[itishappy]

It's both string interpolation and (anonymous) function definition.

    # Function Definition:
    addAndStringify = \num1, num2 ->
        Num.toStr (num1 + num2)

    # String Interpolation:
    "\(greeting) there, \(audience)!"
    # desugars to
    Str.concat greeting (Str.concat " there, " (Str.concat audience "!"))

https://www.roc-lang.org/tutorial