Elm spa example header with shared button (on Click) attribute

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The viewHeader function in my example does not have to be literally in the Main.elm module.
It can be in its own module, taking a msg argument:

module Main exposing (main)

import Header

type Msg
    = HeaderClicked
    | GotHomeMsg Home.Msg


view model =
            ...
            in
            { title = title
            , body = Header.view HeaderClicked :: List.map (Html.map toMsg) body
            }

module Header exposing (view)

view : msg -> Html msg
view onClick = 
    div []
    [ button
        [ onClick msg ]
        []
    ]

or if really needed could have its own Msg which would be mapped like the pages ones.

At the end, all messages are Main ones, for example:

type Msg
    = HeaderMsg Header.Msg
    | GotHomeMsg Home.Msg
    ...


view model =
            ...
            in
            { title = title
            , body = Html.map HeaderMsg viewHeader :: List.map (Html.map toMsg) body
            }

I would strongly prefer the first solution (the msg argument(s)) as long as one or two messages for the header are enough.

Lastly:

Try to structure by types, not by code size.
See The life of a file.