DeriveFoldable treatment of tuples is surprising

[Oleg Grenrus]

For `Traversable` one have to `traverse` over everything:

    traverse @Foo = forall f. Applicative f => (a -> f b) -> Foo a -> f
(Foo b)

    ~= .... -> ((a,a), a) -> f ((b,b), b)

And thus the same behavior for `Foldable`.

You should define:

    data Foo b a = Foo ((b,b), a) deriving Foldable

- Oleg

On 21.03.2017 23:11, David Feuer wrote:
> The point is that there are two reasonable ways to do it, and the
> deriving mechanism, as a rule, does not make choices between
> reasonable alternatives.
>
> On Tue, Mar 21, 2017 at 5:05 PM, Jake McArthur <jake.mcarthur at gmail.com> wrote:
>> I think it's a question of what one considers consistent. Is it more
>> consistent to treat tuples as transparent and consider every component with
>> type `a`, or is it more consistent to treat tuples as opaque and reuse the
>> existing Foldable instance for tuples even if it might cause a compile time
>> error?
>>
>>
>> On Tue, Mar 21, 2017, 4:34 PM David Feuer <david.feuer at gmail.com> wrote:
>>> This seems much too weird:
>>>
>>> *> :set -XDeriveFoldable
>>> *> data Foo a = Foo ((a,a),a) deriving Foldable
>>> *> length ((1,1),1)
>>> 1
>>> *> length $ Foo ((1,1),1)
>>> 3
>>>
>>> I've opened Trac #13465 [*] for this. As I write there, I think the
>>> right thing is to refuse to derive Foldable for a type whose Foldable
>>> instance would currently fold over components of a tuple other than
>>> the last one.
>>>
>>> I could go either way on Traversable instances. One could argue that
>>> since all relevant components *must* be traversed, we should just go
>>> ahead and do that. Or one could argue that we should be consistent
>>> with Foldable and refuse to derive it.
>>>
>>> What do you all think?
>>>
>>> [*] https://ghc.haskell.org/trac/ghc/ticket/13465
>>> _______________________________________________
>>> Glasgow-haskell-users mailing list
>>> Glasgow-haskell-users at haskell.org
>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/glasgow-haskell-users
> _______________________________________________
> ghc-devs mailing list
> ghc-devs at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs


-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 819 bytes
Desc: OpenPGP digital signature
URL: <http://mail.haskell.org/pipermail/ghc-devs/attachments/20170321/68237016/attachment.sig>