Hi Aleksandar, This library for extensible records does use -XDataKinds: http://hackage.haskell.org/package/vinyl. It doesn't have as many definitions as HList, but that might be because more recent extensions are more powerful. Many other libraries are listed http://www.haskell.org/haskellwiki/Extensible_record#Libraries_on_hackage. I don't know of a comparison between all of them. -- Adam

Hi AntC, Thanks for the feedback! On 26/04/13 09:55, AntC wrote:

...

Adam Gundry writes:

Hi,

I am hoping to do a GSoC project this year working on GHC, and have been pointed in the direction of the records issue (in particular, the desire to overload field names).

Heck you're brave!

Or possibly stupid. ;-) That's what I'm trying to figure out. There are certainly other, less controversial, things I might work on! Your questions are the kind of detailed issue that I'm tempted to postpone for the time being, although I guess I can state my prejudices:

Are you sure you want to step into the aggravated issue of changing the dot operator from being function composition?

I'd prefer to leave dot alone and add a new operator for record field projection, mostly because I don't relish trying to modify the parser, but I am open to other opinions.

Are you going to use explicit type application? ("The type of get is very odd.")

GHC desperately needs explicit type application, but that's another syntactic minefield; I don't think it matters much for this proposal, in that it's an implementation detail rather than something that should be exposed to the user.

Are you going to handle type-changing update?

I think we need to, though the right solution might well be to punt for now and implement non-overloaded update alone (i.e. require the user to explicitly specify the record type being updated when there is ambiguity). This also works for multiple update, which is an issue otherwise.

...

The plan would be to implement a solution to the "narrow issue" of overloaded field names, along the lines of Simon PJ's SORF proposal

So has <someone> decided that SORF is the best of those many proposals? I guess it's because it comes with the SPJ ring of confidence?

I should have phrased my message more carefully. By "along the lines of" SORF I didn't mean to exclude adapting it to take into account your work on DORF/TPDORF. As I understand it, they both set out to solve the same basic problem (that Johan outlines so clearly) but differ in the implementation details. I rather meant to contrast the SORF/DORF approach with an approach based solely on name-spacing. Perhaps I should have mentioned that SPJ suggested I look at this, although I don't think we want to be dictatorial about the final result.

Before jumping to that decision, I suggest you/your sponsors consider the implications of the "NewAxioms" stuff in GHC Head [2] to support 'controlled' overlap.

I think overlap is the only extra feature needed to support the DORF or TPDORF proposals. (Plus the syntactic sugar already outlined in that proposal.)

...

This would provide a basis for experimentation with first-class record types.

No: first-class record types needs much more than the SORF proposal. In particular it needs a way to extend an existing record to make a new one; project a subset of fields; and most important to merge two records with some fields in common avoiding doubling-up those fields (aka Relational Natural Join).

The DORF/TPDORF proposals are aimed much better as a step towards first- class record types. [IMO **]

Oleg/Ralf's HList paper covers all the ground for first-class records. It depends heavily on overlaps, which is why the NewAxioms stuff would work in really well.

While the NewAxioms stuff looks interesting, it's at an early stage. Moreover, I'm worried that a records implementation based on desugaring to an encoding (in the style of HList) will inevitably reveal details of that encoding to the user. Petr rightly points out the need for good error messages: these are much easier to generate if the constraints arising from records are solved in GHC, rather than a library like HList. There is certainly a long way from this proposal to full-blown extensible records, and I don't expect to get there in a summer.

AntC

[2] http://hackage.haskell.org/trac/ghc/wiki/NewAxioms

[**] Declaration of interest: I wrote the DORF and TPDORF proposals.

Thanks again for your efforts. From reading the list archives I can see that a lot of thought and work has gone in to the different proposals, which definitely will inform the final design. It would be good if we could finally make progress on the implementation! Best regards, Adam

Hi Petr, On 26/04/13 19:53, Petr Pudlák wrote:

Hi Adam,

very nice idea. As the others, I'm curious why you chose to implement SORF in favor of the other ideas?

As I've commented in a message just now [1], by mentioning SORF I didn't mean to exclude taking on board the other proposals (particularly DORF/TPDORF). I do think a type-based approach is the way forward, with a new sort of constraint that gives a polymorphic type to record projection. The plans based on changing the name-spacing rules either need type-based resolution as well, or require field names to be prefixed with the relevant data type (so they don't fully solve the problem). Also, I'm a type theorist, so obviously the type-based solution is the best one. ;-)

I just read the SORF proposal, and I'm a bit concerned about what error messages would GHC issue when someone would type incorrect code involving such records. Currently Haskell's error messages already pose a barrier for newcomers (like "No instance for (Num (a -> a))"), and if records are converted into those very complicated `Has` instances, type errors would be probably undecipherable even for moderate skilled Haskell users. Considering that records are a basic feature of Haskell and something that people with OOP background are familiar with, this could result in a feature that would without doubts deter many (if not most) newcomers. So do you think it would be possible to implement it in such a way that users get sensible type error messages?

