Discuss possible PR for projection

[joneshf]

Discuss possible PR for projection

[joneshf]

Sorry, I totally missed this. Is this still relevant, or have you figured a way out here? Happy to have the discussion, but don't want to if you're already good.

[bbarker]

@joneshf I think it is still relevant, but not urgent in the sense I have a workaround. But the two proposed functions (project and getSubset) seem generally useful. However, the way I've implemented them may not be ideal.

[joneshf]

Gotcha.

I'm not really sure what you're wanting to do with these two values. Is there a brief example of how you might use these? Based on the names, I'm making the assumption that you've got a big option and you want a smaller one. Is that about right?

Forgive me if you've already figured this out, but I also wonder if this is something that can be solved with row-polymorphism? Sometimes with these problems, if we open the row on a value (not on the type, necessarily), it can solve these problems. Sometimes inference messes up, so it's not a panacea.

For example, if we've got this:

type Person
  = Option
      ( firstName :: String
      , lastName :: String
      )

displayName ::
  Person ->
  String
displayName person = case firstName person, lastName person of
  Just first, Just last -> first <> " " <> last
  Just first, Nothing -> first
  Nothing, Just last -> last
  Nothing, Nothing -> "Anonymous"

firstName ::
  Person ->
  Data.Maybe.Maybe String
firstName person = Option.get (Data.Symbol.SProxy :: _ "firstName") person

lastName ::
  Person ->
  Data.Maybe.Maybe String
lastName person = Option.get (Data.Symbol.SProxy :: _ "lastName") person

But we don't want firstName and lastName to know about the whole person, we can change them to something like:

firstName ::
  forall option.
  Option
    ( firstName :: String
    | option
    ) ->
  Data.Maybe.Maybe String
firstName person = Option.get (Data.Symbol.SProxy :: _ "firstName") person

lastName ::
  forall option.
  Option
    ( lastName :: String
    | option
    ) ->
  Data.Maybe.Maybe String
lastName person = Option.get (Data.Symbol.SProxy :: _ "lastName") person