Hi, This is a change in behavior. Previously GHC was more liberal than Haskell 98 prescribed, and would not default the kind of otherwise unconstrained type variables to *. 7.4 does default to *, so you have to provide kind signatures when you want another kind (particularly in phantom type variables).
Cheers, Pedro On Tue, Dec 27, 2011 at 16:47, Bas van Dijk <v.dijk....@gmail.com> wrote: > On 27 December 2011 17:38, Michael Snoyman <mich...@snoyman.com> wrote: > > Thanks to Mark Wright for pointing this out[1]. > > > > We have the equivalent of the following code in persistent: > > > > {-# LANGUAGE MultiParamTypeClasses #-} > > data Key backend entity = Key > > > > class Monad (b m) => Foo b m where > > func :: b m (Key b m) > > > > This code works fine with GHC 7.0, but I get the following message from > GHC 7.4: > > > > Expecting two more arguments to `b' > > In the type `b m (Key b m)' > > In the class declaration for `Foo' > > > > Is this expected behavior, or a bug? If the former, what would be a > > possible workaround? > > > > Thanks, > > Michael > > > > [1] https://github.com/yesodweb/persistent/issues/31 > > > > _______________________________________________ > > Haskell-Cafe mailing list > > Haskell-Cafe@haskell.org > > http://www.haskell.org/mailman/listinfo/haskell-cafe > > I fixed a similar breakage in the hmatrix library: > > > https://github.com/AlbertoRuiz/hmatrix/commit/a4f38eb196209436f72b938f6355f6e28474bef3 > > I don't know if it's a bug in GHC, but the workaround is to add an > explicit kind signature: > > {-# LANGUAGE KindSignatures, MultiParamTypeClasses #-} > data Key (backend :: * -> * -> *) entity = Key > > class Monad (b m) => Foo b m where > func :: b m (Key b m) > > Cheers, > > Bas > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >
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