Safe Haskell | None |
---|---|
Language | Haskell2010 |
Documentation
type family NFDatas lst :: Constraint where ... Source #
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
and evaluate
x
,
analogous to the difference between return
$!
xthrowIO
and throw
. If the lazy
value x
throws an exception,
will fail to return an
return
$!
xIO
action and will throw an exception instead.
, on the
other hand, always produces an evaluate
xIO
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
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
A class of types that can be fully evaluated.
Since: deepseq-1.1.0.0
Nothing
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 | |
Defined in Control.DeepSeq | |
NFData Char | |
Defined in Control.DeepSeq | |
NFData Double | |
Defined in Control.DeepSeq | |
NFData Float | |
Defined in Control.DeepSeq | |
NFData Int | |
Defined in Control.DeepSeq | |
NFData Int8 | |
Defined in Control.DeepSeq | |
NFData Int16 | |
Defined in Control.DeepSeq | |
NFData Int32 | |
Defined in Control.DeepSeq | |
NFData Int64 | |
Defined in Control.DeepSeq | |
NFData Integer | |
Defined in Control.DeepSeq | |
NFData Natural | Since: deepseq-1.4.0.0 |
Defined in Control.DeepSeq | |
NFData Ordering | |
Defined in Control.DeepSeq | |
NFData Word | |
Defined in Control.DeepSeq | |
NFData Word8 | |
Defined in Control.DeepSeq | |
NFData Word16 | |
Defined in Control.DeepSeq | |
NFData Word32 | |
Defined in Control.DeepSeq | |
NFData Word64 | |
Defined in Control.DeepSeq | |
NFData CallStack | Since: deepseq-1.4.2.0 |
Defined in Control.DeepSeq | |
NFData () | |
Defined in Control.DeepSeq | |
NFData TyCon | NOTE: Prior to Since: deepseq-1.4.0.0 |
Defined in Control.DeepSeq | |
NFData Void | Since: deepseq-1.4.0.0 |
Defined in Control.DeepSeq | |
NFData Unique | Since: deepseq-1.4.0.0 |
Defined in Control.DeepSeq | |
NFData Version | Since: deepseq-1.3.0.0 |
Defined in Control.DeepSeq | |
NFData ThreadId | Since: deepseq-1.4.0.0 |
Defined in Control.DeepSeq | |
NFData ExitCode | Since: deepseq-1.4.2.0 |
Defined in Control.DeepSeq | |
NFData MaskingState | Since: deepseq-1.4.4.0 |
Defined in Control.DeepSeq rnf :: MaskingState -> () # | |
NFData TypeRep | NOTE: Prior to Since: deepseq-1.4.0.0 |
Defined in Control.DeepSeq | |
NFData All | Since: deepseq-1.4.0.0 |
Defined in Control.DeepSeq | |
NFData Any | Since: deepseq-1.4.0.0 |
Defined in Control.DeepSeq | |
NFData CChar | Since: deepseq-1.4.0.0 |
Defined in Control.DeepSeq | |
NFData CSChar | Since: deepseq-1.4.0.0 |
Defined in Control.DeepSeq | |
NFData CUChar | Since: deepseq-1.4.0.0 |
Defined in Control.DeepSeq | |
NFData CShort | Since: deepseq-1.4.0.0 |
Defined in Control.DeepSeq | |
NFData CUShort | Since: deepseq-1.4.0.0 |
Defined in Control.DeepSeq | |
NFData CInt | Since: deepseq-1.4.0.0 |
Defined in Control.DeepSeq | |
NFData CUInt | Since: deepseq-1.4.0.0 |
Defined in Control.DeepSeq | |
