Hi, On Sun, Dec 21, 2008 at 11:45 AM, Luke Palmer <lrpal...@gmail.com> wrote: > 2008/12/21 Iavor Diatchki <iavor.diatc...@gmail.com> >> >> >> g :: TestClass a => a -> Integer >> g = fst (a :: (a -> Integer, a -> Integer)) > > Which I believe needs to be written: > > g :: forall a. TestClass a => a -> Integer > g = fst (a :: (a -> Integer, a -> Integer)) >
quite right! sorry for not testing my code. -iavor >> >> Here we are using another GHC extension called "scoped type variables" >> to associate the "a" in the type signature of "g" with the "a" in the >> type annotation for the value "a". >> >> Hope that this helps, >> Iavor >> >> >> >> >> On Sun, Dec 21, 2008 at 9:21 AM, Maurício <briqueabra...@yahoo.com> >> wrote: >> >>> Why isn't the last line of this code allowed? >> >>> f :: (TestClass a) => a -> Integer >> >>> f = const 1 >> >>> a = (f,f) >> >>> g = fst a >> >>> The only thing I can think about is monomorphism >> >>> restriction, but it's allowed (...) >> > >> >> (...) The reason is that a has type >> >> a :: (TestClass a, TestClass b) => (a,b) >> >> and then when we take 'fst' of this value (as in g) we get >> > >> >> g :: (TestClass a, TestClass b) => a >> >> which is an ambiguous type, (...) >> > >> > Is there some version (i.e., set of extensions) of >> > Haskell where this would be allowed? >> > >> > _______________________________________________ >> > Haskell-Cafe mailing list >> > Haskell-Cafe@haskell.org >> > http://www.haskell.org/mailman/listinfo/haskell-cafe >> > >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe@haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> > >
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