parameters, Html checkbox takes in elm - elm

Question

This is the view snippet taken from elm checkbox examples -
view address model =
div [] <|
span [toStyle model] [text "Hello, how are yo?"]
:: br [] []
:: checkbox address model.red Red "red"
++ checkbox address model.underline Underline "underline"
++ checkbox address model.bold Bold "bold"
checkbox : Address Action -> Bool -> (Bool -> Action) -> String -> List Html
checkbox address isChecked tag name =
[ input
[ type' "checkbox"
, checked isChecked
, on "change" targetChecked (Signal.message address << tag)
]
[]
, text name
, br [] []
]
1) I understand, double colons and double plus are for concatenating lists? How are they different from each other?
2) In the checkbox function in this line, (Signal.message address << tag), what does tag unbind to? Is it Red (or) red? What is this argument used for?
3) What is the type of argument does the address function take?

Answer 1

Answer
double colon (::) adds a single element to the start of a list.
double plus (++) concatenates two lists.
tag would be equal to Red in the when checkbox is called from checkbox address model.red Red "red". Red is a function from Bool to Action. It wraps the event of the check box in this data constructor so that later in the update function, you can distinguish the event of that check box from the events of the other check boxes.
address is not a function. It has type Address Action, meaning it holds the address of a mailbox that can receive Action messages. This "mailbox" is not visible in the code, it's used internally by StartApp, which is used in the main function.
Code references
To keep this question useful even if the linked example changes, these are the relevant code portions I'm referring to:
The update function:
update action model =
case action of
Red bool ->
{ model | red <- bool }
Underline bool ->
{ model | underline <- bool }
Bold bool ->
{ model | bold <- bool }
The Action type:
type Action
= Red Bool
| Underline Bool
| Bold Bool
The main function:
main =
StartApp.start { model = initialModel, view = view, update = update }