magicDict

Mon Apr 26 20:33:53 UTC 2021

I like withDict a lot. It is direct, easy to chain/use, avoids fighting
about direction completely, and even matches the argument order used by
reify in the reflection library.

+1 from me.

On Mon, Apr 26, 2021 at 7:49 AM Simon Peyton Jones <simonpj at microsoft.com>
wrote:

> |  I would like to propose one more option:
> |
> |  withDict :: dt -> (ct => a) -> a
>
> Ah, you mean simply: swap the argument order.   I can see your logic about
> chaining etc.  I'd be fine with this.
>
> Simon
>
> |  -----Original Message-----
> |  From: Krzysztof Gogolewski <krz.gogolewski at gmail.com>
> |  Sent: 26 April 2021 15:35
> |  To: Simon Peyton Jones <simonpj at microsoft.com>
> |  Cc: Spiwack, Arnaud <arnaud.spiwack at tweag.io>; Edward Kmett
> |  <ekmett at gmail.com>; GHC developers <ghc-devs at haskell.org>
> |  Subject: Re: magicDict
> |
> |  I would like to propose one more option:
> |
> |  withDict :: dt -> (ct => a) -> a
> |
> |  1. This is less symmetric than '(ct => a) -> dt -> a'
> |     but in existing applications magicDict gets the arguments
> |     in the reverse order.
> |  2. Easier to chain 'withDict d1 (withDict d2 ...)'.
> |  3. The name is similar to 'withTypeable' or 'withFile',
> |     and avoids arguing which is reify or reflect.
> |
> |  On Mon, Apr 26, 2021 at 9:41 AM Simon Peyton Jones via ghc-devs <ghc-
> |  devs at haskell.org> wrote:
> |  >
> |  > Can we just agree a name, then?   Please correct me if I'm wrong,
> |  but
> |  >
> |  > I think Ed prefers 'reifyDict',
> |  > That is compatible with the existing reflection library Arnaud
> |  > disagrees but isn't going to die in the trenches for this one
> |  > Virtually anything is better than 'magicDict'.
> |  >
> |  >
> |  >
> |  >
> |  >
> |  > So: reifyDict it is?
> |  >
> |  >
> |  >
> |  > Simon
> |  >
> |  >
> |  >
> |  > From: Spiwack, Arnaud <arnaud.spiwack at tweag.io>
> |  > Sent: 26 April 2021 08:10
> |  > To: Edward Kmett <ekmett at gmail.com>
> |  > Cc: Simon Peyton Jones <simonpj at microsoft.com>; GHC developers
> |  > <ghc-devs at haskell.org>
> |  > Subject: Re: magicDict
> |  >
> |  >
> |  >
> |  >
> |  >
> |  >
> |  >
> |  > On Sun, Apr 25, 2021 at 2:20 AM Edward Kmett <ekmett at gmail.com>
> |  wrote:
> |  >
> |  > I speak to much this same point in this old stack overflow response,
> |  though to exactly the opposite conclusion, and to exactly the opposite
> |  pet peeve.
> |  >
> |  >
> |  >
> |  >
> |  https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fstac
> |  >
> |  koverflow.com%2Fa%2F5316014%2F34707&data=04%7C01%7Csimonpj%40micro
> |  >
> |  soft.com%7C87da21fdcc8e4ed6bef508d908c071fb%7C72f988bf86f141af91ab2d7c
> |  >
> |  d011db47%7C1%7C0%7C637550444930791696%7CUnknown%7CTWFpbGZsb3d8eyJWIjoi
> |  >
> |  MC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&
> |  >
> |  sdata=VlRrIEROGj%2BE6%2FuLXBEdfa%2BPWVlHh50dahgjIrw4tQU%3D&reserve
> |  > d=0
> |  >
> |  >
> |  >
> |  > :-)
> |  >
> |  >
> |  >
> |  > I do not feel that I chose the vocabulary without due consideration
> |  of the precise meaning of the words used.
> |  >
> |  >
> |  >
> |  > I didn't mean to imply that you did. Sorry if I did so: written
> |  communication is hard. For what it's worth, I didn't really think that
> |  I would change your mind, either.
> |  >
> |  >
> |  >
> |  > Though it still seems to me that the name `ReifiedMonoid` uses the
> |  word for a different thing than the `reifyMonoid` function does.
> |  >
> |  >
> |  >
> |  > To be explicit:
> |  >
> |  >
> |  >
> |  > Viewing a type as a space, 'reify' in the reflection library takes
> |  some space 'a' and splits it into individual fibers for each term in
> |  'a', finding the appropriate one and handing it back to you as a fresh
> |  type 's' that captures just that singular value. The result is
> |  significantly less abstract, as we gained detail on the type, now
> |  every point in the original space 'a' is a new space. At the type
> |  level the fresh 's' in s `Reifies` a now concretely names exactly one
> |  inhabitant of 'a'.
> |  >
> |  >
> |  >
> |  > On the flip side, 'reflect' in the reflection library forgets this
> |  finer fibration / structure on space, losing the information about
> |  which fiber the answer came from, being forgetful is precisely the
> |  justification of it being the 'reflect' half of the reify -| reflect
> |  pairing.
> |  >
> |  >
> |  >
> |  > I confess I don't necessarily anticipate this changing your mind but
> |  it was not chosen blindly, reflect is the forgetful mapping here,
> |  reification is free and left adjoint to it, at least in the context of
> |  reflection-the-library, where a quantifier is being injected to track
> |  the particular member.
> |  >
> |  >
> |  >
> |  > I've got to admit that I have the hardest time seeing the `s` as
> |  representing an inhabitant of `a`. I'm probably missing something
> |  here.
> |  >
> |  >
> |  >
> |  > I also don't think that a free object construction embodies a
> |  reify/reflect pair completely. It's probably fair to see `reify` as
> |  being the natural mapping from the free object of X to X (the counit
> |  of the adjunction). But reification will not be the unit of the
> |  adjunction, because it's trivial. So there is still a piece missing in
> |  this story.
> |  >
> |  >
> |  >
> |  > Anyway... I've made my point, and I am not too willing to spend too
> |  much time proving Wadler's law correct. So I think I'll stop here,
> |  fascinating a conversation though it is.
> |  >
> |  >
> |  >
> |  > Best,
> |  >
> |  > Arnaud
> |  >
> |  > _______________________________________________
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> |  > ghc-devs at haskell.org
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>
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