A class of types that can be fully evaluated.

Since: deepseq-1.1.0.0

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: deepseq-1.1.0.0

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 ()

Finally, here's an exception safe variant of the readFile' example:

readFile' :: FilePath -> IO String
readFile' fn = bracket (openFile fn ReadMode) hClose $ \h ->
                       evaluate . force =<< hGetContents h

Since: deepseq-1.2.0.0

the deep analogue of $!. In the expression f $!! x, x is fully evaluated before the function f is applied to it.

Since: deepseq-1.2.0.0

Alias for evaluateWHNF . force with a clearer name.

>>> let list = [1..5] :: [Int]
>>> :sprint list
list = _
>>> () <$ evaluateNF list
>>> :sprint list
list = [1,2,3,4,5]

Alias for evaluateWHNF . rnf. Similar to evaluateNF but discards the resulting value.

>>> let list = [1..5] :: [Int]
>>> :sprint list
list = _
>>> evaluateNF_ list
>>> :sprint list
list = [1,2,3,4,5]

Lifted alias for evaluate with a clearer name.

>>> let list = [1..5] :: [Int]
>>> :sprint list
list = _
>>> () <$ evaluateWHNF list
>>> :sprint list
list = 1 : _

Like evaluateWHNF but discards value.

>>> let list = [1..5] :: [Int]
>>> :sprint list
list = _
>>> evaluateWHNF_ list
>>> :sprint list
list = 1 : _