A simpler ORF

[Adam Gundry]

Thanks Iavor,

It's been suggested before that the compiler should simply stop
generating record selectors (see Trac #5972). I'm unconvinced that the
benefit (being able to use the same name repeatedly) is worth the cost
(being unable to use selector functions at all, without writing them out
by hand or using TH).

In particular, as an application programmer I would be dependent on what
my libraries choose to do about records: they might not give me any
selectors at all, they might give me lenses with the same names, etc.
Given a piece of code that uses field names in expressions, I would no
longer know what it means without finding out how it chooses to define
fields.

Consider, however, the current version of the redesigned ORF proposal:

https://ghc.haskell.org/trac/ghc/wiki/Records/OverloadedRecordFields/Redesign

If you ignore all the stuff about the #x syntax (and hence are free to
ignore all the types stuff), you end up with something extremely simple:

 * record field names can be reused
 * selectors and updates can be used only if unambiguous
 * construction and pattern-matching can be used freely

An unambiguous update syntax could be useful, but that's mostly
orthogonal to the ORF proposal (and an easy extension of it). I think
this satisfies your goals 1 and 2.

With the proposed design, however, we also gain:

 * the ability to use overloaded field names, either as selectors or as
lenses, without using TH;
 * a syntax which generalises to other uses, completely separate from
records (e.g. we can turn #x into a Symbol singleton);
 * interoperability with anonymous records proposals.

Moreover, we have an implementation that shows the design works: the
implementation needs some revision to use the #x syntax rather than
renamer magic, and adapt to some finer points of the design, but the
core ideas are all there.

We've been putting this problem off for literally decades on the basis
that more experimentation is needed. Now that we have a
plausible-looking solution in the form of ORF, let's get it into GHC so
people can try it.

Adam


On 29/01/15 05:15, Iavor Diatchki wrote:
> Hello,
> 
> I've been following the various discussions about changes to Haskell's
> record system, and I find that most of the current proposals are fairly
> complex, especially for the benefit they provide.
> 
> I use Haskell records a lot, and I've been wondering if there might be a
> simpler alternative, one that:
>   1. will allow us to reuse field names across records
>   2. does not require any fancy types
>   3. it will not preclude continued research on "the right" way to get
> more type-based record resolution.
> 
> Based on designs I've seen in the past, my experience with Haskell
> records, and discussions with colleagues, I put together a document
> describing a potential design that, I think, satisfies goals 1 to 3:
> 
> https://ghc.haskell.org/trac/ghc/wiki/Records/OverloadedRecordFields/Simple
> 
> I think the proposal should be fairly simple to implement, and I'd be
> willing to do it, if there is enough support from the community.
> 
> Let me know what you think!
> 
> -Iavor


-- 
Adam Gundry, Haskell Consultant
Well-Typed LLP, http://www.well-typed.com/