Lenses and TypeFamilies

问题

I've encountered a problem of using Control.Lens together with
datatypes while using the -XTypeFamilies GHC pragma.

{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE TypeFamilies    #-}

import Control.Lens (makeLenses)

class SomeClass t where
  data SomeData t :: * -> *

data MyData = MyData Int

instance SomeClass MyData where
  data SomeData MyData a = SomeData {_a :: a, _b :: a}

makeLenses ''SomeData

The error message is: reifyDatatype: Use a value constructor to reify a data family instance.

Is there any way to overcome it, maybe using some functional from Control.Lens?

回答1:

The most sensible thing would be to just define those lenses yourself... it's not like it's very difficult:

a, b :: Lens' (SomeData MyData a) a
a = lens _a (\s a' -> s{_a=a'})
b = lens _b (\s b' -> s{_b=b'})

or even

a, b :: Functor f => (a -> f a) -> SomeData MyData a -> f (SomeData MyData a)
a f (SomeData a₀ b₀) = (`SomeData`b₀) <$> f a₀
b f (SomeData a₀ b₀) =   SomeData a₀  <$> f b₀

...which doesn't use anything from the lens library at all, but is fully compatible to all lens combinators.

回答2:

tfMakeLenses generates setters of type t a -> a -> t a for associated datatypes.
There are some places this function can be improved, but it works!

tfMakeLenses :: Name -> DecsQ
tfMakeLenses t = do
  fieldNames <- tfFieldNames t
  let associatedFunNames = associateFunNames fieldNames
  return (map createLens associatedFunNames)
  where createLens :: (Name, Name) -> Dec
        createLens (funName, fieldName) =
          let dtVar  = mkName "dt"
              valVar = mkName "newValue"
              body   = NormalB (LamE [VarP valVar] (RecUpdE (VarE dtVar) [(fieldName, VarE valVar)]))
          in FunD funName [(Clause [VarP dtVar] body [])]

        associateFunNames :: [Name] -> [(Name, Name)]
        associateFunNames [] = []
        associateFunNames (fieldName:xs) = ((mkName . tail . nameBase) fieldName, (mkName . nameBase) fieldName)
                                         : associateFunNames xs

        tfFieldNames t = do
          FamilyI _ ((DataInstD _ _ _ _ ((RecC _ fields):_) _):_) <- reify t
          let fieldNames = flip map fields $ \(name, _, _) -> name
          return fieldNames

来源：https://stackoverflow.com/questions/47946585/lenses-and-typefamilies