Control.DeepSeq

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

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) -> ()