[Haskell-cafe] Principal type in Haskell

Bulat Ziganshin bulat.ziganshin at gmail.com
Thu Jun 22 06:28:14 EDT 2006

Hello william,

Thursday, June 22, 2006, 1:22:32 PM, you wrote:

> GHCi yields type f :: (Eq [a]) => a -> a -> Bool.

> But according to the paper "Type classes: an exploration of the design
> space", predicate Eq [a] should be reduced to Eq a. Is this reduction 
> performed here? What should be the principal type of f?

Ghc, unlike H98, supports instances like this:

instance Eq [MyType] where
  a==b = True

so this extension to type inference allows to use such instance even
if MyType is not in Eq class

Best regards,
 Bulat                            mailto:Bulat.Ziganshin at gmail.com

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