2017-04-09 2:01 GMT+02:00 Ben Franksen <ben.franksen@online.de>:

Am 08.04.2017 um 04:03 schrieb Anthony Clayden:
> [...] So to take your examples from an earlier thread,
> what do you expect Haskell to do here?:
>
> maximum (True,2) => ?
> minimum ((3, 4),5) => ? -- i.e. :: ((Int, Int), Int)
> sum (7,3.14) => ? -- i.e. :: (Int, Double)
> product (Left $ error "Errk") => ? -- i.e. ::
> Either e Int

I (and others) think these should be type errors.

Exactly.

> Do you expect Foldable (a, a) to behave differently
> vs Foldable (b, a) vs Foldable ((a, a), a) vs ...?
>
> Let's get everybody agreed on that.

It would be nice if we could all agree on removing these instances.

Yep. :-)

BTW, I find it remarkable that of those who defend these instances, few
seem to be especially interested in clarifying the OP's question: what
laws exactly do we expect for Foldable? [...]

And even if there are laws which hold for those instances, it doesn't mean that these instances should be defined. Take e.g. Bool: One can define a "Num Bool" instance which respects the usual laws (interpreting Bool basically as a "Word1", just like Word32 etc.), but we do *not* want to have this in the standard language/libraries, and for a good reason: It would make types less useful, removing a part of the usual "If it compiles, it works" safety net...