Records in Haskell

[AntC]

Ian Lynagh <igloo <at> earth.li> writes:

> 
> On Thu, Mar 01, 2012 at 08:52:29PM +0000, AntC wrote:
> > 
> > And you get "In my opinion, this is ugly, ...
> 
> That comment was from strake888, not SPJ?
> 

Thanks Ian, you're right. Specifically, it's 'igloo's tweak to the proposal 
and 'strake888's comment. (I had erroneously thought the whole of that page 
was SPJ's, and I hadn't much re-read it since SPJ posted it.)


> Personally, in the context of Haskell (where the case of the first
> character often determines the behaviour, e.g. a pattern of Foo vs foo),
> I don't think it's too bad.
> 

Hmm. Upper case in an expression always means data constructor (or qualified 
name). (You've possibly not been watching the outrage at changing the meaning 
of `.` ;-)

Also this would be ambiguous:

    object.SubObject.Field.subField

    -- are the `SubObject` and `Field` (private) field selectors,
    -- or a qualified name for subField?
    -- or perhaps SubObject.Field is a qualified private field selector?

Putting parentheses would cut out some of those interpretations, but not all 
of them??


In terms of scope control, I think (I'm guessing rather) you do get similar 
behaviour to DORF, with the added inconvenience of:
* an extra arg to Has (how does the constraint sugar cope?)
    r{ field :: Int } => ...
    r{ Field :: Int } => ...               -- ? does that look odd
                  -- suppose I have two private namespaces
    r{ Field :: Int ::: Field1 } => ...     -- ??
    r{ (Field ::: Field2) :: Int } => ...   -- ???
* something(?) extra in record decls:
    data PublicRecord = Pub { field :: Int }
    data PrivateRecord = Priv { Field :: Int }            -- ?
    data PrivateRecord = Priv { Field :: Int ::: Field2 } -- ??
* a need for equality constraints between Kind and Type
  (that's the ft ~ FieldT bit)
  The class decl and instances are 'punning' on tyvar `ft`
   being both a type and a Kind.
  Is that even contemplated with Kinds?
* a need for something(?) different on record update syntax:
    pubRec{ field = 27 }
    privRec{ Field = 92 }      -- does upper case there look odd to you?
    privRec{ Field = 87 ::: Field2 }

("ugly" is a mild reaction, the more I think about it.)

> 
> But I think you are agreeing that (leaving aside the issue of whether
> the design is reasonable) the above variant would indeed allow the user
> to choose the behaviour of either SORF or DORF.
> 

No, not the "user to choose", but the implementor. We can't possibly try to 
support both approaches.

AntC