deepseq-1.4.2.0: Deep evaluation of data structures

Copyright(c) The University of Glasgow 2001-2009
LicenseBSD-style (see the file LICENSE)
Maintainerlibraries@haskell.org
Stabilitystable
Portabilityportable
Safe HaskellSafe
LanguageHaskell2010

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 #

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 #

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 #

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 #

A class of types that can be fully evaluated.

Since: 1.1.0.0

Methods

rnf :: a -> () #

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 -> () #

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 # 

Methods

rnf :: Bool -> () #

NFData Char # 

Methods

rnf :: Char -> () #

NFData Double # 

Methods

rnf :: Double -> () #

NFData Float # 

Methods

rnf :: Float -> () #

NFData Int # 

Methods

rnf :: Int -> () #

NFData Int8 # 

Methods

rnf :: Int8 -> () #

NFData Int16 # 

Methods

rnf :: Int16 -> () #

NFData Int32 # 

Methods

rnf :: Int32 -> () #

NFData Int64 # 

Methods

rnf :: Int64 -> () #

NFData Integer # 

Methods

rnf :: Integer -> () #

NFData Word # 

Methods

rnf :: Word -> () #

NFData Word8 # 

Methods

rnf :: Word8 -> () #

NFData Word16 # 

Methods

rnf :: Word16 -> () #

NFData Word32 # 

Methods

rnf :: Word32 -> () #

NFData Word64 # 

Methods

rnf :: Word64 -> () #

NFData CallStack #

Since: 1.4.2.0

Methods

rnf :: CallStack -> () #

NFData TypeRep #

NOTE: Only defined for base-4.8.0.0 and later

Since: 1.4.0.0

Methods

rnf :: TypeRep -> () #

NFData () # 

Methods

rnf :: () -> () #

NFData TyCon #

NOTE: Only defined for base-4.8.0.0 and later

Since: 1.4.0.0

Methods

rnf :: TyCon -> () #

NFData Natural #

Since: 1.4.0.0

Methods

rnf :: Natural -> () #

NFData Void #

Defined as rnf = absurd.

