Re: Overlapping and incoherent instances

@Johan: imho, your question does make sense. Say I am issueing a library with instance Bla Char instance Bla a => Bla [a] instance Blubb Char instance Blubb String then, by the proposed distinction, the user of my library would have to write instance {-# OVERLAPPING #-} Bla String but instance {-# OVERLAPPABLE #-} Blubb a => Blubb [a] I would not know why the user has to be burdened with such quibbles. A simple instance {-# OVERLAP #-} Bla String instance {-# OVERLAP #-} Blubb a => Blubb [a] to shut off the compiler complaining about non-unique solutions involving these *additional* instance would do the job. The semantics would be: Whenever an instance adds *new* ambiguities, it has to be marked with OVERLAP. And, there should be monotonicity such that the user can mark any instance as OVERLAP even though there is no actual overlap (otherwise things become mad when switching import ordering etc.). On 30.07.2014 14:55, Johan Tibell wrote:

On Wed, Jul 30, 2014 at 2:50 PM, Ivan Lazar Miljenovic wrote:

...

On 30 July 2014 22:07, Andreas Abel wrote:

...

I am a bit surprised by the distinction you outline below. This is maybe because I am native German, not English. The German equivalent of "overlap", "überschneiden/überlappen", is used exclusively in a symmetrical fashion. It's like in English, if I say "our interests overlap", then it is pointless to ask whether my interest are overlapping yours or are overlapped by yours. I want to alert you to the fact that non-native English speaker might have little understanding for a distinction between "OVERLAPPING" and "OVERLAPPABLE".

Let's try to guess what it meant: Given

A) instance Bla Char B) instance Bla a => Bla [a] C) instance Bla String

you will in context A,B write C as OVERLAPPING, and in context A,C write B as OVERLAPPABLE?

IIUC, B will be OVERLAPPABLE ("you can overlap this") and C will be OVERLAPPING ("I'm overlapping an existing one") whereas C will be plain.

Apologies if this question doesn't make sense.

Can we really talk about overlapping, given that instances can be written in different modules, moved between modules, or removed?

-- Andreas Abel <>< Du bist der geliebte Mensch. Department of Computer Science and Engineering Chalmers and Gothenburg University, Sweden andreas.abel@gu.se http://www2.tcs.ifi.lmu.de/~abel/