Many language implementations come with an ''interactive mode''. This is a [[wp:command-line interpreter|command-line interpreter]] that reads lines from the user and evaluates these lines as statements or expressions. An interactive mode may also be known as a ''command mode'', a ''[[wp:read-eval-print loop|read-eval-print loop]]'' (REPL), or a ''shell''.

Show how to start this mode, then, as a small example of its use, interactively create a function of two strings and a separator that returns the strings separated by two concatenated instances of the separator.

For example, <tt>f('Rosetta', 'Code', ':')</tt> should return <tt>'Rosetta::Code'</tt>

Note: this task is not about creating your own interactive mode.