A type keeping track of changes to a value since it was last saved. This is great for building editing interfaces that keep track of whether the user has made any changes.

Suppose we're writing a text editing application. Saved a assumes that a saved value we can revert to always exists. For our text editing example such a value can for instance be an empty document. If we're saving these documents to some backend, we might want to distinguish between this initial unpersisted value and a value that has been persisted to the backend.

One way to do this is to use different types for persisted and unpersisted documents. For instance, let's assume a document that is persisted on the backend has an id, while one that's not persisted does not. We can define separate types for documents in these different states:

type alias PersistedDoc =
    { id : Int
    , title : String
    -- More properties here
    }

type alias UnpersistedDoc =
    { title : String
    -- More properties here
    }

type Model
    = Persisted (Saved PersistedDoc)
    | Unpersisted (Saved UnpersistedDoc)

When we perform the initial save, we can turn a Saved UnpersistedDoc to a Saved PersistedCo using the map function.

Lets stay with the example of a text editing application a bit longer. At some point we'll want to save a version of the document to a backend, keeping track of the progress of this operation and whether it results in a successful safe or not. The Saved a type has no opinion on what it means for a value to be saved, but the RemoteData is perfect for this use case. We can put the Saved a and RemoteData e a types next to each other on the model:

type alias Model =
    { doc : Saved Doc
    , saveState : RemoteData Http.Error Doc
    }

At this point it looks a bit like by using both Saved a and RemoteData e a we're duplicating state, but really the above solution precisely tracks the tree versions of a document that are relevant while the document is in the process being saved:

1. The last succesfully saved version, which is tracked by Saved a.
2. The version we're currently attempting to save, which is tracked by RemoteData e a.
3. The most recently edited version the user is seeing on the screen, which can already have diverged from the version we're currently saving. This version is also traced by Saved a.

Once our Http request responds that the save was successfull, we can use setSaved to update the last saved value of our document.