[Errk, somehow posted this with no Subject. Trying again.]

On Sun Apr 9 19:24:12 UTC 2017, Sven Panne wrote:

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2017-04-09 2:01 GMT+02:00 Ben Franksen :

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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.

OK. Do y'all think there should be a `toList` method for them? Indeed, should there be a `toList` method available for any data type that doesn't implement `fromList`? Perhaps `toList` should not be a method of Foldable?

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Do you expect Foldable (a, a) to behave differently vs Foldable (b, a) vs Foldable ((a, a), a) vs ...?

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

Yep. :-)

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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? [...]

One of the Laws appears to be that `length` etc behave as `length . toList`. AntC