weird error message from type qualification mistake

[guest]

{-# LANGUAGE GADTs, RankNTypes  #-}

data Parity a where
  Odd :: Int -> forall a. Parity (Int,Int,a)

(the "forall a" is in the wrong place) gives the error message

    Malformed constructor result type:
      forall a :: k_a3Gz. Parity (Int, Int, a)

That k_a3Gz. is a little bit cryptic.

--solrize

[guest]

test case shown above, since wiki formatting messed it up

[simonpj]

Good point. The message from 6.12 is better

T3830.hs:4:3:
    Data constructor `Odd' returns type `forall a.
                                         Parity (Int, Int, a)'
      instead of an instance of its parent type `Parity a'
    In the definition of data constructor `Odd'
    In the data type declaration for `Parity'

Nevertheless, I also added a little more info to HsTyVarBndr to prevent this kind of thing happening again.

Simon