Overloaded record fields

[Simon Peyton-Jones]

|  record projections.  I would prefer to have dot notation for a
|  general, very tightly-binding reverse application, and the type of the record
|  selector for a field f changed to "forall r t. r { f :: t } => r -> t"
|  instead of "SomeRecordType -> t".  Such a general reverse application dot would
|  allow things like "string.toUpper" and for me personally, it would
|  make a Haskell OO library that I'm working on more elegant...

Actually I *hadn't* considered that.   I'm sure it's been suggested before (there has been so much discussion), but I had not really thought about it in the context of our very modest proposal.

We're proposing, in effect, that ".f" is a postfix function with type "forall r t. r { f :: t } => r -> t".   You propose to decompose that idea further, into (a) reverse function application and (b) a first class function f.

It is kind of weird that
	f . g  means    \x. f (g x)
but     f.g    means    g f

but perhaps it is not *more* weird than our proposal.

Your proposal also allows things like

	data T = MkT { f :: Int }

	foo :: [T] -> [Int]
	foo = map f xs

because the field selector 'f' has the very general type you give, but the type signature would be enough to fix it.  Or, if foo lacks a type signature, I suppose we'd infer

	foo :: (r { f::a }) => [r] -> [a]

which is also fine.

It also allows you to use record field names in prefix position, just as now, which is a good thing.  

In fact, your observation allows us to regard our proposal as consisting of two entirely orthogonal parts
  * Generalise the type of record field selectors
  * Introduce period as reverse function application

Both have merit.

Simon

|  -----Original Message-----
|  From: glasgow-haskell-users-bounces at haskell.org [mailto:glasgow-haskell-users-
|  bounces at haskell.org] On Behalf Of Dominique Devriese
|  Sent: 26 June 2013 13:16
|  To: Adam Gundry
|  Cc: glasgow-haskell-users at haskell.org
|  Subject: Re: Overloaded record fields
|  
|  I think it's a good idea to push forward on the records design because
|  it seems futile to hope for an ideal consensus proposal.
|  
|  The only thing I dislike though is that dot notation is special-cased to
|  record projections.  I would prefer to have dot notation for a
|  general, very tightly-binding reverse application, and the type of the record
|  selector for a field f changed to "forall r t. r { f :: t } => r -> t"
|  instead of	
|  "SomeRecordType -> t".  Such a general reverse application dot would
|  allow things like "string.toUpper" and for me personally, it would
|  make a Haskell OO library that I'm working on more elegant...
|  
|  But I guess you've considered such a design and decided against it,
|  perhaps because of the stronger backward compatibility implications of
|  changing the selectors' types?
|  
|  Dominique
|  
|  2013/6/24 Adam Gundry <adam.gundry at strath.ac.uk>:
|  > Hi everyone,
|  >
|  > I am implementing an overloaded record fields extension for GHC as a
|  > GSoC project. Thanks to all those who gave their feedback on the
|  > original proposal! I've started to document the plan on the GHC wiki:
|  >
|  > http://hackage.haskell.org/trac/ghc/wiki/Records/OverloadedRecordFields/Plan
|  >
|  > If you have any comments on the proposed changes, or anything is unclear
|  > about the design, I'd like to hear from you.
|  >
|  > Thanks,
|  >
|  > Adam Gundry
|  >
|  > _______________________________________________
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|  > Glasgow-haskell-users at haskell.org
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