Records in Haskell

[Simon Peyton-Jones]

Frege has a detailed explanation of the semantics of its record implementation, and the language is *very* similar to Haskell. Lets just start by using Frege's document as the proposal. We can start a new wiki page as discussions are needed.

If it's a serious proposal, it needs a page to specify the design.  Currently all we have is a paragraph on http://hackage.haskell.org/trac/ghc/wiki/Records, under "Better name spacing".

As previously stated on this thread, the Frege user manual is available here:
http://code.google.com/p/frege/downloads/detail?name=Language-202.pdf
see Sections 3.2 (primary expressions) and 4.2.1 (Algebraic Data type Declaration - Constructors with labeled fields)

To all those concerned about Records: look at the Frege implementation and poke holes in it.

Well the most obvious issue is this.  3.2 says
e.m = (T.m e) if the expression e has type t and the type constructor
of t is T and there exists a function T.m
But that innocent-looking statement begs the *entire* question!  How do we know if "e has type t?   This is the route ML takes for arithmetic operators: + means integer plus if the argument is of type Int, float plus if the argument is of type Float, and so on.

Haskell type classes were specifically designed to address this situation. And if you apply type classes to the record situation, I think you end up with
http://hackage.haskell.org/trac/ghc/wiki/Records/OverloadedRecordFields

Well, so maybe we can give up on that.  Imagine Frege without the above abbreviation.  The basic idea is that field names are rendered unique by pre-pending the module name.  As I understand it, to record selection one would then be forced to write (T.m e), to select the 'm' field.  That is the, qualification with T is compulsory.   The trouble with this is that it's *already* possible; simply define suitably named fields
  data T = MkE { t_m :: Int, t_n :: Bool }
Here I have prefixed with a (lower case version of) the type name.  So we don't seem to be much further ahead.

Maybe one could make it optional if there is no ambiguity, much like Haskell's existing qualified names.  But there is considerable ambiguity about whether T.m means
  m imported from module T
or
  the m record selector of data type T

Perhaps one could make it work out.  But before we can talk about it we need to see a design. Which takes us back to the question of leadership.

Simon


We only want critiques about
* achieving name-spacing right now
* implementing it in such a way that extensible records could be implemented in its place in the future, although we will not allow that discussion to hold up a records implementation now, just possibly modify things slightly.

Greg Weber

On Thu, Dec 29, 2011 at 2:00 PM, Simon Peyton-Jones <simonpj at microsoft.com<mailto:simonpj at microsoft.com>> wrote:
| The lack of response, I believe, is just a lack of anyone who
| can cut through all the noise and come up with some
| practical way to move forward in one of the many possible
| directions.
You're right.  But it is very telling that the vast majority of responses on
       http://www.reddit.com/r/haskell/comments/nph9l/records_stalled_again_leadership_needed/
were not about the subject (leadership) but rather on suggesting yet more, incompletely-specified solutions to the original problem.  My modest attempt to build a consensus by articulating the simplest solution I could think of, manifestly failed.

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


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