On Oct 14, 2008, at 10:19 PM, Jason Dagit wrote:
On Tue, Oct 14, 2008 at 7:27 AM, Daniel Gorín <[EMAIL PROTECTED]>
wrote:
Hi
After installing ghc 6.10-rc, I have a program that no longer
compiles. I get the dreaded "GADT pattern match...." error, instead :)
Here is a boiled-down example:
[...]
I don't have 6.10 handy to try out your program, but in 6.8 and
older the type error message you're getting means that the compiler
needs more "outside in" help with type checking this.
Usually this means adding type more type signatures on the outside.
For example, maybe you need to give the type signatures inside the
case to make the types inside the pattern matches of the case more
rigid. That probably didn't make a lot of sense :( So here is an
example,
case wit :: {- Try adding a signature here -} of ...
Given that your code has such deep pattern nesting I would argue
that it is in your best interest to add local functions (in a where
clause) along with their explicit type signatures. Start with the
inner most case expressions and convert those to local functions and
work your way out.
I've tried adding some signatures (together with -
XScopedTypeVariables), but with no luck. Why is it that this no
longer compiles? More importantly, how can I make it compile again? :)
I think adding local functions is easier than randomly sprinkling in
the type signatures. It has a nice side-effect that your new code
is often easier to read as well.
Good luck!
Jason
Thanks for the advice!
By using some auxiliary functions I can now compile an alternative
version of the program. And although the resulting program is more
clear, I'd still like to know if this can be achieved be adding only
annotations to the original program. The reason for this is that, for
performance reasons, I depend on the case-of-case transformation
removing every possible case construct. I already verified this is
happening for the original program and I rather keep the code as is
than browse through the generated core again :)
I must say that I also found this thread to be very helpful:
http://thread.gmane.org/gmane.comp.lang.haskell.glasgow.user/15153
I'll make sure the wiki points to it.
For the record the resulting code is this:
{-# LANGUAGE GADTs, EmptyDataDecls #-}
module T where
data S
data M
data Wit t where
S :: Wit S
M :: Wit M
data Impl t a where
I1 :: Maybe a -> Impl S a
I2 :: [a] -> Impl M a
type W_ t a = Wit t -> Impl t a
newtype W t a = Wrap (W_ t a)
unWrap1 :: Impl S a -> Maybe a
unWrap1 (I1 m) = m
unWrap2 :: Impl M a -> [a]
unWrap2 (I2 m) = m
bind :: W t a -> (a -> W t b) -> W_ t b
bind (Wrap w) f = \wit ->
case wit of
S -> I1 $ do a <- unWrap1 (w S)
case (f a) of
Wrap w' -> unWrap1 (w' S)
M -> I2 $ do a <- unWrap2 (w M)
case (f a) of
Wrap w' -> unWrap2 (w' M)
Bye
Daniel
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