Template Haskell doesn't like scoped type variables

[rl@…]

GHC panics is scoped type variables are used in [d| ... |]:

[rl@rl-lap stuff]$ cat R.hs
$(id [d| f (x :: a) = x |])

[rl@rl-lap stuff]$ ghc -fglasgow-exts -c R.hs
ghc-6.4.2: panic! (the `impossible' happened, GHC version 6.4.2):
        Failed binder lookup: a{tv a18c}

Please report this as a compiler bug.  See:
  http://www.haskell.org/ghc/reportabug

[simonpj]

Template Haskell originally only supported Haskell 98, but various non-H98 extensions have since been added. Apparently, TH patterns can contain type signatures (I didn't know that), but there does not seem to be any support for lexical scoping of type variables.

Since GHC itself is in a state of flux wrt lexically scoped type variables, I'm not going to fix this now. I would love some TH enthusiast to take it on.

Meanwhile, just avoid type signatures in patterns.

Simon

[rl@…]

Just for the record, it isn't just type signatures in patterns:

[rl@rl-lap stuff]$ cat R.hs
$(id [d| f :: forall a . a -> a
         f x = (x :: a) |])

[rl@rl-lap stuff]$ ghc -fglasgow-exts -c R.hs
ghc-6.4.2: panic! (the `impossible' happened, GHC version 6.4.2):
        Failed binder lookup: a{tv a189}

[simonmar]

We should fail more gracefully in this case (and perhaps when TH is used with other extensions that aren't supported with TH, too).

[simonpj]

OK, at last GHC fails more gracefully when there are TH errors.

(It still does not accept scoped tyvars for TH, though.)

Simon