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<title>Read–eval–print loop</title>

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<span id="openzim-page-title" class="mw-page-title-main"><span class="mw-page-title-main">Read–eval–print loop</span></span>

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</style><div role="note" class="hatnote navigation-not-searchable">"Repl" redirects here; not to be confused with <a href="Replit" title="Replit">Replit</a>.</div>

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<p>A <b>read–eval–print loop</b> (<b>REPL</b>), also termed an <b>interactive toplevel</b> or <b>language shell</b>, is a simple interactive <a href="Computer_programming" title="Computer programming">computer programming</a> 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.<sup id="cite_ref-Grillmeyer_2013_p.239_1-0" class="reference"><a href="#cite_note-Grillmeyer_2013_p.239-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup> The term usually refers to programming interfaces similar to the classic <a href="Lisp_machine" title="Lisp machine">Lisp machine</a> interactive environment. Common examples include <a href="Command-line_interface" title="Command-line interface">command-line</a> <a href="Shell_(computing)" title="Shell (computing)">shells</a> and similar environments for <a href="Programming_language" title="Programming language">programming languages</a>, and the technique is very characteristic of <a href="Scripting_language" title="Scripting language">scripting languages</a>.<sup id="cite_ref-2" class="reference"><a href="#cite_note-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup>

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<div class="mw-heading mw-heading2"><h2 id="History">History</h2></div>

<p>In 1964, the expression <i>READ-EVAL-PRINT cycle</i> is used by <a href="L._Peter_Deutsch" title="L. Peter Deutsch">L. Peter Deutsch</a> and <a href="Edmund_Berkeley" title="Edmund Berkeley">Edmund Berkeley</a> for an implementation of <a href="Lisp_(programming_language)" title="Lisp (programming language)">Lisp</a> on the <a href="PDP-1" title="PDP-1">PDP-1</a>.<sup id="cite_ref-3" class="reference"><a href="#cite_note-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup> Just one month later, <a href="Project_Mac" class="mw-redirect" title="Project Mac">Project Mac</a> published a report by <a href="Joseph_Weizenbaum" title="Joseph Weizenbaum">Joseph Weizenbaum</a> (the creator of <a href="ELIZA" title="ELIZA">ELIZA</a>, the world's first chatbot) describing a REPL-based language, called OPL-1, implemented in his <a href="SLIP_(programming_language)" title="SLIP (programming language)">Fortran-SLIP</a> language on the <a href="Compatible_Time-Sharing_System" title="Compatible Time-Sharing System">Compatible Time Sharing System (CTSS)</a>.<sup id="cite_ref-4" class="reference"><a href="#cite_note-4"><span class="cite-bracket">[</span>4<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-5" class="reference"><a href="#cite_note-5"><span class="cite-bracket">[</span>5<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-6" class="reference"><a href="#cite_note-6"><span class="cite-bracket">[</span>6<span class="cite-bracket">]</span></a></sup>

</p><p>The 1974 <a href="Maclisp" title="Maclisp">Maclisp</a> reference manual by <a href="David_A._Moon" title="David A. Moon">David A. Moon</a> attests "Read-eval-print loop" on page 89, but does not use the acronym REPL.<sup id="cite_ref-7" class="reference"><a href="#cite_note-7"><span class="cite-bracket">[</span>7<span class="cite-bracket">]</span></a></sup>

</p><p>Since at least the 1980s, the abbreviations <i>REP Loop</i> and <i>REPL</i> are attested in the context of <a href="Scheme_(programming_language)" title="Scheme (programming language)">Scheme</a>.<sup id="cite_ref-8" class="reference"><a href="#cite_note-8"><span class="cite-bracket">[</span>8<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-9" class="reference"><a href="#cite_note-9"><span class="cite-bracket">[</span>9<span class="cite-bracket">]</span></a></sup>

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<div class="mw-heading mw-heading2"><h2 id="Overview">Overview</h2></div>

<p>In a REPL, the user enters one or more expressions (rather than an entire <a href="Translation_unit_(programming)" title="Translation unit (programming)">compilation unit</a>) and the REPL evaluates them and displays the results.<sup id="cite_ref-Grillmeyer_2013_p.239_1-1" class="reference"><a href="#cite_note-Grillmeyer_2013_p.239-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup> The name <i>read–eval–print loop</i> comes from the names of the Lisp primitive functions which implement this functionality:

