Overloaded record fields

[Edward Kmett]

Note: the lens solution already gives you 'reverse function application'
with the existing (.) due to CPS in the lens type.

-Edward

On Wed, Jun 26, 2013 at 4:39 PM, Simon Peyton-Jones
<simonpj at microsoft.com>wrote:

> |  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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