Hi, Simon
Thanks a lot for your mail. It turns out I could have resolved this by
myself (with the help of this thread http://thread.gmane.org/gmane.comp.lang.haskell.glasgow.user/15153
, to be honest). What I was missing was this key part:
bind :: forall a b t. W t a -> (a -> W t b) -> W_ t b
------- the forall brings a,b,t into scope inside bind
So, while I had turned on the ScopedTypeVariables extension, none of
the type variables in question was actually in scope. How embarrassing!
I can't blame anyone but me for this but, anyway, I feel that it may
have helped me if the introduction of Section 8.7.6 of the user manual
were a little more explicit about this. Although the example reads
"f :: forall a. [a] -> [a]", and the text below says "The type
signature for f brings the type variable into scope", the role of the
"forall" is not mentioned until Section 8.7.6.2 (and since I already
knew what the extension was about, and was only looking for the proper
extension name, I didn't make it that far :))
Also, since you are always willing to get examples of confusing error
messages, I wanted to bring this one into attention:
In your case the error message was:
GADT.hs:26:56:
GADT pattern match with non-rigid result type `Maybe a'
Solution: add a type signature
In a case alternative: I1 m' -> m'
In the expression: case w' S of { I1 m' -> m' }
In a case alternative: Wrap w' -> case w' S of { I1 m' -> m' }
This is when ScopedTypeVariables is off. Now, what I found very
confusing at first is that I thought the "a" in 'Maybe a' was
referring to the "a" in 'W t a -> (a -> W t b) -> W_ t b', and I
couldn't see how that could be happening. Once ScopedTypeVariables is
on, one gets 'GADT pattern match with non-rigid result type `Maybe
a1'" and everything makes more sense :)
And maybe the "add a type signature" can be more explicit? Like "add a
type signature that makes the type of the result known at the matching
point". Just a suggestion...
I hope this helps. I'm still trying to find a really good way to
explain the reasoning here. Do pls augment the wiki page with what
you have learned!
I've put some of this in the "Upgrading packages" wiki, and added a
link to the previous thread which I found to be very clear.
Thanks again!
Daniel
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