Since: 1.4.0.0

Methods

rnf :: Void -> () #

NFData Version #

Since: 1.3.0.0

Methods

rnf :: Version -> () #

NFData Unique #

Since: 1.4.0.0

Methods

rnf :: Unique -> () #

NFData ThreadId #

Since: 1.4.0.0

Methods

rnf :: ThreadId -> () #

NFData ExitCode #

Since: 1.4.2.0

Methods

rnf :: ExitCode -> () #

NFData CChar #

Since: 1.4.0.0

Methods

rnf :: CChar -> () #

NFData CSChar #

Since: 1.4.0.0

Methods

rnf :: CSChar -> () #

NFData CUChar #

Since: 1.4.0.0

Methods

rnf :: CUChar -> () #

NFData CShort #

Since: 1.4.0.0

Methods

rnf :: CShort -> () #

NFData CUShort #

Since: 1.4.0.0

Methods

rnf :: CUShort -> () #

NFData CInt #

Since: 1.4.0.0

Methods

rnf :: CInt -> () #

NFData CUInt #

Since: 1.4.0.0

Methods

rnf :: CUInt -> () #

NFData CLong #

Since: 1.4.0.0

Methods

rnf :: CLong -> () #

NFData CULong #

Since: 1.4.0.0

Methods

rnf :: CULong -> () #

NFData CLLong #

Since: 1.4.0.0

Methods

rnf :: CLLong -> () #

NFData CULLong #

Since: 1.4.0.0

Methods

rnf :: CULLong -> () #

NFData CFloat #

Since: 1.4.0.0

Methods

rnf :: CFloat -> () #

NFData CDouble #

Since: 1.4.0.0

Methods

rnf :: CDouble -> () #

NFData CPtrdiff #

Since: 1.4.0.0

Methods

rnf :: CPtrdiff -> () #

NFData CSize #

Since: 1.4.0.0

Methods

rnf :: CSize -> () #

NFData CWchar #

Since: 1.4.0.0

Methods

rnf :: CWchar -> () #

NFData CSigAtomic #

Since: 1.4.0.0

Methods

rnf :: CSigAtomic -> () #

NFData CClock #

Since: 1.4.0.0

Methods

rnf :: CClock -> () #

NFData CTime #

Since: 1.4.0.0

Methods

rnf :: CTime -> () #

NFData CUSeconds #

Since: 1.4.0.0

Methods

rnf :: CUSeconds -> () #

NFData CSUSeconds #

Since: 1.4.0.0

Methods

rnf :: CSUSeconds -> () #

NFData CFile #

Since: 1.4.0.0

Methods

rnf :: CFile -> () #

NFData CFpos #

Since: 1.4.0.0

Methods

rnf :: CFpos -> () #

NFData CJmpBuf #

Since: 1.4.0.0

Methods

rnf :: CJmpBuf -> () #

NFData CIntPtr #

Since: 1.4.0.0

Methods

rnf :: CIntPtr -> () #

NFData CUIntPtr #

Since: 1.4.0.0

Methods

rnf :: CUIntPtr -> () #

NFData CIntMax #

Since: 1.4.0.0

Methods

rnf :: CIntMax -> () #

NFData CUIntMax #

Since: 1.4.0.0

Methods

rnf :: CUIntMax -> () #

NFData All #

Since: 1.4.0.0

Methods

rnf :: All -> () #

NFData Any #

Since: 1.4.0.0

Methods

rnf :: Any -> () #

NFData Fingerprint #

Since: 1.4.0.0

Methods

rnf :: Fingerprint -> () #

NFData SrcLoc #

Since: 1.4.2.0

Methods

rnf :: SrcLoc -> () #

NFData a => NFData [a] # 

Methods

rnf :: [a] -> () #

NFData a => NFData (Maybe a) # 

Methods

rnf :: Maybe a -> () #

NFData a => NFData (Ratio a) # 

Methods

rnf :: Ratio a -> () #

NFData (Ptr a) #

Since: 1.4.2.0

Methods

rnf :: Ptr a -> () #

NFData (FunPtr a) #

Since: 1.4.2.0

Methods

rnf :: FunPtr a -> () #

NFData a => NFData (Identity a) #

Since: 1.4.0.0

Methods

rnf :: Identity a -> () #

NFData a => NFData (Min a) #

Since: 1.4.2.0

Methods

rnf :: Min a -> () #

NFData a => NFData (Max a) #

Since: 1.4.2.0

Methods

rnf :: Max a -> () #

NFData a => NFData (First a) #

Since: 1.4.2.0

Methods

rnf :: First a -> () #

NFData a => NFData (Last a) #

Since: 1.4.2.0

Methods

rnf :: Last a -> () #

NFData m => NFData (WrappedMonoid m) #

Since: 1.4.2.0

Methods

rnf :: WrappedMonoid m -> () #

NFData a => NFData (Option a) #

Since: 1.4.2.0

Methods

rnf :: Option a -> () #

NFData a => NFData (NonEmpty a) #

Since: 1.4.2.0

Methods

rnf :: NonEmpty a -> () #

NFData (Fixed a) #

Since: 1.3.0.0

Methods

rnf :: Fixed a -> () #

NFData a => NFData (Complex a) # 

Methods

rnf :: Complex a -> () #

NFData (StableName a) #

Since: 1.4.0.0

Methods

rnf :: StableName a -> () #

NFData a => NFData (ZipList a) #

Since: 1.4.0.0

Methods

rnf :: ZipList a -> () #

NFData a => NFData (Dual a) #

Since: 1.4.0.0

Methods

rnf :: Dual a -> () #

NFData a => NFData (Sum a) #

Since: 1.4.0.0

Methods

rnf :: Sum a -> () #

NFData a => NFData (Product a) #

Since: 1.4.0.0

Methods

rnf :: Product a -> () #

NFData a => NFData (First a) #

Since: 1.4.0.0

Methods

rnf :: First a -> () #

NFData a => NFData (Last a) #

Since: 1.4.0.0

Methods

rnf :: Last a -> () #

NFData (IORef a) #

NOTE: Only strict in the reference and not the referenced value.

Since: 1.4.2.0

Methods

rnf :: IORef a -> () #

NFData a => NFData (Down a) #

Since: 1.4.0.0

Methods

rnf :: Down a -> () #

NFData (MVar a) #

NOTE: Only strict in the reference and not the referenced value.

Since: 1.4.2.0

Methods

rnf :: MVar a -> () #

NFData (a -> b) #

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) -> () #

(NFData a, NFData b) => NFData (Either a b) # 

Methods

rnf :: Either a b -> () #

(NFData a, NFData b) => NFData (a, b) # 

Methods

rnf :: (a, b) -> () #

(NFData a, NFData b) => NFData (Array a b) # 

Methods

rnf :: Array a b -> () #

(NFData a, NFData b) => NFData (Arg a b) #

Since: 1.4.2.0

Methods

rnf :: Arg a b -> () #

NFData (Proxy k a) #

Since: 1.4.0.0

Methods

rnf :: Proxy k a -> () #

NFData (STRef s a) #

NOTE: Only strict in the reference and not the referenced value.

Since: 1.4.2.0

Methods

rnf :: STRef s a -> () #

(NFData a, NFData b, NFData c) => NFData (a, b, c) # 

Methods

rnf :: (a, b, c) -> () #

NFData a => NFData (Const k a b) #

Since: 1.4.0.0

Methods

rnf :: Const k a b -> () #

(NFData a, NFData b, NFData c, NFData d) => NFData (a, b, c, d) # 

Methods

rnf :: (a, b, c, d) -> () #

(NFData a1, NFData a2, NFData a3, NFData a4, NFData a5) => NFData (a1, a2, a3, a4, a5) # 

Methods

rnf :: (a1, a2, a3, a4, a5) -> () #

(NFData a1, NFData a2, NFData a3, NFData a4, NFData a5, NFData a6) => NFData (a1, a2, a3, a4, a5, a6) # 

Methods

rnf :: (a1, a2, a3, a4, a5, a6) -> () #

(NFData a1, NFData a2, NFData a3, NFData a4, NFData a5, NFData a6, NFData a7) => NFData (a1, a2, a3, a4, a5, a6, a7) # 

Methods

rnf :: (a1, a2, a3, a4, a5, a6, a7) -> () #

(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) # 

Methods

rnf :: (a1, a2, a3, a4, a5, a6, a7, a8) -> () #

(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) # 

Methods

rnf :: (a1, a2, a3, a4, a5, a6, a7, a8, a9) -> () #