deepseq-1.4.1.1: 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 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

Minimal complete definition

Nothing

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 !_ = ()

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 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 Void Source

Defined as rnf = absurd.

Since: 1.4.0.0

Methods

rnf :: Void -> () Source

NFData Unique Source

Since: 1.4.0.0

Methods

rnf :: Unique -> () Source

NFData Natural Source

Since: 1.4.0.0

Methods

rnf :: Natural -> () Source

NFData Version Source

Since: 1.3.0.0

Methods

rnf :: Version -> () Source

NFData ThreadId Source

Since: 1.4.0.0

Methods

rnf :: ThreadId -> () 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 TyCon Source

NOTE: Only defined for base-4.8.0.0 and later

Since: 1.4.0.0

Methods

rnf :: TyCon -> () Source

NFData Fingerprint Source

Since: 1.4.0.0

Methods

rnf :: Fingerprint -> () Source

NFData a => NFData [a] Source 

Methods

rnf :: [a] -> () Source

(Integral a, NFData a) => NFData (Ratio a) Source 

Methods

rnf :: Ratio a -> () Source

NFData (StableName a) Source

Since: 1.4.0.0

Methods

rnf :: StableName a -> () Source

NFData a => NFData (Identity a) Source

Since: 1.4.0.0

Methods

rnf :: Identity 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 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 a => NFData (Down a) Source

Since: 1.4.0.0

Methods

rnf :: Down a -> () Source

NFData a => NFData (Maybe a) Source 

Methods

rnf :: Maybe 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

(Ix a, NFData a, NFData b) => NFData (Array a b) Source 

Methods

rnf :: Array a b -> () Source

NFData a => NFData (Const a b) Source

Since: 1.4.0.0

Methods

rnf :: Const a b -> () Source

NFData (Proxy * a) Source

Since: 1.4.0.0

Methods

rnf :: Proxy * a -> () Source

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

Methods

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