[Haskell-cafe] context deduction in GHC

[MigMit]

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

Отправлено с iPad

> 30 сент. 2014 г., в 21:52, Doug McIlroy <doug at cs.dartmouth.edu> 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
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