Because believe it or not, Show is not a superclass of Num. Отправлено с iPad

30 сент. 2014 г., в 21:52, Doug McIlroy написал(а):

Why does ghci 7.6.3 not like this example, which section 4.3.2 of the language definition says is valid Haskell?

class Foo a class Foo a => Bar a instance (Eq a, Show a) => Foo [a] instance Num a => Bar [a]

Ghci complains, "Could not deduce (Show a) arising from the superclasses of an instance declaration from the context (Num a)". Yet Show is a superclass of Num. Hugs accepts the code.

Doug McIlroy _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe