Safe Haskell	None
Language	Haskell2010

Prologue.Control.DeepSeq

Synopsis

type family NFDatas lst :: Constraint where ...
evaluate :: a -> IO a
force :: NFData a => a -> a
class NFData a where

Documentation

type family NFDatas lst :: Constraint where ... Source #

Equations

NFDatas '[] = ()
NFDatas (a ': as) = (NFData a, NFDatas as)

evaluate :: a -> IO a #

Evaluate the argument to weak head normal form.

evaluate is typically used to uncover any exceptions that a lazy value may contain, and possibly handle them.

evaluate only evaluates to weak head normal form. If deeper evaluation is needed, the force function from Control.DeepSeq may be handy:

evaluate $ force x

There is a subtle difference between evaluate x and return $! x, analogous to the difference between throwIO and throw. If the lazy value x throws an exception, return $! x will fail to return an IO action and will throw an exception instead. evaluate x, on the other hand, always produces an IO action; that action will throw an exception upon execution iff x throws an exception upon evaluation.

The practical implication of this difference is that due to the imprecise exceptions semantics,

(return $! error "foo") >> error "bar"

may throw either "foo" or "bar", depending on the optimizations performed by the compiler. On the other hand,

evaluate (error "foo") >> error "bar"

is guaranteed to throw "foo".

The rule of thumb is to use evaluate to force or handle exceptions in lazy values. If, on the other hand, you are forcing a lazy value for efficiency reasons only and do not care about exceptions, you may use return $! x.

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.

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

class NFData a where #

A class of types that can be fully evaluated.

Since: deepseq-1.1.0.0

Methods

rnf :: a -> () #

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

`Generic` `NFData` deriving

Starting with GHC 7.2, you can automatically derive instances for types possessing a Generic instance.

Note: Generic1 can be auto-derived starting with GHC 7.4

{-# LANGUAGE DeriveGeneric #-}

import GHC.Generics (Generic, Generic1)
import Control.DeepSeq

data Foo a = Foo a String
             deriving (Eq, Generic, Generic1)

instance NFData a => NFData (Foo a)
instance NFData1 Foo

data Colour = Red | Green | Blue
              deriving Generic

instance NFData Colour

Starting with GHC 7.10, the example above can be written more concisely by enabling the new DeriveAnyClass extension:

{-# LANGUAGE DeriveGeneric, DeriveAnyClass #-}

import GHC.Generics (Generic)
import Control.DeepSeq

data Foo a = Foo a String
             deriving (Eq, Generic, Generic1, NFData, NFData1)

data Colour = Red | Green | Blue
              deriving (Generic, NFData)

Compatibility with previous `deepseq` versions

Prior to version 1.4.0.0, the default implementation of the rnf method was defined as

rnf a = seq a ()

However, starting with deepseq-1.4.0.0, the default implementation is based on DefaultSignatures allowing for more accurate auto-derived NFData instances. If you need the previously used exact default rnf method implementation semantics, use

instance NFData Colour where rnf x = seq x ()

