[Haskell-cafe] Re: Re: instance Eq (a -> b)

Edward Kmett ekmett at gmail.com
Wed Apr 21 11:02:58 EDT 2010

On Wed, Apr 21, 2010 at 5:25 AM, Max Rabkin <max.rabkin at gmail.com> wrote:

> On Wed, Apr 21, 2010 at 1:44 AM, Edward Kmett <ekmett at gmail.com> wrote:
> > Eq doesn't state anywhere that the instances should be structural, though
> in
> > general where possible it is a good idea, since you don't have to worry
> > about whether or not functions respect your choice of setoid.
> Wikipedia's definition of structural equality is an object-oriented
> one, but if by structural equality you mean the natural equality on
> algebraic datatypes (as derived automatically), I don't believe this
> is quite the case. If the type is abstract, surely the Eq instance
> need only be a quotient w.r.t. the operations defined on it. Thus, for
> example, two Sets can be considered equal if they contain the same
> elements, rather than having identical tree shapes (except that
> Data.Set exports unsafe functions, like mapMonotonic which has an
> unchecked precondition).

Yes. My point about why falling back on structural equality is a good idea
when possible, is that then you don't have to work so hard to make sure that
x == y =>  f x == f y holds. When your equality instance isn't structural
you need to effectively prove a theorem every time you work with the
structure to avoid violating preconceptions. My post was acknowledging the
expedience of such methods.

I think we are using a lot of words to agree with one another. ;)

-Edward Kmett

> --Max
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