| Portability | portable |
|---|---|
| Stability | stable |
| Maintainer | libraries@haskell.org |
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).
Documentation
deepseq :: NFData a => a -> b -> bSource
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 -> bSource
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 -> aSource
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.
A class of types that can be fully evaluated.
Methods
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).
Instances