read–eval–print loop
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A read–eval–print loop (REPL), also termed an interactive toplevel or language shell, is a simple interactive
computer programming Computer programming is the process of performing a particular computation (or more generally, accomplishing a specific computing result), usually by designing and building an executable computer program. Programming involves tasks such as ana ...
environment 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 classic
Lisp machine Lisp machines are general-purpose computers designed to efficiently run Lisp as their main software and programming language, usually via hardware support. They are an example of a high-level language computer architecture, and in a sense, the ...
interactive environment. Common examples include
command-line A command-line interpreter or command-line processor uses a command-line interface (CLI) to receive commands from a user in the form of lines of text. This provides a means of setting parameters for the environment, invoking executables and pro ...
shells and similar environments for
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s, and the technique is very characteristic of
scripting language A scripting language or script language is a programming language that is used to manipulate, customize, and automate the facilities of an existing system. Scripting languages are usually interpreted at runtime rather than compiled. A scripting ...
s.


History

The first Read-Eval-Print loop, ''cycle'', was invented in 1964 by
L. Peter Deutsch L Peter Deutsch (born Laurence Peter Deutsch on August 7, 1946, in Boston, Massachusetts) is the founder of Aladdin Enterprises and creator of Ghostscript, a free software PostScript and Portable Document Format, PDF interpreter. Deutsch's othe ...
and
Edmund Berkeley Edmund Callis Berkeley (February 22, 1909 – March 7, 1988) was an American computer scientist who co-founded the Association for Computing Machinery (ACM) in 1947. His 1949 book ''Giant Brains, or Machines That Think'' popularized cognitiv ...
for an implementation of
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on the
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.


Overview

In a REPL, the user enters one or more expressions (rather than an entire
compilation unit In C and C++ programming language terminology, a translation unit (or more casually a compilation unit) is the ultimate input to a C or C++ compiler from which an object file is generated. A translation unit roughly consists of a source file aft ...
) and the REPL evaluates them and displays the results. The name ''read–eval–print loop'' comes 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 In computer programming, an S-expression (or symbolic expression, abbreviated as sexpr or sexp) is an expression in a like-named notation for nested list (tree-structured) data. S-expressions were invented for and popularized by the programming la ...
(+ 1 2 3), which is parsed into a
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containing four data elements. * The ''
eval In some programming languages, eval , short for the English evaluate, is a function which evaluates a string as though it were an expression in the language, and returns a result; in others, it executes multiple lines of code as though they had b ...
'' 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. The development environment then returns to the read state, creating a loop, which terminates when the program is closed. REPLs facilitate
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and
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because 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 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 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 # form. The REPL of
CLIM The Common Lisp Interface Manager (CLIM) is a Common Lisp-based programming interface for creating user interfaces, i.e., graphical user interfaces (GUIs). It provides an application programming interface (API) to user interface facilities for the ...
,
SLIME Slime may refer to: Biology * Slime mold, a broad term often referring to roughly six groups of Eukaryotes * Biofilm, an aggregate of microorganisms in which cells adhere to each other and/or to a surface * Slimy (fish), also known as the pony ...
, and the
Symbolics Symbolics was a computer manufacturer Symbolics, Inc., and a privately held company that acquired the assets of the former company and continues to sell and maintain the Open Genera Lisp system and the Macsyma computer algebra system.
Lisp Machine Lisp machines are general-purpose computers designed to efficiently run Lisp as their main software and programming language, usually via hardware support. They are an example of a high-level language computer architecture, and in a sense, the ...
can 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 an interpreter on top of a virtual machine which provides an interface to the compiler. For example, starting with JDK 9,
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included
JShell JShell is a Java read-eval-print loop which was first introduced in the JDK 9. It is tracked by JEP 222 ''jshell: The Java Shell (Read-Eval-Print Loop)''. One reason why JShell was proposed for Java 9 is the lack of a standard interactive environ ...
as 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 a
shell Shell may refer to: Architecture and design * Shell (structure), a thin structure ** Concrete shell, a thin shell of concrete, usually with no interior columns or exterior buttresses ** Thin-shell structure Science Biology * Seashell, a hard o ...
, 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 instantaneous
prototyping A prototype is an early sample, model, or release of a product built to test a concept or process. It is a term used in a variety of contexts, including semantics, design, electronics, and software programming. A prototype is generally used to ...
. Other uses include mathematical calculation, creating documents that integrate scientific analysis (e.g.
IPython IPython (Interactive Python) is a command shell for interactive computing in multiple programming languages, originally developed for the Python programming language, that offers introspection, rich media, shell syntax, tab completion, and histo ...
), interactive software maintenance,
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, and algorithm exploration.


Lisp specifics


Implementation

A minimal definition is: (define (REPL env) (print (eval env (read))) (REPL env) ) where env represents initial eval-uation environment. It is also assumed that env can be destructively updated by eval.


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 In computing and computer programming, exception handling is the process of responding to the occurrence of ''exceptions'' – anomalous or exceptional conditions requiring special processing – during the execution of a program. In general, an ...
. The REPL provides restarts. These restarts can be used, when an error occurs, to go back to a certain REPL level. *
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sensitive 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. * Variables to 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 In computing, direct or immediate mode in an interactive programming system is the immediate execution of commands, statements, or expressions. In many interactive systems, most of these can both be included in programs or executed directly in ...


References


External links

* Paul Graham has written
description of a REPL implementation
in Common Lisp. * Joël Franusi
Online-REPs-and-REPLs list
{{DEFAULTSORT:Read-Eval-Print Loop Command shells Lisp (programming language) Interpreters (computing) User interface techniques