import System.IO
import Control.DeepSeq

main = do
    h <- openFile "f" ReadMode
    s <- hGetContents h
    s `deepseq` hClose h
    return s

> [1,2,undefined] `seq` 3
3

> [1,2,undefined] `deepseq` 3
*** Exception: Prelude.undefined

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.

Since: 1.1.0.0

($!!) :: 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.

Since: 1.2.0.0

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.

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 Source

A class of types that can be fully evaluated.

Since: 1.1.0.0

{-# 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

{-# 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)

rnf a = seq a ()

instance NFData Colour where rnf x = seq x ()

{-# LANGUAGE BangPatterns #-}
instance NFData Colour where rnf !_ = ()