Read-eval-print loop

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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 Python, BASIC, Scheme, 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.

The REPL is commonly misnamed an interpreter. This is an erroneous usage, since many programming languages that use compilation (including bytecode compilation) have REPLs, such as Common Lisp and Python.

Because the print function outputs in the same textual format that the read function uses for input, most results are printed in a form that could (if it's useful) be copied and pasted back into the REPL. However, it's sometimes necessary to print representations of opaque data elements that can't sensibly be read back in -- such as a socket handle or a complex class instance. In these cases, there must exist a syntax for unreadable objects -- in Python, it's the <__module__.class instance> notation; in Common Lisp, the #<whatever> form.

REPL provides many ways to debug Lisp programs, allowing to see and change the values of variables and redefine previously declared functions.

[edit] Implementation

To implement a Lisp REPL, it is necessary only to implement these three functions and an infinite-loop function. (Naturally, the implementation of eval will be complicated, since it must also implement all the primitive functions like car and + and special operators like if.) This done, a basic REPL itself is but a single line of code: (loop (print (eval (read)))).

[edit] External links

• Paul Graham has written a description of a REPL implementation in Common Lisp.