February 2004 - Glasgow-haskell-users

RE: Generics and type classes
by Simon Peyton-Jones 02 Feb '04

02 Feb '04

What you ask is not easy. When you ask for everywhere special you ask to apply special to each node of the tree -- and special requires the MyClass dictionary. But the library code for 'gmapT' and 'everywhere' don't know about MyClass. In particular, to do (gmapT special), poor old gmapT has to find a MyClass dictionary to pass to each call to special -- and it has no way to do that. I bet that you could do what you want by replacing the 'instance MyClass ExampleType1' by mkTs for special, so special uses run-time type dispatch to implement the operations in MyClass. I guess that's what Keeane is suggesting. Simon | -----Original Message----- | From: glasgow-haskell-users-bounces(a)haskell.org [mailto:glasgow-haskell-users- | bounces(a)haskell.org] On Behalf Of MR K P SCHUPKE | Sent: 02 February 2004 11:48 | To: doc(a)drjava.de; glasgow-haskell-users(a)haskell.org | Subject: Re: Generics and type classes | | Because the 'cast' operator used in generics, works on having a concrete | type to cast to. What you need to do is: | | module TypeTest where | | import Data.Generics | | class Data a => MyClass a | | instance MyClass ExampleType1 | instance MyClass ExampleType2 | | special :: ExampleType1 -> ExampleType1 | special = ... | | special2 :: ExampleType2 -> ExampleType2 | special2 = ... | | generic :: MyClass a => a -> a | generic = everywhere (mkT special `extT` special2 ...) | | Regards, | Keean. | _______________________________________________ | Glasgow-haskell-users mailing list | Glasgow-haskell-users(a)haskell.org | http://www.haskell.org/mailman/listinfo/glasgow-haskell-users

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Re: Generics and type classes
by MR K P SCHUPKE 02 Feb '04

02 Feb '04

Because the 'cast' operator used in generics, works on having a concrete type to cast to. What you need to do is: module TypeTest where import Data.Generics class Data a => MyClass a instance MyClass ExampleType1 instance MyClass ExampleType2 special :: ExampleType1 -> ExampleType1 special = ... special2 :: ExampleType2 -> ExampleType2 special2 = ... generic :: MyClass a => a -> a generic = everywhere (mkT special `extT` special2 ...) Regards, Keean.

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RE: linking problems after switching from GHC5 to GHC6
by Simon Marlow 02 Feb '04

02 Feb '04

Thanks; looks like I overlooked that patch. One workaround is to use '-lfoo' rather than 'foo.a' (not forgetting to add the lib directory with -L<dir> if necessary). Cheers, Simon > -----Original Message----- > From: glasgow-haskell-users-bounces(a)haskell.org > [mailto:glasgow-haskell-users-bounces@haskell.org] On Behalf > Of Sigbjorn Finne > Sent: 30 January 2004 16:39 > To: glasgow-haskell-users(a)haskell.org > Subject: Re: linking problems after switching from GHC5 to GHC6 > > This is a 6.x bug; I fixed it in HEAD a while back, > > http://haskell.org/pipermail/cvs-ghc/2003-October/018991.html > > but the change wasn't merged for some reason. > > --sigbjorn > > ----- Original Message ----- > From: "Sven Panne" <Sven.Panne(a)aedion.de> > To: "Volker Wysk" <post(a)volker-wysk.de> > Cc: <glasgow-haskell-users(a)haskell.org> > Sent: Friday, January 30, 2004 08:28 > Subject: Re: linking problems after switching from GHC5 to GHC6 > > > > Volker Wysk wrote: > > > I've put together a minimal program which reproduces the > problem. Save > the > > > three attachments in a new directroy and type "make". I'ts just as > > > described earlier. The trivial program compiles with > GHC5, but not GHC6. > [...] > > > > Hmmm, it looks like GHC is reversing the order of files in > the linking > step. > > Mentioning liba.a *before* Main.o in the linking step is a > workaround... > > > > Cheers, > > S. > > > > > > _______________________________________________ > > Glasgow-haskell-users mailing list > > Glasgow-haskell-users(a)haskell.org > > http://www.haskell.org/mailman/listinfo/glasgow-haskell-users > > _______________________________________________ > Glasgow-haskell-users mailing list > Glasgow-haskell-users(a)haskell.org > http://www.haskell.org/mailman/listinfo/glasgow-haskell-users >

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