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