</p>

<ul><li>The <i>read</i> function accepts an expression from the user, and <a href="Parsing" title="Parsing">parses</a> it into a data structure in memory. For instance, the user may enter the <a href="S-expression" title="S-expression">s-expression</a> <code>(+ 1 2 3)</code>, which is parsed into a <a href="Linked_list" title="Linked list">linked list</a> containing four data elements.</li>

<li>The <i><a href="Eval" title="Eval">eval</a></i> 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 <code>+</code> is called on the arguments <code>1 2 3</code>, yielding the result <code>6</code>.</li>

<li>The <i>print</i> function takes the result yielded by <i>eval</i>, and prints it out to the user. If it is a complex expression, it may be <a href="Prettyprint" class="mw-redirect" title="Prettyprint">pretty-printed</a> to make it easier to understand.</li></ul>

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

</p><p>REPLs facilitate <a href="Exploratory_programming" title="Exploratory programming">exploratory programming</a> and <a href="Debugging" title="Debugging">debugging</a> 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.

</p><p>Because the <i>print</i> function outputs in the same textual format that the <i>read</i> 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 <code><__module__.class instance></code> notation, and in Common Lisp, the <code>#<whatever></code> form. The REPL of <a href="CLIM" class="mw-redirect" title="CLIM">CLIM</a>, <a href="SLIME" title="SLIME">SLIME</a>, and the <a href="Symbolics" title="Symbolics">Symbolics</a> <a href="Lisp_Machine" class="mw-redirect" title="Lisp Machine">Lisp Machine</a> 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.

</p><p>REPLs can be created to support any text-based language. REPL support for compiled languages is usually achieved by implementing an <a href="Interpreter_(computing)" title="Interpreter (computing)">interpreter</a> on top of a virtual machine which provides an interface to the compiler. For example, starting with JDK 9, <a href="Java_(programming_language)" title="Java (programming language)">Java</a> included <a href="JShell" title="JShell">JShell</a> 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.

</p>

<p>As a <a href="Shell_(computing)" title="Shell (computing)">shell</a>, 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 interactive <a href="Software_prototyping" title="Software prototyping">prototyping</a>.<sup id="cite_ref-Binsbergen_10-0" class="reference"><a href="#cite_note-Binsbergen-10"><span class="cite-bracket">[</span>10<span class="cite-bracket">]</span></a></sup> Other uses include mathematical calculation, creating documents that integrate scientific analysis (e.g. <a href="IPython" title="IPython">IPython</a>), interactive software maintenance, <a href="Benchmarking" title="Benchmarking">benchmarking</a>, and algorithm exploration.

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<div class="mw-heading mw-heading2"><h2 id="Lisp_specifics">Lisp specifics</h2></div>

<div class="mw-heading mw-heading3"><h3 id="Implementation">Implementation</h3></div><p>

A minimal definition is: </p><div class="mw-highlight mw-highlight-lang-scheme mw-content-ltr" dir="ltr"><pre><span class="p">(</span><span class="k">define</span><span class="w"> </span><span class="p">(</span><span class="nf">REPL</span><span class="w"> </span><span class="nv">env</span><span class="p">)</span>

<span class="w"> </span><span class="p">(</span><span class="nf">print</span><span class="w"> </span><span class="p">(</span><span class="nb">eval</span><span class="w"> </span><span class="nv">env</span><span class="w"> </span><span class="p">(</span><span class="nb">read</span><span class="p">)))</span>

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<p>where <code>env</code> represents initial <code>eval</code>-uation environment. It is also assumed that <code>env</code> can be destructively updated by <code>eval</code>.

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<div class="mw-heading mw-heading3"><h3 id="Functionality">Functionality</h3></div>

<p>Typical functionality provided by a Lisp REPL includes:

</p>

<ul><li>History of inputs and outputs.</li>

<li>Variables are set for the input expressions and results. These variables are also available in the REPL. For example in Common Lisp <code>*</code> refers to the last result, <code>**</code> and <code>***</code> to the results before that.</li>

<li>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.</li>

<li><a href="Error_handling" class="mw-redirect" title="Error handling">Error handling</a>. The REPL provides restarts. These restarts can be used, when an error occurs, to go back to a certain REPL level.</li>

<li><a href="Mouse_(computing)" class="mw-redirect" title="Mouse (computing)">Mouse</a> sensitive input and output of data objects.</li>

