Re: [Haskell-cafe] Principal type in Haskell

Thanks. Do I therefore able to conclude that none of the reductions using instance declarations are not performed because of potential overlapping instances? william

From: Bulat Ziganshin Reply-To: Bulat Ziganshin To: "william kim" CC: haskell-cafe@haskell.org Subject: Re: [Haskell-cafe] Principal type in Haskell Date: Thu, 22 Jun 2006 14:28:14 +0400

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@gmail.com

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