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 typeWhnfIsNf and decWhnfIsNf). For instance, consider the following types Foo and Bar:

 data Foo a = Foo1
            | Foo2 !Int !String
            | Foo3 (Foo a)
            | Foo4 { fX :: Int, fY :: Char }
            | Foo5 !Bar
            | Foo6 !(String -> Int)
            | Foo a :--: !Bool

 data Bar = Bar0 | Bar1 !Char | Bar2 !Int !Int | Bar3 !Bar

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 (Foo5 _)   = ()
     rnf (Foo6 _)   = ()
     rnf (x :--: _) = x `deepseq` ()

Whereas $(deriveNFData ''Bar) generates the following default NFData instance since Bar is inferred as a WHNF=NF type:

 instance NFData Bar

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.

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])

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 rather limited knowledge and returns Nothing most of the time except for some primitive types and other simple cases.

See also decWhnfIsNf

Try to infer whether a Dec which defines a type which has the property that WHNF=NF for its values. This property is derived statically via the following simple rules:

• newtypes are WHNF=NF if the wrapped type is WHNF=NF
• types are WHNF=NF if the aliased type is WHNF=NF
• Types defined by data are WHNF=NF if all constructors contain only strict fields with WHNF=NF types

Known limitations:

• Doesn't work properly with parametrized declarations (in which case Nothing is returned) or existential types

See also typeWhnfIsNf