<li>Input editing and context specific completion over symbols, pathnames, class names and other objects.</li>

<li>Help and documentation for commands.</li>

<li><a href="Variable_(programming)" class="mw-redirect" title="Variable (programming)">Variables</a> to control the reader. For example, the variable *read-base* controls in which base numbers are read by default.</li>

<li>Variables to control the printer. Example: maximum length or maximum depth of expressions to print.</li>

<li>Additional command syntax. Some REPLs have commands that follow not the s-expression syntax, but often work with Lisp data as arguments.</li>

<li>Graphical REPLs. Some Lisp REPLs (the CLIM Listener is an example) accept also graphical input and output.</li></ul>

<p><br>

</p>

<ul><li><a href="Direct_mode" title="Direct mode">Direct mode</a></li>

<li><a href="Interpreter_(computing)" title="Interpreter (computing)">Interpreter (computing)</a></li></ul>

<div class="mw-heading mw-heading2"><h2 id="References">References</h2></div>

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</style><cite id="CITEREFGrillmeyer2013" class="citation book cs1">Grillmeyer, O. (2013). <a rel="nofollow" class="external text" href="https://books.google.com/books?id=GoXjBwAAQBAJ"><i>Exploring Computer Science with Scheme</i></a>. Undergraduate Texts in Computer Science. Springer New York. p. 239. <a href="ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <bdi>978-1-4757-2937-5</bdi><span class="reference-accessdate">. Retrieved <span class="nowrap">2021-06-26</span></span>. <q>The central component to the Scheme interpreter is the <i>read-eval-print loop</i>. Commands are read in, then evaluated. Finally, the evaluated result is printed.</q></cite></span>

</li>

<li id="cite_note-2"><span class="mw-cite-backlink"><b><a href="#cite_ref-2">^</a></b></span> <span class="reference-text"><cite id="CITEREFHeyPápay2014" class="citation book cs1">Hey, Tony; Pápay, Gyuri (2014). <i>The Computing Universe: A Journey through a Revolution</i>. Cambridge University Press. p. <a rel="nofollow" class="external text" href="https://books.google.com/books?id=q4FIBQAAQBAJ&pg=PA76&dq=repl+scripting">76</a>. <a href="ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <bdi>978-1-316-12322-5</bdi>, "A major characteristic of modern scripting languages is their interactivity, sometimes referred to as a <i>REPL</i> programming environment. ... The characteristics of ease of use and immediate execution with a REPL environment are sometimes taken as the definition of a scripting language."</cite><span class="cs1-maint citation-comment"><code class="cs1-code">{{cite book}}</code>: CS1 maint: postscript (link)</span></span>

</li>

<li id="cite_note-3"><span class="mw-cite-backlink"><b><a href="#cite_ref-3">^</a></b></span> <span class="reference-text"><cite id="CITEREFL._Peter_DeutschEdmund_Berkeley" class="citation cs2">L. Peter Deutsch; Edmund Berkeley, <a rel="nofollow" class="external text" href="http://s3data.computerhistory.org/pdp-1/DEC.pdp_1.1964.102650371.pdf"><i>The LISP Implementation for the PDP-1 Computer</i></a> <span class="cs1-format">(PDF)</span>, p. <a rel="nofollow" class="external text" href="http://s3data.computerhistory.org/pdp-1/DEC.pdp_1.1964.102650371.pdf#page=16">15</a></cite></span>

</li>

<li id="cite_note-4"><span class="mw-cite-backlink"><b><a href="#cite_ref-4">^</a></b></span> <span class="reference-text"><cite id="CITEREFJoseph_Weizenbaum" class="citation cs2">Joseph Weizenbaum, <a rel="nofollow" class="external text" href="https://drive.google.com/file/d/1AHTlN-nCtgMQ2Dnr8t_hP6TAqH-MOfOm"><i>OPL-I: AN OPEN ENDED PROGRAMMING SYSTEM WITHIN CTSS</i></a></cite></span>

</li>

<li id="cite_note-5"><span class="mw-cite-backlink"><b><a href="#cite_ref-5">^</a></b></span> <span class="reference-text">Both of these projects were likely carried out in 1963, well before the respective publications appeared. However, it is impossible to place the dates of invention exactly.</span>

