Re: new-typeable, new cast?

Well, there was another surprise. Haddock can't deal with Proxy for some reason. (It dies with the error "synifyKind". I looked but couldn't figure out what was going on there.) I've posted a bug report on the haddock Trac (http://trac.haskell.org/haddock/ticket/242), but validation currently fails on the branch (data-proxy) with this work in it. So, I can't merge with master. At this point, I'm going to wait until the haddock folks fix that bug, then assuming all is well, I will merge. Richard On 2013-07-23 09:59, Richard Eisenberg wrote:

I think that we really only need (eqT :: (Typeable a, Typeable b) => Maybe (a :=: b)). All of the gcasts can be defined in terms of that function.

However, I concretely propose (and plan to do) this: Leave gcast1 and gcast2 in and undeprecated. Simon is right that there is an infinite family of gcasts, and there may be no great reason to have gcast1 and gcast2 and none of the others, but there also doesn't seem to be a compelling reason to remove them (or deprecate them) and break code. In any case, this debate doesn't seem to be a good reason to hold up wrapping the rest of these details up, because we can always revisit and add or remove functions in the coming weeks. I'm not trying to ramrod my ideas through here, just trying to favor action over inaction -- anyone can feel free to come back and make edits after I push.

Barring another surprise, I believe I'll push before lunch (in the UK).

Richard

On 2013-07-23 03:04, Edward Kmett wrote:

...

If we have gcast1 then can't the others just be applications of that under various wrappers?

In practice we should be able to implement gcast2 by working on a single argument with a product kind, and so on and so forth, but we need at least gcast1 as a base case.

In theory this is the more fundamental operation, and gcast is defineable in terms of gcast1 with a wrapper with a unit-kinded argument, but I wouldn't want to invert the relationship.

This is based on the same trick that Conor uses to show that a single (poly-kinded) type argument is enough for all of the indexed monad machinery, no matter how complicated it gets.

TL;DR We only need gcast1, the others are window-dressing.

-Edward

On Mon, Jul 22, 2013 at 9:31 PM, Simon Peyton-Jones wrote:

...

Harump.  There is *still* an infinite family going on here, isn't there?

gcast :: forall a b c. (Typeable a, Typeable b) => c a -> Maybe (c b)

but what about this

gcast_2 :: forall a b c d e. (Typeable a, Typeable b, Typeable d, Typeable e)              => c a b -> c d e

Then, as you point out, we have gcast1 and 2 (etc).  And there are more.  What about

gcast1' :: forall c t a a'. (Typeable a, Typeable a')        => c (t a) -> Maybe (c (t a'))

One could imagine all sorts of combinations of bits that stay the same (and hence do not need to be Typeable) and bits that change (and hence do need to be Typable).

As Edward says, all these functions can be polykinded, which makes them more useful, but there still seem too many of them.

I wonder if we could instead make a combinatory library that lets us build these functions easily.  It think we are going to offer a function that computes an equality witness:

mkEqWit :: (Typable a, Typeable b) => Maybe (EQ a b)

Now we need to be able to compose witnesses:

appEqWit :: Eq a b -> Eq c d -> Eq (a c) (b d) reflEqWit :: Eq a a

Now, I think you can make all those other casts.

Would that do the job better?

Simon

|  -----Original Message----- |  From: Richard Eisenberg [mailto:eir@cis.upenn.edu] |  Sent: 22 July 2013 10:20 |  To: José Pedro Magalhães |  Cc: Dimitrios Vytiniotis; Simon Peyton-Jones; Stephanie Weirich; |  josepedromagalhaes@gmail.com; libraries@haskell.org; Conor McBride |  Subject: Re: new-typeable, new cast? |

|  There seems to be a small tangle. The proposal includes deprecating |  gcast1 and gcast2 in favor of the poly-kinded gcast. But, there is a |  small discrepancy between these. Here are the type signatures: | |  > gcast :: forall a b c. (Typeable a, Typeable b) => c a -> Maybe (c b) |  > gcast1 :: forall c t t' a. (Typeable (t :: * -> *), Typeable t') |  >        => c (t a) -> Maybe (c (t' a)) | |  The difference is that gcast1 does *not* require the variable `a` to be |  Typeable, whereas defining gcast1 = gcast does require this. Not |  requiring `a` to be Typeable seems correct to me, as the type signature |  of gcast1 requires both uses of `a` to be the same. But, gcast isn't |  smart enough to know that. Here are some ideas of how to proceed: | |  - Keep gcast1 and gcast2 undeprecated. |  - Require clients to add more Typeable constraints (for example, in |  Data.Data) to get their code to compile with gcast. |  - Come up with some other workaround, but none is striking me at the |  moment. | |  Thoughts? | |  Richard | |  On 2013-07-22 09:44, Richard Eisenberg wrote: |  > I was waiting to respond to Shachaf's email saying "pushed", but |  > instead, I have to say "currently validating". |  > |  > Expect this by the end of the day. Sorry it's taken so long! |  > |  > Richard |  > |  > On 2013-07-22 09:23, José Pedro Magalhães wrote: |  >> Thanks for bringing this up again. This was started in my data-proxy |  >> branch of base [1], |  >> but never really finished. We definitely want to have this in 7.8, and |  >> I think there's |  >>  only some minor finishing work to do (check if we have all the |  >> instances we want, |  >> document, etc.). Perhaps you can look through what's there already, |  >> and what you |  >> think is missing? I'm more than happy to accept contributing patches |  >> too :-) |  >> |  >> Thanks, |  >> Pedro |  >> |  >> On Sun, Jul 21, 2013 at 3:09 AM, Shachaf Ben-Kiki |  >> wrote: |  >> |  >>> On Mon, Mar 4, 2013 at 11:12 AM, José Pedro Magalhães |  >>> wrote: |  >>>> Hi, |  >>>> |  >>>> On Mon, Mar 4, 2013 at 4:51 PM, Richard Eisenberg |  >>> wrote: |  >>>>> |  >>>>> Unless I'm missing something, this all looks very |  >>> straightforward. The |  >>>>> implementation is there, already, in the guise of (gcast Refl). |  >>> We would |  >>>>> just shuffle the definitions around a bit. Am I indeed missing |  >>> something? |  >>>> |  >>>> |  >>>> I think that is the case indeed. Though I agree that it would be |  >>> a nice |  >>>> addition to Data.Typeable. |  >>>> |  >>>> |  >>> |  >>> This thread is a few months old now, but it looks like people were |  >>> generally in favor of adding (gcast Refl) to Data.Typeable. I've |  >>> used |  >>> it in real code in at least one place since then (where I just |  >>> defined |  >>> it locally). It doesn't look like it's actually been added, though |  >>> -- |  >>> is it planned to go into HEAD eventually? |  >>> |  >>>     Shachaf |  >> |  >> |  >> |  >> Links: |  >> ------ |  >> [1] https://github.com/ghc/packages-base/tree/data-proxy [1] |  > |  > _______________________________________________ |  > Libraries mailing list |  > Libraries@haskell.org |  > http://www.haskell.org/mailman/listinfo/libraries [2]

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