State of DData

[Wolfgang Jeltsch]

Am Freitag, 26. März 2004 16:37 schrieb Simon Marlow:
> [...]

> I'm afraid it isn't true that all instances of Monad respect the monad
> laws, due to our old friend seq.  See:
>
> http://www.haskell.org/pipermail/haskell/2002-May/009622.html

By the way, is there some effort in restricting seq in order to resolve this 
problem?

> I don't want this discussion to get out of hand.

Me, too. ;-)

> The only point I'm making here is that the Haskell definition doesn't
> *require* that every instance of (==) be equality, in the same way that it
> doesn't *require* that all Monad instances are monads, and all '+'s are
> additions etc.

Hmm, than we have a different interpretation of the standard as far as (==) is 
concerned.  Well, concerning Monad you're probably right because the Report 
only says "should" satisfy instead of "shall" satisfy or whatever.  As far as 
I could see the Report says nothing about the meaning of (+).

> Libraries might rely on properties of these methods, but as far as is
> practical those assumptions should be documented.

The problem is that probably a lot of libraries make assumptions like 
adherance to the Monad laws so that it's maybe difficult to document all 
these assumptions.

> Cheers,
> 	Simon

Sorry, if I was a bit dissentious.

Wolfgang