function doesn't stay polymorphic enough when it is declared to be

I’ll take another pass and simplify this later but I thought I’d post this in case the solution is obvious

This code:

package RecordSet

enum RowEntry[w]:
  REBool(value: w[Bool])
  REInt(value: w[Int])
  REString(value: w[String])

struct RecordField[t](name: String, to_entry: forall w. w[t] -> RowEntry[w])
struct RecordValue[t](value: t)
struct RecordGetter[shape, t](f: forall w. shape[w] -> w[t])

struct RecordSet[shape](fields: shape[RecordField], rows: List[shape[RecordValue]], getters: shape[RecordGetter[shape]])

def restructure(RecordSet(fields, rows, getters), f: shape1[RecordGetter[shape1]] -> (forall w. shape1[w] -> shape2[w], shape2[RecordGetter[shape2]])) -> RecordSet[shape2]:
  (reshaper, new_getters) = f(getters)
  RecordSet(reshaper(fields), rows.map_List(reshaper), new_getters)

results in:

-----------------------------------------------------------------------------
in file: test_workspace/recordset.bosatsu, package RecordSet, type RecordField does not unify with type RecordValue
14|def restructure(RecordSet(fields, rows, getters), f: shape1[RecordGetter[shape1]] -> (forall w. shape1[w] -> shape2[w], shape2[RecordGetter[shape2]])) -> RecordSet[shape2]:
15|  (reshaper, new_getters) = f(getters)
16|  RecordSet(reshaper(fields), rows.map_List(reshaper), new_getters)
                                               ^^^^^^^^

even though reshaper should have type forall w. shape1[w] -> shape2[w]

[johnynek]

glad you have a work around, but that’s obviously annoying. It can see that either of those two types would work, but somehow it can’t widen them to those types at the use site.

This would be nice to improve. Thanks for working through it.

[johnynek]

this could be related to #650