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)) > > > 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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