Yeah, I am familier with that, this is fairly orthogonal actually and both can be used together to good effect. This proposal is more about making records 'lightweight', as in, I have a recursive function that takes 5 or so recursive values and they all have to be passed as independent options (or a tuple) right now which is error prone, by making a lightweight inferred record we get named parameters that are infered just like they were listed as options and CPR analysis will even ensure there is no run time penalty, the fields will be expanded out as positional parameters internally. An issue with OverloadedRecordFields is that it relies on a lot of type system complexity, This builds on the existing DisambiguateRecordFields extension and simply removes the restriction that the records with the same field names be defined in different modules. since field names declared this way don't share a namespace with functions this isn't an issue. Notably, everything to disambiguate fields can take place in the renamer and the fields can be typechecked exactly as if they were top level pattern bindings. Another possible extension is allowing it to infer scoped type variables as well for a parameterized record type. working on something concrete for that with my jhc implementation, the issue is that the record will have to be parameterized by a type to keep them from escaping. Perhaps just a single type parameter that is existentially instantiated, syntax being data R t = R {fa,fb,fc} or putting the data declaration in a local where or let binding. but that will require some more work. (and scoped type variables in jhc are a little iffy at the moment as is) John On Tue, Jun 3, 2014 at 11:33 PM, Carter Schonwald <carter.schonwald@gmail.com> wrote:
Hey John in some respects, this sounds like syntax sugar around https://ghc.haskell.org/trac/ghc/wiki/Records/OverloadedRecordFields have you had a look at that? (its not quite merged into HEAD yet, but that due soon i'm told)
On Wed, Jun 4, 2014 at 2:26 AM, John Meacham <john@repetae.net> wrote:
This is also available as html at
http://repetae.net/computer/jhc/record_inference.html
Record Type Inference =====================
An extension to the named field mechanism that will greatly enhance the utility of them when combined with the existing `DisambiguateRecordFields`, `RecordPuns`, and `RecordWildCards`.
The proposal is to allow the types of record fields to be inferred by the normal type inference engine. It would look like
~~~~ {.haskell} data Rec = Rec {fieldA,fieldB,fieldC}
f Rec { .. } = Rec { .. } where fieldA = True fieldB = 4 ~~~~
This would infer the types `Bool`, `Int`, and `forall a . a` for the fields of the record constructor and `f :: Rec -> Rec` for f. For the purposes of type checking the fields are treated as monomorphic and not generalized but defaulted like normal after typechecking the module. Other than infering the types of the record fields, the records have the normal syntax. The extensions `RecordPuns`, `RecordWildCards` and `DisambiguateRecordFields` will be enabled when record field inference is enabled.
Selector functions will not be created for infered records, as in, the names are field labels and not functions. This means they do not share a namespace with functions and do not conflict with each other. Multiple records may have the same field names in the same module. This means the following is fine.
~~~~ {.haskell} data Rec1 = Rec1 {input, withFoo, withoutFoo } data Rec2 = Rec2 {input, withBar, withoutBar }
f Rec1 { .. } = case input of [] -> Rec1 { .. } (x:xs) -> if hasFoo x then Rec1 { withFoo = x:withFoo, .. } else Rec1 { withoutFoo = x:withoutFoo, .. } ~~~~
Possible extensions -------------------
### as-pattern disambiguation
In order to make the disambiguation of record fields more useful without relying on the type checker for disambiguation, We can declare that variables explicitly bound to a constsructor in a pattern match use that constructor to disambiguate fields for operations on the variable. This is a purely syntactic transformation that can happen before typechecking. It can be used as follows.
~~~~ {.haskell} -- use the input bound by a Rec1 to update the input bound by a Rec2 f r1@Rec1 { input } r2@Rec2 {} = case input of xs | any hasBar xs = f r1 { input = [] } r2 { input } ~~~~
### Field label inference
It is concievable that we may want to infer the fields themselves of a record, as in:
~~~~ {.haskell} -- infer that R has the field labels bob and susan data R = R { ..} f x@R {bob} = R {susan = bob} ~~~~
In order to implement this, a pass through the file will collect every field label that is used with an explicit R constructor and treat the record as if it were declared with those names as infered fields. _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
-- John Meacham - http://notanumber.net/