On 2 mar 2008, at 01.21, Krzysztof Skrzętnicki wrote:

Well, it is simply

...

coerce :: a -> b coerce _ = undefined

so coerce is simply empty function. But still, it is possible to write a function of type (a->b). Well, possibly I didn't write anything particularly new, but please excuse me for I'm still in sort of a shock after I've discovered it.

Yet this function reminds me how little I know so I have a question for you. I didn't took any lectures in Category Theory (for I'm still just a student, and I won't for there are none in my institute), but are there any good (e)books you would recommend for a (future) computer scientist?

For your particular problem you might want to start with this thread (and the linked paper): http://lambda-the-ultimate.org/node/2003

"Krzysztof Skrzętnicki" writes:

Well, it is simply

...

coerce :: a -> b coerce _ = undefined

so coerce is simply empty function. But still, it is possible to write a function of type (a->b). Well, possibly I didn't write anything particularly new, but please excuse me for I'm still in sort of a shock after I've discovered it.

You have to remember that types in Haskell all have undefined as a member (this is an inevitable consequence of allowing arbitrary recursion). So your coerce is returning something that is in the type claimed by the result, so there's nothing shocking about it! -- Jón Fairbairn Jon.Fairbairn@cl.cam.ac.uk