Fix TH bug when deriving ToJSON1/2 or FromJSON1/2 for types that don't mention the parameter

[RyanGlScott]

Before, this code:

newtype Foo a = Foo (Foo Int)

$(deriveToJSON  defaultOptions ''Foo)
$(deriveToJSON1 defaultOptions ''Foo)

Would generate the following ToJSON1 instance:

    deriveToJSON1 defaultOptions ''Foo
  ======>
    instance ToJSON1 Foo where
      liftToJSON
        = \ _tj1_aFns _tjl1_aFnt value_aFnr
            -> case value_aFnr of {
                 Foo arg1_aFnu -> liftToJSON toJSON toJSONList arg1_aFnu }
      liftToEncoding
        = \ _te1_aFnw _tel1_aFnx value_aFnv
            -> case value_aFnv of {
                 Foo arg1_aFny
                   -> liftToEncoding toEncoding toEncodingList arg1_aFny }

Although this typechecks, actually contradicts the strategy that GHC adapts when deriving * -> *-kinded classes (e.g., Functor). Namely, when an argument type doesn't mention the last type parameter at all, GHC doesn't attempt to "tunnel into" the type expression like the above code is doing with the expression liftToJSON toJSON toJSONList.

With the changes in this PR, the generated code would instead be:

    deriveToJSON1 defaultOptions ''Foo
  ======>
    instance ToJSON1 Foo where
      liftToJSON
        = \ _tj1_aFo0 _tjl1_aFo1 value_aFnZ
            -> case value_aFnZ of { Foo arg1_aFo2 -> toJSON arg1_aFo2 }
      liftToEncoding
        = \ _te1_aFo4 _tel1_aFo5 value_aFo3
            -> case value_aFo3 of { Foo arg1_aFo6 -> toEncoding arg1_aFo6 }

[phadej]

Could you add the test for this? If i understood correctly, liftToJSON undefined undefined (Foo 1) should work, but wasn't working previously?

[RyanGlScott]

Good point - I've added a regression test for this property.

[phadej]

it could be directly testCase "PR #445" pr445, no need to wrap in testGroup

[phadej]

Except from a minor style issue, LGTM.

[RyanGlScott]

Good point again, updated.

[bergmark]

Cool, thanks!