or alternatively

instance NFData Colour where rnf = rwhnf

or

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

Instances

NFData Bool
Instance details Defined in Control.DeepSeq Methods rnf :: Bool -> () #
NFData Char
Instance details Defined in Control.DeepSeq Methods rnf :: Char -> () #
NFData Double
Instance details Defined in Control.DeepSeq Methods rnf :: Double -> () #
NFData Float
Instance details Defined in Control.DeepSeq Methods rnf :: Float -> () #
NFData Int
Instance details Defined in Control.DeepSeq Methods rnf :: Int -> () #
NFData Int8
Instance details Defined in Control.DeepSeq Methods rnf :: Int8 -> () #
NFData Int16
Instance details Defined in Control.DeepSeq Methods rnf :: Int16 -> () #
NFData Int32
Instance details Defined in Control.DeepSeq Methods rnf :: Int32 -> () #
NFData Int64
Instance details Defined in Control.DeepSeq Methods rnf :: Int64 -> () #
NFData Integer
Instance details Defined in Control.DeepSeq Methods rnf :: Integer -> () #
NFData Natural	Since: deepseq-1.4.0.0
Instance details Defined in Control.DeepSeq Methods rnf :: Natural -> () #
NFData Ordering
Instance details Defined in Control.DeepSeq Methods rnf :: Ordering -> () #
NFData Word
Instance details Defined in Control.DeepSeq Methods rnf :: Word -> () #
NFData Word8
Instance details Defined in Control.DeepSeq Methods rnf :: Word8 -> () #
NFData Word16
Instance details Defined in Control.DeepSeq Methods rnf :: Word16 -> () #
NFData Word32
Instance details Defined in Control.DeepSeq Methods rnf :: Word32 -> () #
NFData Word64
Instance details Defined in Control.DeepSeq Methods rnf :: Word64 -> () #
NFData CallStack	Since: deepseq-1.4.2.0
Instance details Defined in Control.DeepSeq Methods rnf :: CallStack -> () #
NFData ()
Instance details Defined in Control.DeepSeq Methods rnf :: () -> () #
NFData TyCon	NOTE: Only defined for `base-4.8.0.0` and later Since: deepseq-1.4.0.0
Instance details Defined in Control.DeepSeq Methods rnf :: TyCon -> () #
NFData Void	Defined as `rnf = absurd`. Since: deepseq-1.4.0.0
Instance details Defined in Control.DeepSeq Methods rnf :: Void -> () #
NFData Unique	Since: deepseq-1.4.0.0
Instance details Defined in Control.DeepSeq Methods rnf :: Unique -> () #
NFData Version	Since: deepseq-1.3.0.0
Instance details Defined in Control.DeepSeq Methods rnf :: Version -> () #
NFData ThreadId	Since: deepseq-1.4.0.0
Instance details Defined in Control.DeepSeq Methods rnf :: ThreadId -> () #
NFData ExitCode	Since: deepseq-1.4.2.0
Instance details Defined in Control.DeepSeq Methods rnf :: ExitCode -> () #
NFData TypeRep	NOTE: Only defined for `base-4.8.0.0` and later Since: deepseq-1.4.0.0
Instance details Defined in Control.DeepSeq Methods rnf :: TypeRep -> () #
NFData All	Since: deepseq-1.4.0.0
Instance details Defined in Control.DeepSeq Methods rnf :: All -> () #
NFData Any	Since: deepseq-1.4.0.0
Instance details Defined in Control.DeepSeq Methods rnf :: Any -> () #
NFData CChar	Since: deepseq-1.4.0.0
Instance details Defined in Control.DeepSeq Methods rnf :: CChar -> () #
NFData CSChar	Since: deepseq-1.4.0.0
Instance details Defined in Control.DeepSeq Methods rnf :: CSChar -> () #
NFData CUChar	Since: deepseq-1.4.0.0
Instance details Defined in Control.DeepSeq Methods rnf :: CUChar -> () #
NFData CShort	Since: deepseq-1.4.0.0
Instance details Defined in Control.DeepSeq Methods rnf :: CShort -> () #
NFData CUShort	Since: deepseq-1.4.0.0
Instance details Defined in Control.DeepSeq Methods rnf :: CUShort -> () #
NFData CInt	Since: deepseq-1.4.0.0
Instance details Defined in Control.DeepSeq Methods rnf :: CInt -> () #
NFData CUInt	Since: deepseq-1.4.0.0
Instance details Defined in Control.DeepSeq Methods rnf :: CUInt -> () #
NFData CLong	Since: deepseq-1.4.0.0
Instance details Defined in Control.DeepSeq Methods rnf :: CLong -> () #
NFData CULong	Since: deepseq-1.4.0.0
Instance details Defined in Control.DeepSeq Methods rnf :: CULong -> () #
NFData CLLong	Since: deepseq-1.4.0.0
Instance details Defined in Control.DeepSeq Methods rnf :: CLLong -> () #
