singletons.hs

{-
I have a type as the following:
```data Foo =
    Foo1 (Either String Int)
  | Foo2 (Either String Char)
  | Foo3 (Either String Bar)
...
```
There may end up being a lot of these. All will have `Either String a` inside. There are a bunch of functions taking a `Foo` argument. In most cases, the behaviour of how to treat a `Left` is the same, but for one, where it's important to know the constructor. I can't thus pull the `Either` out to the top level. Nor can I easily just turn the constructor into a tag, because it determines the type inside the `Right`

Am I missing a nice way to structure this?
-}

{-# LANGUAGE TypeFamilies, DataKinds, GADTs, TemplateHaskell #-}

import Data.Singletons
import Data.Singletons.TH

data Tag = Tag1 | Tag2 | Tag3
type family Foo (x :: Tag) :: * where 
  Foo 'Tag1 = Int
  Foo Tag2 = Char
  Foo Tag3 = Int

data Bar = forall (a :: Tag). Bar (Sing a) (Either String (Foo a))

$(genSingletons [''Tag])

bar = Bar (sing :: Sing Tag1) (Right 1)
bar2 = Bar (sing :: Sing Tag2) (Left "hsidoh")

consumeLefts :: Bar -> Maybe String
consumeLefts (Bar _ (Left a)) = Just a
consumeLefts _ = Nothing

consumeLeftsCare :: Bar -> String
consumeLeftsCare (Bar STag1 _) = "tag1"
consumeLeftsCare _ = "not tag1"