[Haskell-cafe] Return of the revenge of the revisit of the extensible records, reiterated

[Carter Schonwald]

very cool work! Thanks for sharing! The ordering on symbols to get rid of
the permutation issue is slick.

you should perhaps also share on the ghc-devs list, or maybe throw your
design ideas on a page on the ghc wiki, to document them?

-Carter


On Wed, Nov 27, 2013 at 3:08 AM, Atze van der Ploeg <atzeus at gmail.com>wrote:

> Actually, after your email I played around with variants and it turns out
> they don't work at all. I'll rework the variants. The records work
> beatifully though.
>
> > First, decomp is like lookup to me, so I expected the Right
> > constructor for when that lookup succeeds.
>
> Agreed!
>
> > Second, (Label :: Label "x") is a pretty unacceptable way to write
> > what is just x in other languages. One idea would be to
> > follow <
> http://hackage.haskell.org/package/HList-0.3.0.1/docs/Data-HList-Labelable.html
> >
> > and create values/labels which will do something like:
> >
> >  x .=. 1 -- call to inj perhaps?
> >
> >  v ^? x  -- call to decomp
> >
> > Another idea is to make `x stand for (Label :: Label "x"),
> > much like 'x and ''x in template haskell. Trying out
> > a good syntax by using a quasiquoter or preprocessor
> > before getting something into ghc is probably worth doing.
> > One example that has not been as useful as originally
> > thought it would be is:
> > <
> http://hackage.haskell.org/package/HList-0.3.0.1/docs/Data-HList-RecordPuns.html
> >
>
> Yes, that is currently the most painful bit of the syntax. It should be
> possible to adopt HList labelable. I would like a small syntactic extension
> that allows 'x for (Label :: Label "x") indeed. I'll probably hack this up
> later.
>
> > On a somewhat related note, would your strategy of
> > having sorted labels give better compile times for
> > for code which uses records that are a bit larger
> > than a toy example:
> > <http://code.haskell.org/~aavogt/xmonad-hlist/>
>
> Depends, as far as I understand HList record sometimes require searching
> for a permutation of l such that l~l' which seems expensive to me. This is
> not necessary if we keep the row sorted. For projections and decompositions
> the performance is (theoretically) the same: linear searching in a list
> (sorted or unsorted list)  is O(n).
>
> The real benefit of keeping the row sorted is that { x = 0 , y = 0 } and {
> y = 0, x = 0 } have the same type. When a row is not sorted, as in HList,
> then if we for example have an instance Eq for a row (because all elements
> support Eq) then for using (==) both arguments would have to the same order
> in the row or we need a manual call to a permutation function. When keeping
> the row ordered, this is not necessary. The same kind of problem occurs
> when we fix the type of a function to a specific row:
> ( using whishful syntax )
> f :: Rec [ x = Int , y = Int ] -> Int
>
> If the row is not ordered, then f { y = 0 , x = 0 } will not typecheck and
> will require a manual call to permute the row.
>
> Cheers!
>
> Atze
>
> >
> >
> >
> > Regards,
> > Adam Vogt
> >
> > On Tue, Nov 26, 2013 at 12:09 PM, Atze van der Ploeg <atzeus at gmail.com>
> wrote:
> > > Dear all,
> > >
> > > Extensible records have been a long outstanding feature request for
> GHC.
> > > Using the new closed type families and type literals, it is actually
> > > possible to implement Daan Leijen's "`Extensible Records with Scoped
> Labels"
> > > system as a library.
> > >
> > > I have implemented this library at https://github.com/atzeus/openrec(the
> > > documentation is at http://homepages.cwi.nl/~ploeg/openrecvardocs/).
> The
> > > only thing it requires is the availability of a closed type family that
> > > compares two type level symbols (i.e. the ordering on strings):
> > > type family (m :: Symbol) <=.? (n :: Symbol) :: Bool
> > > patches to GHC that implement this built-in closed type family are
> also at
> > > the github site.
> > >
> > > I would like to generate some discussion about:
> > > * Is this the interface we would like for open records and variants?
> > > * Would it be worthwhile to invest in syntactic sugar for open record
> > > operations?
> > > * Any comments on the interface and or its implementation?
> > >
> > > Cheers!
> > >
> > > Atze van der Ploeg
> > >
> > > _______________________________________________
> > > Haskell-Cafe mailing list
> > > Haskell-Cafe at haskell.org
> > > http://www.haskell.org/mailman/listinfo/haskell-cafe
> > >
>
>
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