Html Msg vs Html msg?

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The lower case msg is called a type variable.

The upper case Msg is called a concrete type - its something your application has defined.

So we might have:

view : Html msg
view = div [] []

and

type Msg = Click

view : Html Msg
view = div [ onClick Click ] []

The first one doesn’t produce any events, so its type variable is not bound to anything yet.
The second one produces events of your Msg type, and its type variable is bound to that.

Also note that this compiles ok:

innerView : Html msg
innerView = div [] []

view : Html Msg
view = div [ onClick Click ] [ innerView ]

Here innerView needs to have the type Html Msg to be included in some HTML with that type. That’s ok because the free msg variable can be bound to Msg and the type checking passes.

Make sense?

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