How to make lenses for records with type-families [duplicate] - haskell

Question

This question already has answers here:
How to derive instances for records with type-families
(2 answers)
Closed 6 years ago.
Here's what I've got, which is not compiling:
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE StandaloneDeriving #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE FunctionalDependencies #-}
import Data.Text as T
import Data.Int (Int64)
import Control.Lens
type family Incoming validationResult baseType
type instance Incoming Validated baseType = baseType
type instance Incoming ValidationErrors baseType = Either [T.Text] baseType
data Validated
data ValidationErrors
data Tag = Tag {unTag :: T.Text} deriving (Eq, Show)
data NewTag f = NewTag
{
ntClientId :: Incoming f Int64
, ntTag :: Incoming f Tag
}
$(makeLensesWith abbreviatedFields ''NewTag)
Compilation error:
27 3 error error:
• Illegal type synonym family application in instance:
Incoming f_a1Kvx Int64
• In the instance declaration for
‘HasClientId (NewTag f_a1Kvx) (Incoming f_a1Kvx Int64)’ (intero)
27 3 error error:
• Illegal type synonym family application in instance:
Incoming f_a1Kvx Tag
• In the instance declaration for
‘HasTag (NewTag f_a1Kvx) (Incoming f_a1Kvx Tag)’ (intero)

Answer 1

The problem here is that makeLensesFor will try to generate an instance as follows:
instance HasClientId (NewTag f) (Incoming f Int64) where
....
This, however, is an error because you cannot create an instance for the result of a type family application. To avoid this, we can write the instance manually for each of the two possible choices for f:
-- generate lenses _foo for each record selector foo
-- (in this case, generates _ntClientId and _ntTag lenses)
makeLensesWith (lensRules & lensField .~ mappingNamer (\x -> ['_' : x])) ''NewTag
class HasClientId s a | s -> a where
clientId :: Lens' s a
instance HasClientId (NewTag Validated) Int64 where
clientId = _ntClientId
instance HasClientId (NewTag ValidationErrors) (Either [T.Text] Int64) where
clientId f a = f (ntClientId a) <&> \ntClientId' -> a { ntClientId = ntClientId' }
class HasTag s a | s -> a where
tag :: Lens' s a
instance HasTag (NewTag Validated) Tag where
tag = _ntTag
instance HasTag (NewTag ValidationErrors) (Either [T.Text] Tag) where
tag = _ntTag