GHC support for the new "record" package

[Simon Peyton Jones]

|  "wired" into record selectors, which can't be undone later. I think we
|  can fix some of that by desugaring record definitions to:
|  
|  data T = MkT {x :: Int}
|  
|  instance FieldSelector "T" T Int where
|       fieldSelector (MkT x) = x
|  
|  Then someone can, in a library, define:
|  
|  instance FieldSelector x r a => IV x (r -> a) where
|       iv = fieldSelector
|  
|  Now that records don't mention IV, we are free to provide lots of
|  different instances, each capturing some properties of each field,
|  without committing to any one style of lens at this point. Therefore,
|  we could have record desugaring also produce:
|  
|  instance FieldSetter "T" T Int where
|      fieldSet v (T _) = T v
|  
|  And also:
|  
|  instance FieldSTAB "T" T Int where
|      fieldSTAB = ... the stab lens ...

OK, I buy this.  

We generate FieldSelector instances where possible, and FieldSetter instances where possible (fewer cases).

Fine.



Cutting to the chase, if we are beginning to converge, could someone (Adam, Neil?) modify the Redesign page https://ghc.haskell.org/trac/ghc/wiki/Records/OverloadedRecordFields/Redesign to focus on plan B only; and add this FieldGetter/Setter stuff?

It's confusing when we have too many things in play.  I'm sick at the moment, so I'm going home to bed -- hence handing off in a hopeful way to you two.

I have added Edwards "import Field(x)" suggestion under syntax, although I don't really like it.

One last thing: Edward, could you live with lenses coming from #x being of a newtype (Lens a b), or stab variant, rather than actually being a higher rank function etc?  Of course lens composition would no longer be function composition, but that might not be so terrible; ".." perhaps.  It would make error messages vastly more perspicuous. And, much as I love lenses, I think it's a mistake not to abstraction; it dramatically limits your future wiggle room.



I really think we are finally converging.

Simon