NFData CULLong	Since: deepseq-1.4.0.0
Instance details Defined in Control.DeepSeq Methods rnf :: CULLong -> () #
NFData CBool	Since: deepseq-1.4.3.0
Instance details Defined in Control.DeepSeq Methods rnf :: CBool -> () #
NFData CFloat	Since: deepseq-1.4.0.0
Instance details Defined in Control.DeepSeq Methods rnf :: CFloat -> () #
NFData CDouble	Since: deepseq-1.4.0.0
Instance details Defined in Control.DeepSeq Methods rnf :: CDouble -> () #
NFData CPtrdiff	Since: deepseq-1.4.0.0
Instance details Defined in Control.DeepSeq Methods rnf :: CPtrdiff -> () #
NFData CSize	Since: deepseq-1.4.0.0
Instance details Defined in Control.DeepSeq Methods rnf :: CSize -> () #
NFData CWchar	Since: deepseq-1.4.0.0
Instance details Defined in Control.DeepSeq Methods rnf :: CWchar -> () #
NFData CSigAtomic	Since: deepseq-1.4.0.0
Instance details Defined in Control.DeepSeq Methods rnf :: CSigAtomic -> () #
NFData CClock	Since: deepseq-1.4.0.0
Instance details Defined in Control.DeepSeq Methods rnf :: CClock -> () #
NFData CTime	Since: deepseq-1.4.0.0
Instance details Defined in Control.DeepSeq Methods rnf :: CTime -> () #
NFData CUSeconds	Since: deepseq-1.4.0.0
Instance details Defined in Control.DeepSeq Methods rnf :: CUSeconds -> () #
NFData CSUSeconds	Since: deepseq-1.4.0.0
Instance details Defined in Control.DeepSeq Methods rnf :: CSUSeconds -> () #
NFData CFile	Since: deepseq-1.4.0.0
Instance details Defined in Control.DeepSeq Methods rnf :: CFile -> () #
NFData CFpos	Since: deepseq-1.4.0.0
Instance details Defined in Control.DeepSeq Methods rnf :: CFpos -> () #
NFData CJmpBuf	Since: deepseq-1.4.0.0
Instance details Defined in Control.DeepSeq Methods rnf :: CJmpBuf -> () #
NFData CIntPtr	Since: deepseq-1.4.0.0
Instance details Defined in Control.DeepSeq Methods rnf :: CIntPtr -> () #
NFData CUIntPtr	Since: deepseq-1.4.0.0
Instance details Defined in Control.DeepSeq Methods rnf :: CUIntPtr -> () #
NFData CIntMax	Since: deepseq-1.4.0.0
Instance details Defined in Control.DeepSeq Methods rnf :: CIntMax -> () #
NFData CUIntMax	Since: deepseq-1.4.0.0
Instance details Defined in Control.DeepSeq Methods rnf :: CUIntMax -> () #
NFData Fingerprint	Since: deepseq-1.4.0.0
Instance details Defined in Control.DeepSeq Methods rnf :: Fingerprint -> () #
NFData SrcLoc	Since: deepseq-1.4.2.0
Instance details Defined in Control.DeepSeq Methods rnf :: SrcLoc -> () #
NFData IntSet
Instance details Defined in Data.IntSet.Internal Methods rnf :: IntSet -> () #
NFData Pos
Instance details Defined in Text.Megaparsec.Pos Methods rnf :: Pos -> () #
NFData InvalidPosException
Instance details Defined in Text.Megaparsec.Pos Methods rnf :: InvalidPosException -> () #
NFData SourcePos
Instance details Defined in Text.Megaparsec.Pos Methods rnf :: SourcePos -> () #
NFData Doc
Instance details Defined in Text.PrettyPrint.HughesPJ Methods rnf :: Doc -> () #
NFData TextDetails
Instance details Defined in Text.PrettyPrint.Annotated.HughesPJ Methods rnf :: TextDetails -> () #
NFData a => NFData [a]
Instance details Defined in Control.DeepSeq Methods rnf :: [a] -> () #
NFData a => NFData (Maybe a)
Instance details Defined in Control.DeepSeq Methods rnf :: Maybe a -> () #
NFData a => NFData (Ratio a)
Instance details Defined in Control.DeepSeq Methods rnf :: Ratio a -> () #
NFData (Ptr a)	Since: deepseq-1.4.2.0
Instance details Defined in Control.DeepSeq Methods rnf :: Ptr a -> () #
NFData (FunPtr a)	Since: deepseq-1.4.2.0
Instance details Defined in Control.DeepSeq Methods rnf :: FunPtr a -> () #
NFData a => NFData (Complex a)
Instance details Defined in Control.DeepSeq Methods rnf :: Complex a -> () #
NFData (Fixed a)	Since: deepseq-1.3.0.0
Instance details Defined in Control.DeepSeq Methods rnf :: Fixed a -> () #
NFData a => NFData (Min a)	Since: deepseq-1.4.2.0
Instance details Defined in Control.DeepSeq Methods rnf :: Min a -> () #
NFData a => NFData (Max a)	Since: deepseq-1.4.2.0
Instance details Defined in Control.DeepSeq Methods rnf :: Max a -> () #
NFData a => NFData (First a)	Since: deepseq-1.4.2.0
