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

[Sebastiaan Joosten]

I'd like to go for ~, but unfortunately it is in use as a prefix operator (for Lazy pattern matching) and it would be a lot more work to implement than the current :=: / ==. Someone would have to use the same trick as was used for unary/binary minus. I've no idea on how to change the lexer for that, so I'll just go with +1 on Edward's suggestion: ==


On Sep 29, 2013, at 7:21 , Carter Schonwald <carter.schonwald at gmail.com> wrote:

> so a term of type a==b lets you locally introduce the hypothesis that a~b in the local types?
> (just making sure i understand this).
> 
> whats use case for the type level boolean equality? Naively, it seems like that could be derived from a typelevel " Maybe (a==b)'' plus a type level version of the "maybe" combinator
> 
> 
> On Sun, Sep 29, 2013 at 12:10 AM, Richard Eisenberg <eir at cis.upenn.edu> wrote:
> -1 from me.
> 
> Shachaf stated my argument correctly -- I think that the (:=:) operator means something quite different from the term-level (==) operator, and the name should reflect this. I do like thinking about a better name, though, and I'm happy enough if I'm outvoted here.
> 
> Richard
> 
> On Sep 28, 2013, at 10:08 PM, Shachaf Ben-Kiki wrote:
> 
> > On Sat, Sep 28, 2013 at 6:57 PM, Edward Kmett <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
> >>
> >
> > +1. For what it's worth, I suggested that name before, and Richard
> > Eisenberg suggested that == should be for type-level Boolean equality:
> > <http://markmail.org/message/3yifytgt2k3cfwws>. I'm not convinced,
> > though -- this seems fundamental enough to deserve the simplest name
> > possible.
> >
> > (I'm using that link because the haskell.org mailing list archive
> > seems to be gone... Hopefully that comes back, eventually.)
> >
> >    Shachaf
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