You're right to raise this as an issue. I think it is essential to get good error messages, or at least no worse than we have already! Rather than "No instance for Has r l t" we want it to say something like "Type r has no field l". This justifies special treatment of Has constraints by the compiler, rather than simply desugaring to an encoding. Constraint-based type inference means this shouldn't be too difficult. The current description of SORF doesn't really distinguish between the user's perspective and the implementer's. This is something that we should change (so it's clear how much users will need to understand); but perhaps I'll wait and see if my GSoC proposal is accepted first!

Best regards, Petr

Thanks for your comments, Adam [1] http://www.haskell.org/pipermail/haskell-cafe/2013-April/107876.html

Hi Johan, On 26/04/13 20:46, Johan Tibell wrote:

Hi Adam,

Since we have already had *very* long discussions on this topic, I'm worried that I might open a can of worms be weighing in here, but the issue is important enough to me that I will do so regardless.

I'm the one busily opening this particular can. It's good to know it's an important one though! Thanks for characterising the problem so neatly:

Instead of endorsing one of the listed proposals directly, I will emphasize the problem, so we don't lose sight of it. The problem people run into *in practice* and complain about in blog posts, on Google+, or privately when we chat about Haskell over beer, is that they would like to write a record definition like this one:

data Employee = Employee { id :: Int, name :: String }

printId :: Employee -> IO () printId emp = print $ id emp

but since that doesn't work well in Haskell today due to name collisions, the best practice today is to instead write something like:

data Employee = Employee { employeeId :: Int, employeeName :: String }

printId :: Employee -> IO () printId emp = print $ employeeId emp

The downsides of the latter have been discussed elsewhere, but briefly they are:

* Overly verbose when there's no ambiguity. * Ad-hoc prefix is hard to predict (i.e. sometimes abbreviations of the data type name are used).

The important requirement, which might seem a bit obvious, is that any solution to this problem better not be *even more* verbose than the second code snippet above. If I understand the SORF proposal correctly, you would write:

data Employee = Employee { id :: Int, name :: String }

printId :: Employee -> IO () printId emp = print $ emp.id http://emp.id

Is that correct or do you have to replace 'Employee' with 'r { id :: Int }' in the type signature of 'printId'?

That's correct. The most general type (inferred if the annotation is omitted) will be something like printId :: r { id :: Int } => r -> IO () but you are free to declare a more specific type in the usual way, much as if the constraint was 'Show r', say.

The discussions about an overhauled record system also involve lots of talk about record sub-typing, extensible records, and other more advanced features. I'd like to point out that there doesn't seem to be a great demand for these features. They might be nice-to-haves or might fall out naturally from a solution to the namespacing problem above, but they are in fact not needed to solve the common problem people have with the Haskell record system.

Thanks, I take your point. My proposal is to implement a good solution to the problem you've outlined; I don't think we should go all the way to extensible records just yet, if at all.

Cheers, Johan

All the best, Adam

Johan Tibell writes:

The discussions about an overhauled record system also involve lots of talk about record sub-typing, extensible records, and other more advanced features. I'd like to point out that there doesn't seem to be a great demand for these features. ...

Sorry, Johan, I really have to disagree with that. There's lot's of Haskell to SQL interfaces that build on HList and its extensible record ideas (HDBC for example). But the usability is not good (as Petr points out, and as Oleg/Ralf admitted back in the paper). The type error messages are long and obscure.

... They might be nice-to-haves or might fall out naturally from a solution to the namespacing problem above, but they are in fact not needed to solve the common problem people have with the Haskell record system.

"the common problem people have" is that the record system is unusable [IMO] so doesn't get 'stretched' to see what other difficulties it has. There are all sorts of alternative systems (including Lenses) built with Template Haskell (and chewing gum and gaffer tape: that's how desperately bad is the current situation ;-). I'm saying that many people find the Haskell record system 'as is' so dysfunctional that they give up on it! I feel strongly that as soon as we get past the name collissions, there'll be other blockages to using it. I'd be interested to hear if there are any who can remember the Trex system, and how (un)usable it was? AntC