Instance details Defined in Control.DeepSeq Methods rnf :: First a -> () #
NFData a => NFData (Last a)	Since: deepseq-1.4.2.0
Instance details Defined in Control.DeepSeq Methods rnf :: Last a -> () #
NFData m => NFData (WrappedMonoid m)	Since: deepseq-1.4.2.0
Instance details Defined in Control.DeepSeq Methods rnf :: WrappedMonoid m -> () #
NFData a => NFData (Option a)	Since: deepseq-1.4.2.0
Instance details Defined in Control.DeepSeq Methods rnf :: Option a -> () #
NFData (StableName a)	Since: deepseq-1.4.0.0
Instance details Defined in Control.DeepSeq Methods rnf :: StableName a -> () #
NFData a => NFData (ZipList a)	Since: deepseq-1.4.0.0
Instance details Defined in Control.DeepSeq Methods rnf :: ZipList a -> () #
NFData a => NFData (Identity a)	Since: deepseq-1.4.0.0
Instance details Defined in Control.DeepSeq Methods rnf :: Identity a -> () #
NFData (IORef a)	NOTE: Only strict in the reference and not the referenced value. Since: deepseq-1.4.2.0
Instance details Defined in Control.DeepSeq Methods rnf :: IORef a -> () #
NFData a => NFData (First a)	Since: deepseq-1.4.0.0
Instance details Defined in Control.DeepSeq Methods rnf :: First a -> () #
NFData a => NFData (Last a)	Since: deepseq-1.4.0.0
Instance details Defined in Control.DeepSeq Methods rnf :: Last a -> () #
NFData a => NFData (Dual a)	Since: deepseq-1.4.0.0
Instance details Defined in Control.DeepSeq Methods rnf :: Dual a -> () #
NFData a => NFData (Sum a)	Since: deepseq-1.4.0.0
Instance details Defined in Control.DeepSeq Methods rnf :: Sum a -> () #
NFData a => NFData (Product a)	Since: deepseq-1.4.0.0
Instance details Defined in Control.DeepSeq Methods rnf :: Product a -> () #
NFData a => NFData (Down a)	Since: deepseq-1.4.0.0
Instance details Defined in Control.DeepSeq Methods rnf :: Down a -> () #
NFData (MVar a)	NOTE: Only strict in the reference and not the referenced value. Since: deepseq-1.4.2.0
Instance details Defined in Control.DeepSeq Methods rnf :: MVar a -> () #
NFData a => NFData (NonEmpty a)	Since: deepseq-1.4.2.0
Instance details Defined in Control.DeepSeq Methods rnf :: NonEmpty a -> () #
NFData a => NFData (IntMap a)
Instance details Defined in Data.IntMap.Internal Methods rnf :: IntMap a -> () #
NFData a => NFData (Tree a)
Instance details Defined in Data.Tree Methods rnf :: Tree a -> () #
NFData a => NFData (Seq a)
Instance details Defined in Data.Sequence.Internal Methods rnf :: Seq a -> () #
NFData a => NFData (FingerTree a)
Instance details Defined in Data.Sequence.Internal Methods rnf :: FingerTree a -> () #
NFData a => NFData (Digit a)
Instance details Defined in Data.Sequence.Internal Methods rnf :: Digit a -> () #
NFData a => NFData (Node a)
Instance details Defined in Data.Sequence.Internal Methods rnf :: Node a -> () #
NFData a => NFData (Elem a)
Instance details Defined in Data.Sequence.Internal Methods rnf :: Elem a -> () #
NFData a => NFData (Set a)
Instance details Defined in Data.Set.Internal Methods rnf :: Set a -> () #
NFData a => NFData (DList a)
Instance details Defined in Data.DList Methods rnf :: DList a -> () #
NFData a => NFData (Hashed a)
Instance details Defined in Data.Hashable.Class Methods rnf :: Hashed a -> () #
NFData (Vector a)
Instance details Defined in Data.Vector.Primitive Methods rnf :: Vector a -> () #
NFData (Vector a)
Instance details Defined in Data.Vector.Storable Methods rnf :: Vector a -> () #
NFData (Vector a)
Instance details Defined in Data.Vector.Unboxed.Base Methods rnf :: Vector a -> () #
NFData a => NFData (HashSet a)
Instance details Defined in Data.HashSet Methods rnf :: HashSet a -> () #
NFData a => NFData (Vector a)
Instance details Defined in Data.Vector Methods rnf :: Vector a -> () #
NFData t => NFData (ErrorItem t)
Instance details Defined in Text.Megaparsec.Error Methods rnf :: ErrorItem t -> () #
NFData a => NFData (ErrorFancy a)
Instance details Defined in Text.Megaparsec.Error Methods rnf :: ErrorFancy a -> () #
NFData s => NFData (State s)
Instance details Defined in Text.Megaparsec.State Methods rnf :: State s -> () #
