a -> () Source #
NFData a => NFData (First a) Source #	Since: 1.4.2.0
Methods rnf :: First a -> () Source #
NFData a => NFData (Last a) Source #	Since: 1.4.2.0
Methods rnf :: Last a -> () Source #
NFData m => NFData (WrappedMonoid m) Source #	Since: 1.4.2.0
Methods rnf :: WrappedMonoid m -> () Source #
NFData a => NFData (Option a) Source #	Since: 1.4.2.0
Methods rnf :: Option a -> () Source #
NFData a => NFData (NonEmpty a) Source #	Since: 1.4.2.0
Methods rnf :: NonEmpty a -> () Source #
NFData (Fixed a) Source #	Since: 1.3.0.0
Methods rnf :: Fixed a -> () Source #
NFData a => NFData (Complex a) Source #
Methods rnf :: Complex a -> () Source #
NFData (StableName a) Source #	Since: 1.4.0.0
Methods rnf :: StableName a -> () Source #
NFData a => NFData (ZipList a) Source #	Since: 1.4.0.0
Methods rnf :: ZipList a -> () Source #
NFData a => NFData (Dual a) Source #	Since: 1.4.0.0
Methods rnf :: Dual a -> () Source #
NFData a => NFData (Sum a) Source #	Since: 1.4.0.0
Methods rnf :: Sum a -> () Source #
NFData a => NFData (Product a) Source #	Since: 1.4.0.0
Methods rnf :: Product a -> () Source #
NFData a => NFData (First a) Source #	Since: 1.4.0.0
Methods rnf :: First a -> () Source #
NFData a => NFData (Last a) Source #	Since: 1.4.0.0
Methods rnf :: Last a -> () Source #
NFData (IORef a) Source #	NOTE: Only strict in the reference and not the referenced value. Since: 1.4.2.0
Methods rnf :: IORef a -> () Source #
NFData a => NFData (Down a) Source #	Since: 1.4.0.0
Methods rnf :: Down a -> () Source #
NFData (MVar a) Source #	NOTE: Only strict in the reference and not the referenced value. Since: 1.4.2.0
Methods rnf :: MVar a -> () Source #
NFData (a -> b) Source #	This instance is for convenience and consistency with `seq`. This assumes that WHNF is equivalent to NF for functions. Since: 1.3.0.0
Methods rnf :: (a -> b) -> () Source #
(NFData a, NFData b) => NFData (Either a b) Source #
Methods rnf :: Either a b -> () Source #
(NFData a, NFData b) => NFData (a, b) Source #
Methods rnf :: (a, b) -> () Source #
(NFData a, NFData b) => NFData (Array a b) Source #
Methods rnf :: Array a b -> () Source #
(NFData a, NFData b) => NFData (Arg a b) Source #	Since: 1.4.2.0
Methods rnf :: Arg a b -> () Source #
NFData (Proxy k a) Source #	Since: 1.4.0.0
Methods rnf :: Proxy k a -> () Source #
NFData (STRef s a) Source #	NOTE: Only strict in the reference and not the referenced value. Since: 1.4.2.0
Methods rnf :: STRef s a -> () Source #
(NFData a, NFData b, NFData c) => NFData (a, b, c) Source #
Methods rnf :: (a, b, c) -> () Source #
NFData a => NFData (Const k a b) Source #	Since: 1.4.0.0
Methods rnf :: Const k a b -> () Source #
(NFData a, NFData b, NFData c, NFData d) => NFData (a, b, c, d) Source #
Methods rnf :: (a, b, c, d) -> () Source #
(NFData a1, NFData a2, NFData a3, NFData a4, NFData a5) => NFData (a1, a2, a3, a4, a5) Source #
Methods rnf :: (a1, a2, a3, a4, a5) -> () Source #
(NFData a1, NFData a2, NFData a3, NFData a4, NFData a5, NFData a6) => NFData (a1, a2, a3, a4, a5, a6) Source #
Methods rnf :: (a1, a2, a3, a4, a5, a6) -> () Source #
(NFData a1, NFData a2, NFData a3, NFData a4, NFData a5, NFData a6, NFData a7) => NFData (a1, a2, a3, a4, a5, a6, a7) Source #
Methods rnf :: (a1, a2, a3, a4, a5, a6, a7) -> () Source #
(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) Source #
Methods rnf :: (a1, a2, a3, a4, a5, a6, a7, a8) -> () Source #
(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) Source #
Methods rnf :: (a1, a2, a3, a4, a5, a6, a7, a8, a9) -> () Source #

Documentation

Generic NFData deriving

Compatibility with previous deepseq versions

Generic NFData deriving

Compatibility with previous deepseq versions

`Generic` `NFData` deriving

Compatibility with previous `deepseq` versions

`Generic` `NFData` deriving

Compatibility with previous `deepseq` versions