Control.DeepSeq

deepseq-1.3.0.1: Deep evaluation of data structures

Portability | portable
Stability | stable
Maintainer | libraries@haskell.org
Safe Haskell | Safe

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).

Synopsis

- deepseq :: NFData a => a -> b -> b
- ($!!) :: NFData a => (a -> b) -> a -> b
- force :: NFData a => a -> a
- class NFData a where
  - rnf :: a -> ()

# 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.

($!!) :: NFData a => (a -> b) -> a -> b

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

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

A class of types that can be fully evaluated.

Methods

rnf :: a -> ()

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

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) |