How to get rid of CoRec after a melt

[MMesch]

Hi all,

I'm trying to get rid of the "value" :-> CoRec ElField '[("field1", type1), ("field2", type2)] column in favour of one column called "key" storing the labels and "value" storing the values. The first is quite easy:

type CollapsedFields = '["field1" :-> Int, "field2" :-> Int]
fld :: CoRec ElField CollapsedFields -> String
fld = foldCoRec func
    where
        func :: forall (a :: (Symbol, *)). a ∈ CollapsedFields => ElField a -> String
        func x = case x of
            Field _ -> getLabel x

but the latter is really difficult because GHC somehow needs to know that all fields are Integers—or know what to do if not. This therefore fails:

type CollapsedFields = '["field1" :-> Int, "field2" :-> Int]
fld :: CoRec ElField CollapsedFields -> Int
fld = foldCoRec func
    where
        func :: forall (a :: (Symbol, *)). a ∈ CollapsedFields => ElField a -> Int
        func x = case x of
            Field _ -> getField x

with

• Could not deduce: t ~ Int
  from the context: (a ~ '(s, t), KnownSymbol s)
    bound by a pattern with constructor:
               Field :: forall (s :: Symbol) t.
                        KnownSymbol s =>
                        t -> ElField '(s, t),
             in a case alternative
    at xxxx
  ‘t’ is a rigid type variable bound by
    a pattern with constructor:
      Field :: forall (s :: Symbol) t.
               KnownSymbol s =>
               t -> ElField '(s, t),
    in a case alternative
    at xxxx
  Expected type: ElField '(s, Int)
    Actual type: ElField a
• In the first argument of ‘getField’, namely ‘x’
  In the expression: getField x
  In a case alternative: Field _ -> getField x (typecheck -Wdeferred-type-errors)

The match function that Vinyl provides seems not to work with ElField.

Any ideas how to do this?

[idontgetoutmuch]

This seems somewhat related: #186