GADT Type Checking GHC 6.10 versus older GHC

Dominic Steinitz dominic.steinitz at
Sat Nov 22 03:58:02 EST 2008

Jason Dagit <dagit <at>> writes:

> On Fri, Nov 21, 2008 at 8:57 AM, Simon Peyton-Jones <simonpj <at>> wrote:
> You need a type signature for the case expression.  As Daniel says, this is
worth a
> Thanks Simon.  I had read that several times in the past and I've pointed it
out to others.  It's still relevant but, my question was about whether or not
examples like the one posted are really in error or if GHC is just being overly
strict now.
> In my case, we had rigid type signatures all over the place.  The wiki
document says that the type must be rigid at the point of the match.  I guess
that's what we were violating.  If the code I posted isn't supposed to type
check then I would like to report, as user feedback, that GADTs have become

This is really worrying. I have spent some considerable time using GADTs to
structure a library. I haven't tried 6.10 yet because it's not available in a
packaged form for my flavour of linux. I will put some work into doing this
today and report back.


More information about the Glasgow-haskell-users mailing list