Using variant for Unions

[hendrikniemann]

I am currently struggling with union types. One idea was to return a tuple of output type and value from a resolve type function. The problem is, that this does not work out in the type signature:

resolveType :: forall t. OutputType m t => a -> forall b. Tupe (t b) b

Furthermore, it is not introspectable which output types are potentially returned from the function.

Another idea could be to require a record of possible output types in the union type definition:

aType :: GQL.ObjectType Context AValue
aType = -- ...

bType :: GQL.ObjectType Context BValue
bType = -- ...

myUnionType :: GQL.UnionType Context (Variant (a :: AValue, b :: BValue))
myUnionType = GQL.union "MyUnion" { a: aType, b: bType }

Now this type's parent value has type: (Variant (a :: AValue, b :: BValue)). We can now make usage of this type more convenient using cmap as described in #23:

myUnionType :: GQL.UnionType Context (Either AValue BValue)
myUnionType =
  cmap (either (inj (SProxy :: SProxy "a")) (inj (SProxy :: SProxy "b"))) $
    GQL.union "MyUnion" { a: aType, b: bType }

Instead of Either we could use any other sum type.