NFData CLong | Since: deepseq-1.4.0.0 |
Defined in Control.DeepSeq | |
NFData CULong | Since: deepseq-1.4.0.0 |
Defined in Control.DeepSeq | |
NFData CLLong | Since: deepseq-1.4.0.0 |
Defined in Control.DeepSeq | |
NFData CULLong | Since: deepseq-1.4.0.0 |
Defined in Control.DeepSeq | |
NFData CBool | Since: deepseq-1.4.3.0 |
Defined in Control.DeepSeq | |
NFData CFloat | Since: deepseq-1.4.0.0 |
Defined in Control.DeepSeq | |
NFData CDouble | Since: deepseq-1.4.0.0 |
Defined in Control.DeepSeq | |
NFData CPtrdiff | Since: deepseq-1.4.0.0 |
Defined in Control.DeepSeq | |
NFData CSize | Since: deepseq-1.4.0.0 |
Defined in Control.DeepSeq | |
NFData CWchar | Since: deepseq-1.4.0.0 |
Defined in Control.DeepSeq | |
NFData CSigAtomic | Since: deepseq-1.4.0.0 |
Defined in Control.DeepSeq rnf :: CSigAtomic -> () # | |
NFData CClock | Since: deepseq-1.4.0.0 |
Defined in Control.DeepSeq | |
NFData CTime | Since: deepseq-1.4.0.0 |
Defined in Control.DeepSeq | |
NFData CUSeconds | Since: deepseq-1.4.0.0 |
Defined in Control.DeepSeq | |
NFData CSUSeconds | Since: deepseq-1.4.0.0 |
Defined in Control.DeepSeq rnf :: CSUSeconds -> () # | |
NFData CFile | Since: deepseq-1.4.0.0 |
Defined in Control.DeepSeq | |
NFData CFpos | Since: deepseq-1.4.0.0 |
Defined in Control.DeepSeq | |
NFData CJmpBuf | Since: deepseq-1.4.0.0 |
Defined in Control.DeepSeq | |
NFData CIntPtr | Since: deepseq-1.4.0.0 |
Defined in Control.DeepSeq | |
NFData CUIntPtr | Since: deepseq-1.4.0.0 |
Defined in Control.DeepSeq | |
NFData CIntMax | Since: deepseq-1.4.0.0 |
Defined in Control.DeepSeq | |
NFData CUIntMax | Since: deepseq-1.4.0.0 |
Defined in Control.DeepSeq | |
NFData Fingerprint | Since: deepseq-1.4.0.0 |
Defined in Control.DeepSeq rnf :: Fingerprint -> () # | |
NFData SrcLoc | Since: deepseq-1.4.2.0 |
Defined in Control.DeepSeq | |
NFData IntSet | |
Defined in Data.IntSet.Internal | |
NFData Pos | |
Defined in Text.Megaparsec.Pos | |
NFData InvalidPosException | |
Defined in Text.Megaparsec.Pos rnf :: InvalidPosException -> () # | |
NFData SourcePos | |
Defined in Text.Megaparsec.Pos | |
NFData Doc | |
Defined in Text.PrettyPrint.HughesPJ | |
NFData TextDetails | |
Defined in Text.PrettyPrint.Annotated.HughesPJ rnf :: TextDetails -> () # | |
NFData a => NFData [a] | |
Defined in Control.DeepSeq | |
NFData a => NFData (Maybe a) | |
Defined in Control.DeepSeq | |
NFData a => NFData (Ratio a) | |
Defined in Control.DeepSeq | |
NFData (Ptr a) | Since: deepseq-1.4.2.0 |
Defined in Control.DeepSeq | |
NFData (FunPtr a) | Since: deepseq-1.4.2.0 |
Defined in Control.DeepSeq | |
NFData a => NFData (Complex a) | |
Defined in Control.DeepSeq | |
NFData (Fixed a) | Since: deepseq-1.3.0.0 |
Defined in Control.DeepSeq | |
NFData a => NFData (Min a) | Since: deepseq-1.4.2.0 |
Defined in Control.DeepSeq | |
NFData a => NFData (Max a) | Since: deepseq-1.4.2.0 |
Defined in Control.DeepSeq | |
NFData a => NFData (First a) | Since: deepseq-1.4.2.0 |
