Hi.
Here's a way that seems to work for me. I haven't tested in detail.
There may be problems, or also easier ways to achieve the same. The
DataKinds extension isn't essential. I've just used it for fun.
Cheers,
Andres
--
Andres Löh, Haskell Consultant
Well-Typed LLP, http://www.well-typed.com
{-# LANGUAGE TypeFamilies, ConstraintKinds, KindSignatures, TypeSynonymInstances, DataKinds #-}
module Resolve where
import GHC.Exts
import Control.Applicative
import Control.Monad
data DataBase = DataBase -- specific type not relevant here
data Resolved = Res | Unres
-- provides operation to transform an unresolved `Foo_ Unres` to a resolved `Foo_ Res`
class Resolvable e where
type ResFun e (a :: Resolved) :: * -- replace the 'Resolved' argument in 'e' with 'a'
type ResArg e (a :: Resolved) :: Constraint -- force the 'Resolved' argument in 'e' to be 'a'
resolve :: (ResArg e 'Unres) => DataBase -> e -> Either String (ResFun e 'Res)
-- trivial /resolvable/ type
data Foo_ (r :: Resolved) = Foo
deriving Show
instance Resolvable (Foo_ r) where
type ResFun (Foo_ r) a = Foo_ a
type ResArg (Foo_ r) a = r ~ a
resolve _ x = return Foo
-- Maybe (polymorphic 0 or 1 element container)
resolveMaybe :: (Resolvable e, ResArg e 'Unres) => DataBase -> Maybe e -> Either String (Maybe (ResFun e 'Res))
resolveMaybe db (Just x) = Just <$> resolve db x
resolveMaybe db Nothing = pure Nothing
instance Resolvable e => Resolvable (Maybe e) where
type ResFun (Maybe e) a = Maybe (ResFun e a)
type ResArg (Maybe e) a = ResArg e a
resolve db x = resolveMaybe db x
-- Pairs
resolvePair :: (Resolvable e0, Resolvable e1, ResArg e0 'Unres, ResArg e1 'Unres)
=> DataBase -> (e0, e1) -> Either String (ResFun e0 'Res, ResFun e1 'Res)
resolvePair db (x,y) = (,) <$> resolve db x <*> resolve db y
instance (Resolvable e0, Resolvable e1) => Resolvable (e0, e1) where
type ResFun (e0, e1) a = (ResFun e0 a, ResFun e1 a)
type ResArg (e0, e1) a = (ResArg e0 a, ResArg e1 a)
resolve db x = resolvePair db x
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