Re: GHC support for the new "record" package

There is a problem with the old TRex syntax. In a world with kind signatures and rank-2 types, it would appear that type Point2D = Rec ( x :: Coord, y :: Coord) is ambiguous. Is Coord a kind signature being applied to x and y which are type variables brought into scope implicitly as type Point2D = forall (x :: Coord, y :: Coord) => Rec (x, y) would make more explicit? e.g. type Lens s t a b = Functor f => (a -> f b) -> s -> f t works today in ghc, even though f isn't explicitly scoped and elaborates to: type Lens s t a b = forall f. Functor f => (a -> f b) -> s -> f t -Edward On Wed, Jan 28, 2015 at 4:48 PM, Nikita Volkov wrote:

Chris, this is great! Looks like we can even get rid of the Rec prefix!

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A phrase in round braces and with :: is itself unambiguous in the type context. -

A phrase in round braces with = symbols is unambiguous in the expression context.

Concerning the pattern context a solution needs to be found though. But the two points above are enough for me to fall in love with this direction! The {| braces had a too icky of a touch to them and the plain { required the user to choose whether to use the standard record syntax or anonymous one on the module scale, but not both. ​

2015-01-29 0:26 GMT+03:00 Christopher Done :

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There’s too much to absorb in this discussion at the moment and I’m late to the party anyway, but I would like to make a small note on syntax. Given that this is very similar to TRex both in behaviour and syntactic means of construction, why not just take TRex’s actual syntax? http://en.wikipedia.org/wiki/Hugs#Extensible_records

type Point2D = Rec (x::Coord, y::Coord) point2D = (x=1, y=1) :: Point2D (#x point)

It seems like it wouldn’t create any syntactical ambiguities (which is probably why the Hugs developers chose it).

Ciao

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For those who haven't seen this, Nikita Volkov proposed a new approach to anonymous records, which can be found in the "record" package on Hackage: http://hackage.haskell.org/package/record

It had a *lot* of attention on Reddit: http://nikita-volkov.github.io/record/

Now, the solution is very nice and lightweight, but because it is implemented outside GHC it relies on quasi-quotation (amazing that it can be done at all!). It has some limitations because it needs to parse Haskell syntax, and Haskell is big. So we could make this a lot smoother, both for the implementation and the user, by directly supporting anonymous record syntax in GHC. Obviously we'd have to move the library code into base too.

This message is by way of kicking off the discussion, since nobody else seems to have done so yet. Can we agree that this is the right thing and should be directly supported by GHC? At this point we'd be aiming for 7.12.

Who is interested in working on this? Nikita?

There are various design decisions to think about. For example, when

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quasi-quote brackets are removed, the syntax will conflict with the existing record syntax. The syntax ends up being similar to Simon's 2003

On 20 January 2015 at 22:44, Simon Marlow wrote: the proposal

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http://research.microsoft.com/en-us/um/people/simonpj/Haskell/records.html

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(there are major differences though, notably the use of lenses for selection and update).

I created a template wiki page: https://ghc.haskell.org/trac/ghc/wiki/Records/Volkov

Cheers, Simon _______________________________________________ ghc-devs mailing list ghc-devs@haskell.org http://www.haskell.org/mailman/listinfo/ghc-devs

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