[Haskell-cafe] Overlapping/Incoherent instances
wren ng thornton
wren at freegeek.org
Sun Oct 12 17:58:35 EDT 2008
Don Stewart wrote:
> A quick, ad hoc search reveals, for the "OverlappingInstances" flag,
> (some of these may only mention it in docs or in test suites),
Ah yes, logfloat is using them too, for some of the auxiliary stuff.
* PartialOrd was designed to fix certain brokennesses of the Prelude's
Ord. Naturally any totally ordered type is also partially ordered, so
we'd like to have |Ord a => PartialOrd a| with the obvious
implementation. However the Ord instances for Float and Double are
lies because of NaN values.
I derive the |Ord a => PartialOrd a| instance because I'm a bad little
Haskeller who wanted to make things easier for users of the library.
This usage of OverlappingInstances seems fairly common when people
introduce new classes to the hierarchy, particularly when they are
relaxing requirements of the official classes. That is, there are some
default implementations we'd like to give but cannot for some reason,
and OverlappingInstances allows us to give them.
* Transfinite uses it for similar reasons to fix brokenness of the
realToFrac function when converting transfinite values into the Rational
type. That is, |Transfinite a => RealToFrac a Rational| is not
inhabitable for any |a| since Rational cannot absorb transfinite values.
Both of these seem like good changes for haskell' in order to correct
the behavior of Float and Double (and because there are many more
partially ordered types than totally ordered ones). Alas I've not had a
chance to file the suggestions.
 Given that Ord is more popular. Conversely, if the Prelude is
changed, we may wish to require PartialOrd for all Ord instances, and
thing give Ord a default implementation that removes the Just from the
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