Hello world example from elm-lang.org

Main.elm

module Main exposing (..)

import Html exposing (Html, span, text)
import Html.Attributes exposing (class)


main : Html a
main =
    span [ class "welcome-message" ] [ text "Hello, World!" ]
    

Just came across this while picking up Elm. Do you think you could explain what a is? I'm finding very limited documentation on main in terms of Program options, and Html's docs don't seem to document the Html function it exposes.

EDIT: Just came across the answer in the official docs

So the type List a is saying "I know I have a list, but it could be filled with anything". The lower-case a is called a type variable, meaning that there are no constraints in our program that pin this down to some specific type. In other words, the type can vary based on how it is used.