Getting Elm installed. We will install the Elm CLI with npm. This gives us access to
elm-reactor. The repl lets us play with expressions at the command-line, and reactor starts a development server which builds Elm files on-demand.
Here I am in an empty folder and I want to get Elm installed. Elm's binaries are available through npm. I can install them just by typing $ npm install -g elm. Then I wait for a while. There it is. Now I can type elm. It should be in my path.
I can also type elm-reactor to start the dev server, or I can type elm-repl to start a Read-eval-print-loop, and inside of here I can test it out, foo = 1, looks good, bar = 2, and foo + bar = 3. We have elm installed.