Replace RowToList / Row.Cons type classes with purescript-heterogeneous

[thomashoneyman]

Environment

• purescript-halogen 4.0.0
• purescript-halogen-formless 0.2.0

Problem

There are a huge number of boilerplate RowToList classes used to produce various transformations on heterogeneous records. This involves lots of code duplication and non-obvious type class errors for end users, and it's easy to get the constraints wrong and only realize the error later (the class will compile, but not work as expected once put into use on a particular record).

For example, there are 4 variations on essentially this same class:

class InputFieldsToFormFields (xs :: RL.RowList) (row :: # Type) (to :: # Type) | xs -> to where
  inputFieldsToFormFieldsBuilder :: RLProxy xs -> Record row -> FromScratch to

instance inputFieldsToFormFieldsNil :: InputFieldsToFormFields RL.Nil row () where
  inputFieldsToFormFieldsBuilder _ _ = identity

instance inputFieldsToFormFieldsCons
  :: ( IsSymbol name
     , Row.Cons name (InputField e i o) trash row
     , InputFieldsToFormFields tail row from
     , Row1Cons name (FormField e i o) from to
     )
  => InputFieldsToFormFields (RL.Cons name (InputField e i o) tail) row to where
  inputFieldsToFormFieldsBuilder _ r =
    first <<< rest
    where
      _name = SProxy :: SProxy name
      val = transform $ Record.get _name r
      rest = inputFieldsToFormFieldsBuilder (RLProxy :: RLProxy tail) r
      first = Builder.insert _name val
      transform (InputField input) = FormField
        { input
        , touched: false
        , result: Nothing
        }

Solution

Fortunately, @natefaubion has released purescript-heterogeneous, which might provide a way to replace a few of these classes with more straightforward, less-boilerplatey implementations.

[thomashoneyman]

It would be ideal if this allowed Formless to reduce the number of constraints present on the component and various helper functions in order to speed up compilation times.

[thomashoneyman]

Unable to implement, in the end, due to issues with functional dependencies and using heterogeneous as component constraints. Reverted to the original RowList approach.