Read–eval–print loop

"Repl" redirects here. Not to be confused withReplit.

Aread–eval–print loop(REPL), also termed aninteractive toplevelorlanguage shell,is a simple interactivecomputer programmingenvironment that takes single user inputs, executes them, and returns the result to the user; a program written in a REPL environment is executed piecewise.The term usually refers to programming interfaces similar to the classicLisp machineinteractive environment. Common examples includecommand-lineshellsand similar environments forprogramming languages,and the technique is very characteristic ofscripting languages.

History

In 1964, the expressionREAD-EVAL-PRINT cycleis used byL. Peter DeutschandEdmund Berkeleyfor an implementation ofLispon thePDP-1.

Since at least the 1980s, the abbreviationsREP LoopandREPLare attested in the context ofScheme.

Overview

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

• Thereadfunction accepts an expression from the user, andparsesit into a data structure in memory. For instance, the user may enter thes-expression(+ 1 2 3),which is parsed into alinked listcontaining four data elements.
• Theevalfunction 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 arguments1 2 3,yielding the result6.
• Theprintfunction takes the result yielded byeval,and prints it out to the user. If it is a complex expression, it may bepretty-printedto make it easier to understand.

The development environment then returns to the read state, creating a loop, which terminates when the program is closed.

REPLs facilitateexploratory programminganddebuggingbecause the programmer can inspect the printed result before deciding what expression to provide for the next read. The read–eval–print loop involves the programmer more frequently than the classic edit–compile–run–debug cycle.

Because theprintfunction outputs in the same textual format that thereadfunction uses for input, most results are printed in a form that could be copied and pasted back into the REPL. However, it is sometimes necessary to print representations of elements that cannot 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 is the<__module__.class instance>notation, and in Common Lisp, the#<whatever>form. The REPL ofCLIM,SLIME,and theSymbolicsLisp Machinecan also read back unreadable objects. They record for each output which object was printed. Later when the code is read back, the object will be retrieved from the printed output.

REPLs can be created to support any text-based language. REPL support for compiled languages is usually achieved by implementing aninterpreteron top of a virtual machine which provides an interface to the compiler. For example, starting with JDK 9,JavaincludedJShellas a command-line interface to the language. Various other languages have third-party tools available for download that provide similar shell interaction with the language.

Uses

As ashell,a REPL environment allows users to access relevant features of an operating system in addition to providing access to programming capabilities. The most common use for REPLs outside of operating system shells is for interactiveprototyping.Other uses include mathematical calculation, creating documents that integrate scientific analysis (e.g.IPython), interactive software maintenance,benchmarking,and algorithm exploration.

Lisp specifics

Implementation

A minimal definition is:

(define(REPLenv)
(print(evalenv(read)))
(REPLenv))

whereenvrepresents initialeval-uation environment. It is also assumed thatenvcan be destructively updated byeval.

Functionality

Typical functionality provided by a Lisp REPL includes:

• History of inputs and outputs.
• Variables are set for the input expressions and results. These variables are also available in the REPL. For example in Common Lisp*refers to the last result,**and***to the results before that.
• Levels of REPLs. In many Lisp systems if an error occurs during the reading, evaluation or printing of an expression, the system is not thrown back to the top level with an error message. Instead a new REPL, one level deeper, is started in the error context. The user can then inspect the problem, fix it and continue – if possible. If an error occurs in such a debug REPL, another REPL, again a level deeper, is started. Often the REPL offers special debug commands.
• Error handling.The REPL provides restarts. These restarts can be used, when an error occurs, to go back to a certain REPL level.
• Mousesensitive input and output of data objects.
• Input editing and context specific completion over symbols, pathnames, class names and other objects.
• Help and documentation for commands.
• Variablesto control the reader. For example, the variable *read-base* controls in which base numbers are read by default.
• Variables to control the printer. Example: maximum length or maximum depth of expressions to print.
• Additional command syntax. Some REPLs have commands that follow not the s-expression syntax, but often work with Lisp data as arguments.
• Graphical REPLs. Some Lisp REPLs (the CLIM Listener is an example) accept also graphical input and output.

See also

• Direct mode
• Interpreter (computing)

External links

• Paul Grahamhas written adescription of a REPL implementationin Common Lisp.
• Joël FranusicOnline-REPs-and-REPLs list