Re: [Haskell-cafe] GADT and instance deriving

On 25/05/13 06:06, Alexander Solla wrote:

On Fri, May 24, 2013 at 10:41 AM, Niklas Hambüchen mailto:mail@nh2.me> wrote:

On Sat 25 May 2013 00:37:59 SGT, TP wrote: > Is this the right way to go? Is there any other solution?

I believe whether it's right or just depends on what you want to express.

> Do you confirm that tilde in s~s1 means "s has the same type as s1"?

It means: Both your s and s1 are "Eq"s but not necessarily the same one.

No, it doesn't. s1 ~ s2 means the types are the same. ~ is the "equality constraint".

http://www.haskell.org/ghc/docs/7.4.1/html/users_guide/equality-constraints....

To say that s1 and s2 are Eq's, but not necessarily the same one, we would write a constraint of the form:

Sorry, I didn't formulate that clearly: I meant to describe what the problem in the complaint about s1 ~ s2 is, not what s1 ~ s2 means.

Your first example allows that, so you could have one with an Int and one with a String inside (both are Eqs).

...

Nope. It would perform (Just 1) == (cast "hello"), which is completely possible, since (cast "hello") has the same type as (Just 1).

That's why I said "your first example"; there is no cast in it.