First draft of a Savable package for elm to complement RemoteData

Savable.elm

module Savable exposing (..)

type Savable a = Saved a | Unsaved a a | Saving a a

update : Savable a -> a -> Savable a
update savable new =
  case savable of
    Saved old -> Unsaved old new
    Unsaved old _ -> Unsaved old new
    Saving old _ -> Unsaved old new

saving : Savable a -> Savable a
saving savable =
  case savable of
    Unsaved old new -> Saving old new
    _ -> savable

saved : a -> Savable a -> Savable a
saved savedValue savable =
  let savedIfEqual newValue savedValue =
        if savedValue == newValue then Saved newValue
        else Unsaved savedValue newValue
  in case savable of
    Saved value -> Saved savedValue
    Unsaved _ newValue -> savedIfEqual newValue savedValue
    Saving _ newValue -> savedIfEqual newValue savedValue

failSaving : Savable a -> Savable a
failSaving savable =
  case savable of
    Saving old new -> Unsaved old new
    _ -> savable

getNewValue : Savable a -> a
getNewValue savable =
  case savable of
    Saved value -> value
    Unsaved old new -> new
    Saving old new -> new

getOldValue : Savable a -> a
getOldValue savable =
  case savable of
    Saved value -> value
    Unsaved old new -> old
    Saving old new -> old

Thoughts:

• xilnocas on elm-slack recommended a new Union type that includes All the flags from RemoteData as well as the ones here (which I like but it would require significant copypasta and refactoring of the RemoteData library to make it similarly robust)
• I've considered rounding out the flags with SaveFailed a a err where err is a second type variable (similar to RemoteData)