On Thu Mar 2 16:30:21 UTC 2017 Henning Thielemann wrote:
On Thu, 2 Mar 2017, Andreas Abel wrote:
Today a student came to me wondering why a certain function produced a regular result, where he had expected an error. Turned out he had used `concat`, but not on a lists of lists as he had thought, but on a lists of `Either a [b]`.
That is, he did (concat (xs :: [Either a [b]]))? Does GHC actually accept that? If at all, it would not have to do with Foldable.
Good questions Henning. They've got swamped. I found Andreas' description a bit unclear. We have: concat :: Foldable t => t [a] -> [a] So the `concat` you put above would give an error. To get a result, the student must have put (concat (xs :: Either a [b])) And got result either `[]` from (Left _) or a genuine List from (Right ys). Either way, I'd be grumpy: this hasn't and couldn't concatenate anything. Two thoughts: 1) The FTP hasn't done a very thorough job. Why is `[a]` hard-coded in the signature for concat? I would expect anything specific to Lists to be genericised: concat :: (Foldable t1, Foldable t2) => t1 (t2 a) -> t2 a 2) It's obvious why FTP didn't go that far: (concat (xs :: [Either a [b]])) -- as you speculated -- would be daft. concat only applies for structures that can be concatenated. But that surely includes more than Lists(?) So this is essentially the same complaint as for `length`: Why have an instance of Foldable which always returns length 1? (Or for some instances returns length 0 or 1.) `concat` and `length` should not be in Foldable. They should be in a separate class `Consable` (say), which would be a sub-class of Foldable. Note that `length` used not to be in Foldable. `concat` was, but restricted to Lists, as now. If there's a need for a method in Foldable to return 1 for a tuple, or 0..1 for Maybe/Either, don't call it `length`, call it `cardinality` (say). (This is the same approach as for separate `map`, `fmap`, `foldMap`.) That isn't breaking genericity/polymorphism/library rationalisation, `length` was never previously available for non-List Foldables. So no code could have been using it. `concat` was always in the wrong place, but Foldable (when it was a bit of a rag-bag) used to be good enough. AntC