[Template-haskell] Language.Haskell.THSyntax data types deriving Eq

[Ian Lynagh]

On Wed, Aug 13, 2003 at 10:37:05AM +1000, Sean Seefried wrote:
> On Wed, 13 Aug 2003, Ian Lynagh wrote:
> 
> > This opens up the issue of how much difference should be allowed in
> > things before they stop being equal, e.g. should "Let ds1 in e" and
> > "Let ds2 in e" be equal if they have the same statements in a different
> > order? What about alpha renaming of variables in expressions? It doesn't
> > take many acceptable differences before it starts to get rather
> > complicated, not to mention confusing to the user when some differences
> > are accepted but others aren't.
> >
> > I think the best choice is that two datastructures are equal (==) iff
> > they are identical.
> 
> That's certainly the simplest solution but I'm not convinced it's the
> best. Couldn't you simply write a "canonical-ise" function which takes an
> expression and puts it into some sort of canonical form and then tests for
> equality. However, there are many issues to consider like you said.
> Eta-reduction would another to think about.

Ultimately you hit the undecidability of the equivalence of Haskell
expressions. The principle of least surprise suggests to me going for
the other extreme rather than trying to find the best spot in the
middle.

This is useful for "Has this changed after I applied a function to it?"
but not "Are these definitely the same?" which I think are the two
generally useful functions. It's also the easier to implement and the
more efficient of the two.


Thanks
Ian