Re: [Haskell-beginners] Confused by type constraints

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

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

...

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

...

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.