Defined in Control.DeepSeq | |
NFData a => NFData (Last a) | Since: deepseq-1.4.2.0 |
Defined in Control.DeepSeq | |
NFData m => NFData (WrappedMonoid m) | Since: deepseq-1.4.2.0 |
Defined in Control.DeepSeq rnf :: WrappedMonoid m -> () # | |
NFData a => NFData (Option a) | Since: deepseq-1.4.2.0 |
Defined in Control.DeepSeq | |
NFData (StableName a) | Since: deepseq-1.4.0.0 |
Defined in Control.DeepSeq rnf :: StableName a -> () # | |
NFData a => NFData (ZipList a) | Since: deepseq-1.4.0.0 |
Defined in Control.DeepSeq | |
NFData a => NFData (Identity a) | Since: deepseq-1.4.0.0 |
Defined in Control.DeepSeq | |
NFData (IORef a) | NOTE: Only strict in the reference and not the referenced value. Since: deepseq-1.4.2.0 |
Defined in Control.DeepSeq | |
NFData a => NFData (First a) | Since: deepseq-1.4.0.0 |
Defined in Control.DeepSeq | |
NFData a => NFData (Last a) | Since: deepseq-1.4.0.0 |
Defined in Control.DeepSeq | |
NFData a => NFData (Dual a) | Since: deepseq-1.4.0.0 |
Defined in Control.DeepSeq | |
NFData a => NFData (Sum a) | Since: deepseq-1.4.0.0 |
Defined in Control.DeepSeq | |
NFData a => NFData (Product a) | Since: deepseq-1.4.0.0 |
Defined in Control.DeepSeq | |
NFData a => NFData (Down a) | Since: deepseq-1.4.0.0 |
Defined in Control.DeepSeq | |
NFData (MVar a) | NOTE: Only strict in the reference and not the referenced value. Since: deepseq-1.4.2.0 |
Defined in Control.DeepSeq | |
NFData a => NFData (NonEmpty a) | Since: deepseq-1.4.2.0 |
Defined in Control.DeepSeq | |
NFData a => NFData (IntMap a) | |
Defined in Data.IntMap.Internal | |
NFData a => NFData (Tree a) | |
NFData a => NFData (Seq a) | |
Defined in Data.Sequence.Internal | |
NFData a => NFData (FingerTree a) | |
Defined in Data.Sequence.Internal rnf :: FingerTree a -> () # | |
NFData a => NFData (Digit a) | |
Defined in Data.Sequence.Internal | |
NFData a => NFData (Node a) | |
Defined in Data.Sequence.Internal | |
NFData a => NFData (Elem a) | |
Defined in Data.Sequence.Internal | |
NFData a => NFData (Set a) | |
Defined in Data.Set.Internal | |
NFData a => NFData (DList a) | |
Defined in Data.DList | |
NFData a => NFData (Hashed a) | |
Defined in Data.Hashable.Class | |
NFData (Vector a) | |
Defined in Data.Vector.Primitive | |
NFData (Vector a) | |
Defined in Data.Vector.Storable | |
NFData (Vector a) | |
Defined in Data.Vector.Unboxed.Base | |
NFData a => NFData (HashSet a) | |
Defined in Data.HashSet | |
NFData a => NFData (Vector a) | |
Defined in Data.Vector | |
NFData t => NFData (ErrorItem t) | |
Defined in Text.Megaparsec.Error | |
NFData a => NFData (ErrorFancy a) | |
Defined in Text.Megaparsec.Error rnf :: ErrorFancy a -> () # | |
NFData s => NFData (State s) | |
Defined in Text.Megaparsec.State | |
NFData s => NFData (PosState s) | |
Defined in Text.Megaparsec.State | |
NFData a => NFData (Doc a) | |
Defined in Text.PrettyPrint.Annotated.HughesPJ | |
NFData a => NFData (AnnotDetails a) | |
