Records in Haskell

[Simon Peyton-Jones]

Beyond that, it would really help namespacing in general to appropriately extend the module system to allow multiple modules to be declared within a single file -- or, better yet, "submodules". I know that this introduces a few corner cases that need to be thought through -- what happens with overlapping declarations, for example. But I tend to think the path here is relatively straightforward and obvious, and the added expressive power should make namespacing issues much more tractable. Like the type-level strings proposal, this isn't about implementing records as such -- rather, it's about generally extending the expressive power of the language so that record systems--among other things--are easier to write.

I'm agnostic about nested modules.  In principle they would be a good thing.  However, for the application to records in particular, I'm not sure people would relish saying this
                module M where
                               module T where
                                                data T = MkT { x,y :: Int }
                                module S where
                                                data S = MkS { x,y : Int }

The trouble is that, as ever, it is easier to say "add nested modules" than it is to say what that means.  For example, if I say
                import qualified M
(where M is defined as above), are the data types called M.T, M.S, or M.T.T and M.T.T?  What if I import unqualified?

Without a design it's hard to debate the pros and cons of different approaches to the record question.  If anyone is seriously advocating nested modules, the first step is to work out a concrete design, in detail.

Simon

From: Gershom Bazerman [mailto:gershomb at gmail.com]
Sent: 31 December 2011 19:12
To: Simon Peyton-Jones
Cc: Greg Weber; glasgow-haskell-users at haskell.org
Subject: Re: Records in Haskell

On Dec 31, 2011, at 1:28 PM, Simon Peyton-Jones wrote:
The trouble is that I just don't have the bandwidth (or, if I'm honest, the motivation) to drive this through to a conclusion. And if no one else does either, perhaps it isn't *that* important to anyone.  That said, it clearly is *somewhat* important to a lot of people, so doing nothing isn't very satisfactory either.

Usually I feel I know how to move forward, but here I don't.

Simon
It seems to me that there's only one essential missing language feature, which is appropriately-kinded type-level strings (and, ideally, the ability to reflect these strings back down to the value level). Given that, template haskell, and the HList bag of tricks, I'm confident that  a fair number of elegant records packages can be crafted. Based on that experience, we can then decide what syntactic sugar would be useful to elide the TH layer altogether.

Beyond that, it would really help namespacing in general to appropriately extend the module system to allow multiple modules to be declared within a single file -- or, better yet, "submodules". I know that this introduces a few corner cases that need to be thought through -- what happens with overlapping declarations, for example. But I tend to think the path here is relatively straightforward and obvious, and the added expressive power should make namespacing issues much more tractable. Like the type-level strings proposal, this isn't about implementing records as such -- rather, it's about generally extending the expressive power of the language so that record systems--among other things--are easier to write.

Cheers,
Gershom
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