Read-eval-print loop - Wikipedia, the free encyclopedia

Read-eval-print loop - Wikipedia, the free encyclopedia

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 command line shells and similar environments for Smalltalk, Perl, Scala, Python, Ruby, Haskell, APL, BASIC, J, 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.

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