Documentation
deriveNFData :: Name -> Q [Dec]Source
Derive NFData
instance for simple Data
-declarations
Example usage for deriving NFData
instance for the type TypeName
:
$(deriveNFData ''TypeName)
The derivation tries to avoid evaluation of strict fields whose
types have the WHNF=NF property (see also whnfIsNf
). For
instance, consider the following type Foo
:
data Foo a = Foo1 | Foo2 !Int !String | Foo3 (Foo a) | Foo4 { fX :: Int, fY :: Char } | Foo a :--: !Bool
By invoking $(deriveNFData ''Foo)
the generated NFData
instance
will be equivalent to:
instance NFData a => NFData (Foo a) where rnf Foo1 = () rnf (Foo2 _ x) = x `deepseq` () rnf (Foo3 x) = x `deepseq` () rnf (Foo4 x y) = x `deepseq` y `deepseq` () rnf (x :--: _) = x `deepseq` ()
Known issues/limitations:
-
TypeName
must be a properdata
typename (use theGeneralizedNewtypeDeriving
extension fornewtype
names) - Does not support existential types yet (i.e. use of the
forall
keyword) - Does not always detect phantom type variables (e.g. for
data Foo a = Foo0 | Foo1 (Foo a)
) which causes those to requireNFData
instances.
deriveNFDatas :: [Name] -> Q [Dec]Source
Plural version of deriveNFData
Convenience wrapper for deriveNFData
which allows to derive
multiple NFData
instances for a list of TypeName
s, e.g.:
$(deriveNFData [''TypeName1, ''TypeName2, ''TypeName3])
whnfIsNf :: Type -> Maybe BoolSource
Try to infer whether type has the property that WHNF=NF for its values.
A result of Nothing
means it is not known whether the
property holds for the given type. Just True
means that the
property holds.
This function has currently a very limited knowledge and returns
Nothing
most of the time except for some primitive types.