Problem Statement

Many language implementations come with an interactive mode. This is a 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 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   (the 3rd argument).

should return

This task is   not   about creating your own interactive mode.

Let's start with the solution:

$ brat
# Interactive Brat
brat:1> f = { a, b, s | a + s + s + b }
#=> function: 0xb737ac08
brat:2> f "Rosetta" "Code" ":"
#=> Rosetta::Code
brat:3> quit
Exiting