Higher-kinded data (HKD) types with less pain.
This library takes your regular Haskell data types and produces additional higher-kinded versions, along with term- and type-level conversion functions.
I wanted to leverage the power of higher-kinded data as an internal
implementation detail of my code, without burdening others with the extra type
parameter, Identity wrapping (or poor type inference from the type family
unwrapping Identity), and overall increase in complexity.
With this library, I can work with my data in a higher-kinded fashion temporarily, and then convert back to regular Haskell data types when I'm done.
In other words, this:
import Higher
data Person = Person
{ name :: Text
, age :: Word8
}
$(higher ''Person)...turns into this:
import Higher
data Person = Person
{ name :: Text
, age :: Word8
}
data PersonB f = PersonB
{ nameB :: f Text
, ageB :: f Word8
}
instance Higher Person PersonB where
toHKD :: Person -> PersonB Identity
toHKD (Person x0 x1) = PersonB (Identity x0) (Identity x1)
fromHKD :: PersonB Identity -> Person
fromHKD (PersonB x0 x1) = Person (runIdentity x0) (runIdentity x1)
instance FunctorB PersonB where
bmap :: (forall a. f a -> g a) -> PersonB f -> PersonB g
bmap f (PersonB x0 x1) = PersonB (f x0) (f x1)
instance TraversableB PersonB where
btraverse :: Applicative e => (forall a. f a -> e (g a)) -> PersonB f -> e (PersonB g)
btraverse f (PersonB x0 x1) = pure PersonB <*> f x0 <*> f x1
instance ConstraintsB PersonB where
type AllB c PersonB = (c Text, c Word8)
baddDicts :: AllB c PersonB => PersonB f -> PersonB (Product (Dict c) f)
baddDicts (PersonB x0 x1) = PersonB (Pair Dict x0) (Pair Dict x1)
-- Other `barbies` type class instances coming soonYou can customize how the type constructor, data constructors, and fields are
named with higherWith.
Take a look at the test suite for examples.
Check out higgledy if you prefer using
Generic over Template Haskell.
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