NFData a => NFData (Doc a)
Instance details Defined in Text.PrettyPrint.Annotated.HughesPJ Methods rnf :: Doc a -> () #
NFData a => NFData (AnnotDetails a)
Instance details Defined in Text.PrettyPrint.Annotated.HughesPJ Methods rnf :: AnnotDetails a -> () #
NFData (a -> b)	This instance is for convenience and consistency with `seq`. This assumes that WHNF is equivalent to NF for functions. Since: deepseq-1.3.0.0
Instance details Defined in Control.DeepSeq Methods rnf :: (a -> b) -> () #
(NFData a, NFData b) => NFData (Either a b)
Instance details Defined in Control.DeepSeq Methods rnf :: Either a b -> () #
(NFData a, NFData b) => NFData (a, b)
Instance details Defined in Control.DeepSeq Methods rnf :: (a, b) -> () #
(NFData k, NFData v) => NFData (HashMap k v)
Instance details Defined in Data.HashMap.Base Methods rnf :: HashMap k v -> () #
(NFData k, NFData a) => NFData (Map k a)
Instance details Defined in Data.Map.Internal Methods rnf :: Map k a -> () #
(NFData a, NFData b) => NFData (Array a b)
Instance details Defined in Control.DeepSeq Methods rnf :: Array a b -> () #
(NFData a, NFData b) => NFData (Arg a b)	Since: deepseq-1.4.2.0
Instance details Defined in Control.DeepSeq Methods rnf :: Arg a b -> () #
NFData (Proxy a)	Since: deepseq-1.4.0.0
Instance details Defined in Control.DeepSeq Methods rnf :: Proxy a -> () #
NFData (STRef s a)	NOTE: Only strict in the reference and not the referenced value. Since: deepseq-1.4.2.0
Instance details Defined in Control.DeepSeq Methods rnf :: STRef s a -> () #
(NFData t, NFData e) => NFData (ParseError t e)
Instance details Defined in Text.Megaparsec.Error Methods rnf :: ParseError t e -> () #
(NFData k, NFData v) => NFData (Leaf k v)
Instance details Defined in Data.HashMap.Base Methods rnf :: Leaf k v -> () #
NFData (MVector s a)
Instance details Defined in Data.Vector.Unboxed.Base Methods rnf :: MVector s a -> () #
(NFData a1, NFData a2, NFData a3) => NFData (a1, a2, a3)
Instance details Defined in Control.DeepSeq Methods rnf :: (a1, a2, a3) -> () #
NFData a => NFData (Const a b)	Since: deepseq-1.4.0.0
Instance details Defined in Control.DeepSeq Methods rnf :: Const a b -> () #
NFData (a :~: b)	Since: deepseq-1.4.3.0
Instance details Defined in Control.DeepSeq Methods rnf :: (a :~: b) -> () #
NFData b => NFData (Tagged s b)
Instance details Defined in Data.Tagged Methods rnf :: Tagged s b -> () #
(NFData a1, NFData a2, NFData a3, NFData a4) => NFData (a1, a2, a3, a4)
Instance details Defined in Control.DeepSeq Methods rnf :: (a1, a2, a3, a4) -> () #
(NFData1 f, NFData1 g, NFData a) => NFData (Product f g a)	Since: deepseq-1.4.3.0
Instance details Defined in Control.DeepSeq Methods rnf :: Product f g a -> () #
(NFData1 f, NFData1 g, NFData a) => NFData (Sum f g a)	Since: deepseq-1.4.3.0
Instance details Defined in Control.DeepSeq Methods rnf :: Sum f g a -> () #
NFData (a :~~: b)	Since: deepseq-1.4.3.0
Instance details Defined in Control.DeepSeq Methods rnf :: (a :~~: b) -> () #
(NFData a1, NFData a2, NFData a3, NFData a4, NFData a5) => NFData (a1, a2, a3, a4, a5)
Instance details Defined in Control.DeepSeq Methods rnf :: (a1, a2, a3, a4, a5) -> () #
(NFData1 f, NFData1 g, NFData a) => NFData (Compose f g a)	Since: deepseq-1.4.3.0
Instance details Defined in Control.DeepSeq Methods rnf :: Compose f g a -> () #
(NFData a1, NFData a2, NFData a3, NFData a4, NFData a5, NFData a6) => NFData (a1, a2, a3, a4, a5, a6)
Instance details Defined in Control.DeepSeq Methods rnf :: (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)
Instance details Defined in Control.DeepSeq Methods rnf :: (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)
Instance details Defined in Control.DeepSeq Methods rnf :: (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)
Instance details Defined in Control.DeepSeq Methods rnf :: (a1, a2, a3, a4, a5, a6, a7, a8, a9) -> () #

Documentation

Generic NFData deriving

Compatibility with previous deepseq versions

`Generic` `NFData` deriving

Compatibility with previous `deepseq` versions