Re: [Haskell-cafe] Restricted type classes

On Tuesday 07 September 2010 05:22:55, David Menendez wrote:

On Mon, Sep 6, 2010 at 10:22 PM, wren ng thornton wrote:

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On 9/6/10 1:33 PM, David Menendez wrote:

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For that matter, can you even describe what pure is intended to do without reference to<*>  or join?

As already stated: fmap f . pure = pure . f

That's pretty general. For lists, the functions having that property include const [], \x -> [x,x], and repeat.

In fact, I think *every* appropriately-typed function satisfies that law. Does anyone know of a counter-example?

class Functor f => Pointed f where point :: a -> f a -- satisfying fmap f . point = point . f notQuitePure :: Pointed f => a -> f a notQuitePure _ = point undefined fmap (const True) . notQuitePure = point . const True But I don't see how to violate that law without introducing undefined on the RHS.