On Wed, Jul 13, 2011 at 7:56 AM, ARJANEN Loïc Jean David
If I compile without any option, I get as expected an error because of the type synonym instance. If I remove the instance for String, I also get as expected an error because Char is not an instance of Dummy. But if I add (as suggested in the first error message) the GHC's option -XTypeSynonymInstances, I get an error because of overlapping instances, even if Char is still not an instance of Dummy and I don't see how String could be eligible for the (Dummy t) => Dummy [t] instance...
This is one of the trickier bits about Haskell typeclasses. The instance:
instance (Dummy t) => Dummy [t]
can be read in English as: "The type [t] is an instance of Dummy. Also, using this instance is a compilation failure if t is not an instance of Dummy." More succinctly, the information to the left of the (=>) is not used at all in instance resolution. It does, however, bring the constraint into scope in the body of the instance declaration, so it isn't useless. Antoine
Thanks in advance for your help, ARJANEN Loic.
_______________________________________________ Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners