[Haskell-cafe] Derivation of Eq given Ord

[Andreas Marth]

I have to admit that I didn't test it before my first response.
But now I did and can verify that it in deed "it does relay an error from
deriving module to using [module/ghci]".
So if I don't do any comparisons on Ego everything succeeds.
If there is any comparision then ghc (6.6 in my case) returns the
appropriate error.

Still a bit strange for me, but now atleast I know it. :-)

Andreas

----- Original Message -----
From: "Isaac Dupree" <isaacdupree at charter.net>
To: "Andreas Marth" <Andreas-Haskell at gmx.net>
Cc: "Dougal Stanton" <ithika at gmail.com>; "haskell-cafe"
<haskell-cafe at haskell.org>
Sent: Thursday, August 09, 2007 8:06 PM
Subject: Re: [Haskell-cafe] Derivation of Eq given Ord


> Andreas Marth wrote:
> > I would say that qualifies as a bug because it relays an error from
compile
> > time to run time.
>
> It doesn't relay anything to run time - ghci has to _compile_ the
> expressions you give it too. If you _compile something_ successfully,
> you will know that _it_ will not fail in the stated way (which is all
> you need to know for a normal program with 'main').
>
> However it does relay an error from deriving module to using
> [module/ghci], which may not be right.  HOWEVER, I can't reproduce it -
> ghc (6.6.1) always tells me
>      No instance for (Eq Foo)
>        arising from the superclasses of an instance declaration
> whether data or newtype, deriving Ord, no Eq instance, and no using
> operations at all, and Hugs does too (in different words).
>
> Isaac