deepseq- Deep evaluation of data structures

Copyright(c) The University of Glasgow 2001-2009
LicenseBSD-style (see the file LICENSE)
Safe HaskellSafe



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

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).




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.


($!!) :: 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.


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.


class NFData a where Source

A class of types that can be fully evaluated.


Minimal complete definition



rnf :: a -> () Source

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

The default implementation of rnf is

rnf a = a `seq` ()

which may be convenient when defining instances for data types with no unevaluated fields (e.g. enumerations).


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 Version


NFData a => NFData [a] 
(Integral a, NFData a) => NFData (Ratio a) 
NFData (Fixed a)


(RealFloat a, NFData a) => NFData (Complex a) 
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.


(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 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)