[Proposal] Renaming (:=:) to (==)

Thu Oct 3 08:16:26 UTC 2013

PS: on the specifics of (:=:) vs (==), I am alive to the fact that all these different sorts of equality are quite confusing to our poor programmers.  We might have

                f :: (a~b) => (c == d) -> blah

where the (a~b) is an implicitly-passed witness and the (c == d) is an explicitly-passed one.  But both witness the same sort of equality.  There seems to be a reasonable case for making them look related.  Currently I lean towards (:~:) thus

                f :: (a~b) => (c :~: d) -> blah

Simon

From: Libraries [mailto:libraries-bounces at haskell.org] On Behalf Of Simon Peyton-Jones
Sent: 03 October 2013 09:10
To: Richard Eisenberg; Edward Kmett
Cc: Haskell Libraries
Subject: RE: [Proposal] Renaming (:=:) to (==)

I'm not aware of a wide poll on the names in TypeLits, so we shouldn't necessarily just follow that lead. That said, the above proposal about `?` seems sensible to me. If we decide to do this, we should find somewhere (where??) to articulate this.

Negotiating names is not much fun, but they stay with us for a long time.  And Richard, might you find 20 mins to throw up a wiki page (on the GHC Trac or Haskell wiki, doesn't matter too much) giving the exported signature of TypeLits and related modules (singletons?), together with a summary of the main open naming issues?  Should be mainly cut-and-paste.  That would be really helpful.

Simon

From: Libraries [mailto:libraries-bounces at haskell.org] On Behalf Of Richard Eisenberg
Sent: 03 October 2013 04:07
To: Edward Kmett
Cc: Haskell Libraries
Subject: Re: [Proposal] Renaming (:=:) to (==)

Thanks for pointing this out, Edward. I think consistency within the type level is more important than consistency between the type level and the term level. So, if we settle on a convention that a symbol ending in `?` means Boolean-valued and other symbols mean constraints, I'm all for making the change to (==).

I'm not aware of a wide poll on the names in TypeLits, so we shouldn't necessarily just follow that lead. That said, the above proposal about `?` seems sensible to me. If we decide to do this, we should find somewhere (where??) to articulate this.

Richard

On Oct 2, 2013, at 9:28 PM, Edward Kmett <ekmett at gmail.com<mailto:ekmett at gmail.com>> wrote:

GHC.TypeLits code looks to be using (<=?) as the boolean valued version of the predicate and (<=) for the assertion.

This points to a coming disagreement over style across the different parts of GHC itself, if we're saying that the principle reason for not using (==) is that we want it to be the boolean valued version.

-Edward

On Mon, Sep 30, 2013 at 4:03 PM, Carter Schonwald <carter.schonwald at gmail.com<mailto:carter.schonwald at gmail.com>> wrote:
Agreed.

On Monday, September 30, 2013, Edward A Kmett wrote:
I think if someone went through the effort of writing a patch so you could at least introduce local operator names with an explicit forall, like with ScopedTypeVariables and the proposed explicit type applications then it'd probably be accepted.

Sent from my iPhone

On Sep 30, 2013, at 2:25 PM, Conal Elliott <conal at conal.net<mailto:conal at conal.net>> wrote:
-1.
I'm hoping we don't get more deeply invested in the syntactic change in GHC 7.6 that removed the possibility of symbolic type variables ("~>", "*", "+", etc). I had a new job and wasn't paying attention when SPJ polled the community. From my perspective, the loss has much greater scope than the gain for type level naturals. I'd like to keep the door open to the possibility of bringing back the old notation with the help of a language pragma. It would take a few of us to draft a proposal addressing details.
Not at all meaning to start a syntax debate on this thread. Just an explanation of my -1 for the topic at hand.
- Conal

-- Conal

On Sat, Sep 28, 2013 at 9:57 PM, Edward Kmett <ekmett at gmail.com<mailto:ekmett at gmail.com>> wrote:
As part of the discussion about Typeable, GHC 7.8 is going to include a Data.Type.Equality module that provides a polykinded type equality data type.

I'd like to propose that we rename this type to (==) rather than the (:=:) it was developed under.

We are already using (+), (-), (*), etc. at the type level in type-nats, so it would seem to fit the surrounding convention.

I've done the work of preparing a patch, visible here:

https://github.com/ekmett/packages-base/commit/fb47f8368ad3d40fdd79bdeec334c0554fb17110

Thoughts?

Normally, I'd let this run the usual 2 week course, but we're getting down to the wire for 7.8's release. Once 7.8 ships, we'd basically be stuck with the current name forever.

Discussion Period: 1 week

-Edward Kmett

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