[Haskell-cafe] Specification for Eq?
rl at cse.unsw.edu.au
Sun Mar 16 21:36:36 EDT 2008
> Roman Leshchinskiy wrote:
>> Should the report say something like "a valid Eq instance must ensure
>> that x == y implies f x == f y for all f"?
>> Probably not, since this requires structural equality which is not
>> what you want for ADTs. Should it be "for all f which are not part of
>> the implementation of the type"? That's a non-requirement if the
>> report doesn't specify what the "implementation" is. So what should it
> "for all exported f"
This forces me to confine the implementation of my ADT to a single
module instead of a package. Also (just to be nitpicky :-), it doesn't
deal with methods of classes of which my ADT is an instance since I
don't export those.
It's quite interesting that so far in this discussion, nobody seems to
have to come up with a clear and practically useful (in this context, of
course) definition of observation. I suspect that this is because in
practice, we can and, more importantly, want to observe a lot more than
in theory. For instance, something like serialisation usually wouldn't
even be mentioned in a theoretical paper about a data structure but is
absolutely necessary for writing actual programs.
>> If the representation is stored on the disk, for instance, then it
>> becomes observable, even if it's still hidden in the sense that you
>> can't do anything useful with it other than read it back.
> The trick here is to blame any observable differences on the
> nondeterminism of the IO monad
> serialize :: MyADT -> IO String
> It only guarantees to print out a "random" representation. Of course, in
> reality, serialize just prints the internal representation at hand, but
> we may not know that.
Hmm, I understand what you're saying but... So we go to all the trouble
of placing quite severe restrictions on (==) and now we can't even rely
on them when reasoning about effects?
Also, this requires that I artificially embed my perfectly pure
serialisation function in IO. This doesn't really make reasoning about
it easier but ultimately, isn't that what this is all about?
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