REPL

                                   REPL

A read–eval–print loop (REPL), also termed an interactive toplevel or language shell, is a simple, interactive computer programming environment that takes single user inputs (i.e., single expressions), evaluates them, and returns the result to the user; a program written in a REPL environment is executed piecewise.

In a REPL, the user enters one or more expressions (rather than an entire compilation unit) and the REPL evaluates them and displays the results. The name read–eval–print loopcomes from the names of the Lisp primitive functions which implement this functionality:

• The read function accepts an expression from the user, and parses it into a data structure in memory. For instance, the user may enter the s-expression (+ 1 2 3), which is parsed into a linked list containing four data elements.
• The eval function takes this internal data structure and evaluates it. In Lisp, evaluating an s-expression beginning with the name of a function means calling that function on the arguments that make up the rest of the expression. So the function + is called on the arguments 1 2 3, yielding the result 6.
• The print function takes the result yielded by eval, and prints it out to the user. If it is a complex expression, it may be pretty-printed to make it easier to understand. In this example, though, the number 6 does not need much formatting to print.