Defined in Text.PrettyPrint.Annotated.HughesPJ rnf :: AnnotDetails a -> () # | |
NFData (a -> b) | This instance is for convenience and consistency with Since: deepseq-1.3.0.0 |
Defined in Control.DeepSeq | |
(NFData a, NFData b) => NFData (Either a b) | |
Defined in Control.DeepSeq | |
(NFData a, NFData b) => NFData (a, b) | |
Defined in Control.DeepSeq | |
(NFData k, NFData v) => NFData (HashMap k v) | |
Defined in Data.HashMap.Base | |
(NFData k, NFData a) => NFData (Map k a) | |
Defined in Data.Map.Internal | |
(NFData a, NFData b) => NFData (Array a b) | |
Defined in Control.DeepSeq | |
(NFData a, NFData b) => NFData (Arg a b) | Since: deepseq-1.4.2.0 |
Defined in Control.DeepSeq | |
NFData (Proxy a) | Since: deepseq-1.4.0.0 |
Defined in Control.DeepSeq | |
NFData (STRef s a) | NOTE: Only strict in the reference and not the referenced value. Since: deepseq-1.4.2.0 |
Defined in Control.DeepSeq | |
(NFData (Token s), NFData e) => NFData (ParseError s e) | |
Defined in Text.Megaparsec.Error rnf :: ParseError s e -> () # | |
(NFData s, NFData (Token s), NFData e) => NFData (ParseErrorBundle s e) | |
Defined in Text.Megaparsec.Error rnf :: ParseErrorBundle s e -> () # | |
(NFData k, NFData v) => NFData (Leaf k v) | |
Defined in Data.HashMap.Base | |
NFData (MVector s a) | |
Defined in Data.Vector.Unboxed.Base | |
NFData (MVector s a) | |
Defined in Data.Vector.Storable.Mutable | |
NFData (MVector s a) | |
Defined in Data.Vector.Primitive.Mutable | |
(NFData a1, NFData a2, NFData a3) => NFData (a1, a2, a3) | |
Defined in Control.DeepSeq | |
NFData a => NFData (Const a b) | Since: deepseq-1.4.0.0 |
Defined in Control.DeepSeq | |
NFData (a :~: b) | Since: deepseq-1.4.3.0 |
Defined in Control.DeepSeq | |
NFData b => NFData (Tagged s b) | |
Defined in Data.Tagged | |
(NFData a1, NFData a2, NFData a3, NFData a4) => NFData (a1, a2, a3, a4) | |
Defined in Control.DeepSeq | |
(NFData1 f, NFData1 g, NFData a) => NFData (Product f g a) | Since: deepseq-1.4.3.0 |
Defined in Control.DeepSeq | |
(NFData1 f, NFData1 g, NFData a) => NFData (Sum f g a) | Since: deepseq-1.4.3.0 |
Defined in Control.DeepSeq | |
NFData (a :~~: b) | Since: deepseq-1.4.3.0 |
Defined in Control.DeepSeq | |
(NFData a1, NFData a2, NFData a3, NFData a4, NFData a5) => NFData (a1, a2, a3, a4, a5) | |
Defined in Control.DeepSeq | |
(NFData1 f, NFData1 g, NFData a) => NFData (Compose f g a) | Since: deepseq-1.4.3.0 |
Defined in Control.DeepSeq | |
(NFData a1, NFData a2, NFData a3, NFData a4, NFData a5, NFData a6) => NFData (a1, a2, a3, a4, a5, a6) | |
Defined in Control.DeepSeq | |
(NFData a1, NFData a2, NFData a3, NFData a4, NFData a5, NFData a6, NFData a7) => NFData (a1, a2, a3, a4, a5, a6, a7) | |
Defined in Control.DeepSeq | |
(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) | |
Defined in Control.DeepSeq | |
(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) | |
Defined in Control.DeepSeq |