Hi, Today after half a decade I have found `MonadZip` in `base`. The laws state: ``` liftM (f *** g) (mzip ma mb) = mzip (liftM f ma) (liftM g mb) liftM (const ()) ma = liftM (const ()) mb ==> munzip (mzip ma mb) = (ma, mb) ``` The question is why does ``` mzip /= liftM2 (,) ``` in plain words. Notably we can see lack of IO instance which I'm guessing has something to do asynchronous exceptions but I'm probably wrong…. Rather than investigating for next 30 minutes, I thought I'd just sample existing knowledge (that I could not find out from Google). In what circumstances can we not replace `liftM2 (,)` with `mzip`? -- Mateusz K.
The question is why does
``` mzip /= liftM2 (,)
```
I don't have any evidence, but my gut feeling is that indeed mzip == liftM2 (,) == liftA2 (,), but that MonadZip just predates many advances, and that it's not used often enough to warrant changing. Especially because changing stuff in base comes with huge costs and long debates. Today I would expect something similar to look somewhat more like this: ``` class Applicative f => Unzippative f where unzipF :: f (a,b) => (f a, f b) ``` I personally can't remember a single time that function would have come in handy, so I'm happy with Applicative. And I'm projecting that experience onto others and drawing the conclusion of "Meh.". But if you find out more after
investigating for next 30 minutes
I'd be interested to hear. ;) Cheers, MarLinn
What about:
bisequence :: (Functor f, Bifunctor g) => f (g a b) -> g (f a) (f b)
On Apr 20, 2017 12:21 PM, "MarLinn"
The question is why does
``` mzip /= liftM2 (,)
```
I don't have any evidence, but my gut feeling is that indeed mzip == liftM2 (,) == liftA2 (,), but that MonadZip just predates many advances, and that it's not used often enough to warrant changing. Especially because changing stuff in base comes with huge costs and long debates.
Today I would expect something similar to look somewhat more like this:
``` class Applicative f => Unzippative f where unzipF :: f (a,b) => (f a, f b) ```
I personally can't remember a single time that function would have come in handy, so I'm happy with Applicative. And I'm projecting that experience onto others and drawing the conclusion of "Meh.".
But if you find out more after
investigating for next 30 minutes
I'd be interested to hear. ;)
Cheers, MarLinn _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.
On 04/20/2017 08:19 PM, MarLinn wrote:
The question is why does
``` mzip /= liftM2 (,)
```
I don't have any evidence, but my gut feeling is that indeed mzip == liftM2 (,) == liftA2 (,), but that MonadZip just predates many advances, and that it's not used often enough to warrant changing. Especially because changing stuff in base comes with huge costs and long debates.
Normally I would agree but in case of adding instances, there are no real costs or long debates. Indeed to add a lawful instance you could just send a diff to GHC and see it in next version. In my experience there are only arguments where there are multiple possible "reasonable" instances &c. Hence my question in this case. Even if it was legacy, IO was around in 2011 and there seems to be no reason why it wouldn't have been one of the default instances.
Today I would expect something similar to look somewhat more like this:
``` class Applicative f => Unzippative f where unzipF :: f (a,b) => (f a, f b) ```
I personally can't remember a single time that function would have come in handy, so I'm happy with Applicative. And I'm projecting that experience onto others and drawing the conclusion of "Meh.".
I think we have a bunch across projects.
But if you find out more after
investigating for next 30 minutes
I'd be interested to hear. ;)
Me too! Maybe on weekend if there are no good replies.
Cheers, MarLinn _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.
-- Mateusz K.
Instances for MonadZip not always defined as "liftM2 (,)". Compare:
liftM2 (,) (Alt [1..2]) (Alt ("foo")) Alt {getAlt = [(1,'f'),(1,'o'),(1,'o'),(2,'f'),(2,'o'),(2,'o')]}
mzip (Alt [1..2]) (Alt ("foo")) Alt {getAlt = [(1,'f'),(2,'o')]}
2017-04-20 23:07 GMT+03:00 Mateusz Kowalczyk
On 04/20/2017 08:19 PM, MarLinn wrote:
The question is why does
``` mzip /= liftM2 (,)
```
I don't have any evidence, but my gut feeling is that indeed mzip == liftM2 (,) == liftA2 (,), but that MonadZip just predates many advances, and that it's not used often enough to warrant changing. Especially because changing stuff in base comes with huge costs and long debates.
Normally I would agree but in case of adding instances, there are no real costs or long debates. Indeed to add a lawful instance you could just send a diff to GHC and see it in next version. In my experience there are only arguments where there are multiple possible "reasonable" instances &c. Hence my question in this case.
Even if it was legacy, IO was around in 2011 and there seems to be no reason why it wouldn't have been one of the default instances.
Today I would expect something similar to look somewhat more like this:
``` class Applicative f => Unzippative f where unzipF :: f (a,b) => (f a, f b) ```
I personally can't remember a single time that function would have come in handy, so I'm happy with Applicative. And I'm projecting that experience onto others and drawing the conclusion of "Meh.".
I think we have a bunch across projects.
But if you find out more after
investigating for next 30 minutes
I'd be interested to hear. ;)
Me too! Maybe on weekend if there are no good replies.
Cheers, MarLinn _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.
-- Mateusz K. _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.
participants (4)
-
Dmitry Olshansky -
MarLinn -
Mateusz Kowalczyk -
Theodore Lief Gannon