Re: [Haskell-beginners] Confused by type constraints

On 07/06/2011 12:03, Daniel Fischer wrote:

On Tuesday 07 June 2011, 10:51:00, Guy wrote:

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On 06/06/2011 17:31, Daniel Schoepe wrote:

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On Sun, 05 Jun 2011 19:53:50 +0300, Guy wrote:

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So in my original example, why isn't "instance Foo (Bar f)" destructing Bar - and making the constraint available?

The "Bar" in "instance Foo (Bar f)" is a type constructor, not a data constructor like the X in "s (X x)". Hence "instance Foo (Bar f)" isn't really deconstructing anything, but applying the type constructor Bar to f, whereas "s (X x)" matches a _value_ of type X.

This might be a bit clearer if you name type and data constructors differently, e.g.:

data Bar f a = Foo f => MkBar { bar :: f a }

Cheers, Daniel

Thanks, I (think I) understand your explanation, but ... why can't GHC infer that Bar must always be a Show, seeing as this is the only constructor?

undefined :: Bar f a

You get the Foo constraint from the constructor MkBar, but there is a value which doesn't use the constructor, so you can't get it from the type alone.

So if GHC were to infer the type constraint, it could be violated by undefined. Would this prevent any real code from working?