DeriveFoldable treatment of tuples is surprising
anthony_clayden at clear.net.nz
Sat Mar 25 03:08:36 UTC 2017
> On Wed Mar 22 13:54:05 UTC 2017, Twan van Laarhoven wrote:
>> On 2017-03-21 21:34, David Feuer wrote:
>> This seems much too weird:
>> *> :set -XDeriveFoldable
>> *> data Foo a = Foo ((a,a),a) deriving Foldable
>> *> length ((1,1),1)
>> *> length $ Foo ((1,1),1)
*> length $ Just ((1, 1), 1)
*> length $ Just (1, 1)
*> length (1, 1)
> This is not unique to tuples, consider:
> > :set -XDeriveFoldable
> > data Foo a = Foo [[a]] deriving Foldable
> > length [[1,2]]
> > length $ Foo [[1,2]]
> length $ Just [[1, 2]]
Does the behaviour of other methods within Foldable
seem surprising for DeriveFoldable Foo a = Foo ((a, a), a)?
Did the FTP change touch DeriveFoldable?
(Silly question, yes it must have: `length` didn't used to
be in Foldable.)
> On Tue Mar 21 21:29:20 UTC 2017, Edward Kmett wrote:
> In general I think the current behavior is the least
surprising as it
> "walks all the a's it can" and is the only definition
> further extension with Traversable. Right now there are no
> provided by base that violate the "walk all the a's"
?? Are there instances in base
where the `a` appears more than once on RHS?
I can see only List and Array.
How would I have formed that "walk all the a's" intuition?
(For most of the Prelude instances,
`length`s result is hard-coded as 0 or 1.)
*> length (((1, 2), 3) :: Num a => ((a, a), a))
Doesn't seem to "walk all the a's" there.
I find pretty much all of these results surprising.
(Admittedly for different reasons in different cases.)
I think this is a good reason `length` should not be in
"lengthiness" just doesn't fit the abstraction.
More information about the Glasgow-haskell-users