[Ur] Polymorphic variants and JSON

[Edward Z. Yang]

I think there is an omission in Urweb's typeclass resolution for variants.

ezyang at javelin:~/Dev/logitext/meta$ urweb tests/testJson -dumpTypes
[snip]
 json_phoneType :
 Json.json
  (Basis.variant
    ([#Mobile = Basis.string, #LandLine = {}, #Secret = {}]))
[snip]
ezyang at javelin:~/Dev/logitext/meta$ urweb tests/testJson
/home/ezyang/Dev/logitext/meta/tests/testJson.ur:61:54-61:62: Can't resolve type class instance
Class constraint: 
Basis.show
 (Basis.variant
   ([#Mobile = Basis.string, #LandLine = {}, #Secret = {}]))
/home/ezyang/Dev/logitext/meta/tests/testJson.ur:49:38-49:50: Can't resolve type class instance
Class constraint: 
{Mobile : Json.json Basis.string, LandLine : Json.json {}, 
  Secret : Json.json {}}
/home/ezyang/Dev/logitext/meta/tests/testJson.ur:40:42-40:53: Can't resolve type class instance
Class constraint: 
{Type_ :
  Json.json
   (Basis.variant
     ([#Mobile = Basis.string, #LandLine = {}, #Secret = {}])), 
  Number : Json.json Basis.string}

Cheers,
Edward

Excerpts from Adam Chlipala's message of Sat Apr 14 20:15:10 -0400 2012:
> Edward Z. Yang wrote:
> > Excerpts from Edward Z. Yang's message of Sat Apr 14 19:29:32 -0400 2012:
> >> This particular permutation gives me:
> >>
> >>      /home/ezyang/Dev/logitext/meta/json.ur:300:55-300:69: Can't resolve type class instance
> >>      Class constraint:  Top.folder[[Type]] rest
> >>
> >> and I don't know how to spell the implicit arguments correctly or what
> >> folder I should be giving it (it's /almost/ 'fl' except that the
> >> entries in front need to be lopped off).  Is there a version of fold which
> >> also gives me folders?
> > Here's an idea: fold once to get a record of folders, and fold again
> > with that. But maybe the unification won't be clever enough.
> 
> Oh, yeah; I didn't think of this issue.  It's probably easier to stop 
> using [Variant.weaken]; instead, you can choose your accumulator type to 
> be polymorphic in an extra type-level record to concatenate into the 
> index to [variant].  (This general pattern appears a few times in 
> variant.ur, including in the first function definition.)