Documentation

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 proper data typename (use the GeneralizedNewtypeDeriving extension for newtype 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 require NFData 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 TypeNames, e.g.:

 $(deriveNFData [''TypeName1, ''TypeName2, ''TypeName3])

whnfIsNf :: Type -> Maybe Bool Source

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.