</li>

<li id="cite_note-6"><span class="mw-cite-backlink"><b><a href="#cite_ref-6">^</a></b></span> <span class="reference-text">There is a complex and interesting relationship between Lisp and SLIP, both being eponymous "list processing languages" invented by MIT-related academics: <cite id="CITEREFShrager2024" class="citation cs2">Shrager, Jeff (2024), <i>ELIZA Reinterpreted: The world's first chatbot was not intended as a chatbot at all</i>, <a href="ArXiv_(identifier)" class="mw-redirect" title="ArXiv (identifier)">arXiv</a>:<span class="id-lock-free" title="Freely accessible"><a rel="nofollow" class="external text" href="https://arxiv.org/abs/2406.17650">2406.17650</a></span></cite></span>

</li>

<li id="cite_note-7"><span class="mw-cite-backlink"><b><a href="#cite_ref-7">^</a></b></span> <span class="reference-text"><cite id="CITEREFDavid_A._Moon1974" class="citation cs2">David A. Moon (April 8, 1974), <a rel="nofollow" class="external text" href="https://www.softwarepreservation.org/projects/LISP/MIT/Moon-MACLISP_Reference_Manual-Apr_08_1974.pdf"><i>MACLISP Reference Manual</i></a> <span class="cs1-format">(PDF)</span>, p. 89</cite></span>

</li>

<li id="cite_note-8"><span class="mw-cite-backlink"><b><a href="#cite_ref-8">^</a></b></span> <span class="reference-text"><cite id="CITEREFSmith1988" class="citation book cs1">Smith, Jerry D. (1988). <a rel="nofollow" class="external text" href="https://archive.org/details/introductiontosc0000smit/page/8"><i>An introduction to Scheme</i></a>. Englewood Cliffs, N.J. : Prentice Hall. p. <a rel="nofollow" class="external text" href="https://archive.org/details/introductiontosc0000smit/page/8">8</a>. <a href="ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <bdi>978-0-13-496712-7</bdi>.</cite></span>

</li>

<li id="cite_note-9"><span class="mw-cite-backlink"><b><a href="#cite_ref-9">^</a></b></span> <span class="reference-text"><cite id="CITEREFHanson1986" class="citation web cs1">Hanson, Chris (1986). <a rel="nofollow" class="external text" href="https://github.com/jaseemabid/mit-scheme/blob/47c68ef5fa41821c3f52f886ef69f99d232d10bb/v7/src/runtime/rep.scm">"rep.scm -- Initial 1986 revision of MIT-Scheme"</a>. <i>GitHub</i><span class="reference-accessdate">. Retrieved <span class="nowrap">11 June</span> 2023</span>.</cite></span>

</li>

<li id="cite_note-Binsbergen-10"><span class="mw-cite-backlink"><b><a href="#cite_ref-Binsbergen_10-0">^</a></b></span> <span class="reference-text"><cite id="CITEREFvan_BinsbergenVerano_MerinoJeanjeanvan_der_Storm2020" class="citation conference cs1">van Binsbergen, L. Thomas; Verano Merino, Mauricio; Jeanjean, Pierre; van der Storm, Tijs; Combemale, Benoit; Barais, Olivier (2020-11-17). "A principled approach to REPL interpreters". <a rel="nofollow" class="external text" href="https://hal.inria.fr/hal-02968938/file/onward2020-repls.pdf"><i>Proceedings of the 2020 ACM SIGPLAN International Symposium on New Ideas, New Paradigms, and Reflections on Programming and Software</i></a> <span class="cs1-format">(PDF)</span>. New York, NY, USA: ACM. pp. <span class="nowrap">84–</span>100. <a href="Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<a rel="nofollow" class="external text" href="https://doi.org/10.1145%2F3426428.3426917">10.1145/3426428.3426917</a>. <a href="ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <bdi>978-1-4503-8178-9</bdi>.</cite></span>

</li>

</ol></div></div>

<div class="mw-heading mw-heading2"><h2 id="External_links">External links</h2></div>

<ul><li><a href="Paul_Graham_(programmer)" title="Paul Graham (programmer)">Paul Graham</a> has written a <a rel="nofollow" class="external text" href="http://www.paulgraham.com/rootsoflisp.html">description of a REPL implementation</a> in Common Lisp.</li>

<li>Joël Franusic <a rel="nofollow" class="external text" href="http://joel.franusic.com/Online-REPs-and-REPLs">Online-REPs-and-REPLs list</a></li></ul></div><div><div class="zim-footer">

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