err

foo.hs:30:8: error:
    • Couldn't match type ‘Inner mode f’ with ‘Inner mode f0’
      Expected type: ArgF mode -> Inner mode f0
        Actual type: ArgF mode -> Inner mode f
      NB: ‘Inner’ is a non-injective type family
      The type variable ‘f0’ is ambiguous
    • In the ambiguity check for ‘doc’
      To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
      In the type signature: doc :: Sel f -> String
   |
30 | doc :: Sel f -> String

foo.hs

{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE RankNTypes #-}

data Mode = Field | Descr
type family Inner mode field where
  Inner Field f = f
  Inner Descr f = String


data ArgF (f :: Mode) = ArgF
  { field1 :: Inner f Char
  , field2 :: Inner f Int
  }

someArg :: ArgF Field
someArg = ArgF
  { field1 = 'c'
  , field2 = 42 }

argDescr :: ArgF Descr
argDescr = ArgF
  { field1 = "Some field 1"
  , field2 = "Another fancy field"
  }

type Sel f = forall mode. (ArgF mode -> Inner mode f)

doc :: Sel f -> String
doc sel = sel argDescr