Struggling with Context in Elm Architecture

[Adam Waselnuk]

I am having a hard time wrapping my head around Context as used in the Elm architecture example 4. Consider the following code:

-- http://package.elm-lang.org/packages/elm-lang/core/3.0.0/Signal#forwardTo

forwardTo: Address b -> (a -> b) -> Address a

-- Encounter.elm

viewCharacter : Signal.Address Action -> (ID, Character.Model) -> Html

viewCharacter address (id, model) =

  let

    context = 

      Character.Context

        (Signal.forwardTo address (ModifyCharacter id))

        (Signal.forwardTo address (always (RemoveCharacter id)))

  in

    Character.view context model

-- Character.elm

type alias Context =

  { actions : Signal.Address Action

  , remove : Signal.Address ()

  }

view : Context -> Model -> Html

view context character =

  div

    [ class "character" ]

    [

      input

        [

          class "character-level"

        , type' "number"

        , value (toString character.level)

        , on "input" targetValue (\level -> Signal.message context.actions (ModifyLevel (safeStrToLevel level)))

        ] []

    , input

        [

          class "character-name"

        , type' "text"

        , value (character.name)

        , on "input" targetValue (\name -> Signal.message context.actions (ModifyName name))

        ] []

    , button

        [ onClick context.remove () ]

        [ text "Remove" ]

    ]

What is going on with the Context type alias? In my Encounter module I define a context and pass it two Signal Addresses (I think) but in the Character model it is defined as a record. This is really tripping me up. I don't understand how the type is resolving properly or why my Character view gains access to a context record. Can anyone clarify this or point me to some documentation that explains in more detail what is happening?

[Max Goldstein]

I define a context and pass it two Signal Addresses (I think) but in the Character model it is defined as a record.

 

Type aliasing a record defines a record constructor function, which takes the fields of the record in order. So you're calling it as a function that creates a record of that type.

[Adam Waselnuk]

Thank you so much Max that 100% clarifies it! Do you know where I might find documentation on that? 

[Max Goldstein]

Unfortunately the official docs aren't the best. I'll refer you to this page (...which I wrote...).