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

Minimal complete definition

Nothing

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 !_ = ()

Instances

NFData Bool

NFData Char

NFData Double

NFData Float

NFData Int

NFData Int8

NFData Int16

NFData Int32

NFData Int64

NFData Integer

NFData Word

NFData Word8

NFData Word16

NFData Word32

NFData Word64

NFData ()

NFData Unique

Since: 1.4.0.0

NFData Version

Since: 1.3.0.0

NFData ThreadId

Since: 1.4.0.0

NFData CChar

Since: 1.4.0.0

NFData CSChar

Since: 1.4.0.0

NFData CUChar

Since: 1.4.0.0

NFData CShort

Since: 1.4.0.0

NFData CUShort

Since: 1.4.0.0

NFData CInt

Since: 1.4.0.0

NFData CUInt

Since: 1.4.0.0

NFData CLong

Since: 1.4.0.0

NFData CULong

Since: 1.4.0.0

NFData CLLong

Since: 1.4.0.0

NFData CULLong

Since: 1.4.0.0

NFData CFloat

Since: 1.4.0.0

NFData CDouble

Since: 1.4.0.0

NFData CPtrdiff

Since: 1.4.0.0

NFData CSize

Since: 1.4.0.0

NFData CWchar

Since: 1.4.0.0

NFData CSigAtomic

Since: 1.4.0.0

NFData CClock

Since: 1.4.0.0

NFData CTime

Since: 1.4.0.0

NFData CUSeconds

Since: 1.4.0.0

NFData CSUSeconds

Since: 1.4.0.0

NFData CFile

Since: 1.4.0.0

NFData CFpos

Since: 1.4.0.0

NFData CJmpBuf

Since: 1.4.0.0

NFData CIntPtr

Since: 1.4.0.0

NFData CUIntPtr

Since: 1.4.0.0

NFData CIntMax

Since: 1.4.0.0

NFData CUIntMax

Since: 1.4.0.0

NFData All

Since: 1.4.0.0

NFData Any

Since: 1.4.0.0

NFData Fingerprint

Since: 1.4.0.0

NFData a => NFData [a]

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

NFData (StableName a)

Since: 1.4.0.0

NFData (Fixed a)

Since: 1.3.0.0

NFData a => NFData (Complex a)

NFData a => NFData (ZipList a)

Since: 1.4.0.0

NFData a => NFData (Dual a)

Since: 1.4.0.0

NFData a => NFData (Sum a)

Since: 1.4.0.0

NFData a => NFData (Product a)

Since: 1.4.0.0

NFData a => NFData (First a)

Since: 1.4.0.0

NFData a => NFData (Last a)

Since: 1.4.0.0

NFData a => NFData (Down a)

Since: 1.4.0.0

NFData a => NFData (Maybe 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

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

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

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

NFData a => NFData (Const a b)

Since: 1.4.0.0

NFData (Proxy * a)

Since: 1.4.0.0

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

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

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

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

(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)

(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)