Adam Gundry
writes: On 04/07/13 12:27, AntC wrote: H-R fields are a limitation because we can't update them either. So I think it's a fair question whether supporting h-r polymorphism is worth the limitations?
Yes, higher rank polymorphism is bound to cause trouble with polymorphic projections, and perhaps it won't matter if we limit ourselves to one or the other.
So with h-r fields, let's stratify the requirements: * The current Plan tries to support holding h-r fields in a way backwards-compatible with H98 records. Why? We know that OverloadedFields are going to break some stuff. It's a question of which stuff is more important to not break. * I think the real requirement is to hold an h-r value in a record, accessible by field name. Consider TPDORF does this: (see example based on one in SPJ's SORF http://hackage.haskell.org/trac/ghc/wiki/Records/TypePunningDeclaredOverloa dedRecordFields#Implementation:theHasclasswithmethodsgetandsetandpunnedType s at 'Higher-Ranked polymorphic fields' ) -- must wrap h-r values in a newtype to put them in a record. newtype Rev = Rev (forall a. [a] -> [a]) data HR = HR { rev :: Rev } -- Has class takes 2 args, with type family for GetResult type instance GetResult r "rev" = Rev instance Has HR "rev" where getFld HR{ rev = (Rev x) } = Rev x -- can't unwrap here, 'cos can't spec Polymorphic Then user code must unwrap the newtype at point of applying. I think this approach also allows update for h-r values (providing they're wrapped) -- but I must admit I rather ran out of steam with the prototype. TPDORF also supported type-changing update for parametric polymorphic fields -- but with limitations. To get round those you would have to revert to H98 record update -- just as the current Plan. So I'm tending to the conclusion that cunning though it is to use "a functional-dependency-like mechanism (but using equalities) for the result type", that is actually giving too much of a headache. **bikeshed: I do like the proposed sugar for constraints (r { f :: t }) => ... But how does that play if `Has` only needs 2 args? AntC