Read-eval-print loop

A read-eval-print loop (REPL), also known as an interactive toplevel, is a simple, interactive computer programming environment. The term is most usually used to refer to a Lisp interactive environment, but can be applied to the similar environments for Smalltalk, Python, Haskell, APL, BASIC, J, Scheme, TCL, and other languages as well.

In a REPL, the user may enter expressions, which are then evaluated, and the results displayed. The name read-eval-print loop comes from the names of the Lisp primitive functions which implement this functionality:

The read function accepts a single 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.