Talk:Read–eval–print loop

Attention needed?

What sort of attention is needed on this article exactly? — The Storm Surfer 00:28, 20 Jun 2005 (UTC)

The attention that Fubar Obfusco already gave. --MarSch 14:03, 20 Jun 2005 (UTC)

Thanks for the clarification :) — The Storm Surfer 18:11, 20 Jun 2005 (UTC)

I think there's a species of microtroll who go around posting "This article is inadequate" whines in little boxes all over Wikipedia.

The tragedy is, they don't seem to be capable of actually doing anything useful in their copious free time.

If there's anything wrong with this fine article, it's probably the overabundance of click-throughs for the ignorant. Anybody interested in REPL IDEs is likely to be pretty well informed on computation basics -- but don't tell the microtrolls that!

David Lloyd-Jones (talk) 06:59, 28 August 2014 (UTC)

As someone coming to this article with fresh eyes, I agree it needs attention. Why is there a "Lisp Specifics" section that's nearly half the entire article? Who cares so much about LISP in particular? Among modern readers I would expect the most familiar/useful REPL to know about would be Python, which is barely mentioned. Discussion of REPLs in other modern languages like C#, Lua, JavaScript, etc. would all merit more space than LISP -- almost nobody is coming to the REPL article to learn "how to implement a REPL in LISP"; that content belongs on a LISP page, if anywhere. As written, the article gives the impression that a REPL is primarily a feature of LISP (and yet something that you as a LISP user have to implement yourself). Wiked120521 (talk) 14:21, 23 July 2023 (UTC)

REPL is Lisp jargon that describes a command loop that calls the Lisp functions read, eval, and print. In Lisp, it is usually literally implemented that way. The term drifted to other languages whose implementations usually work differently, but it still refers back to Lisp. 2601:644:8501:AAF0:0:0:0:9BB0 (talk) 09:23, 11 July 2024 (UTC)

Seriously, we’re at the level Trumpist level or narcissistic tyranny of the insanely clueless right now, where the sane are called “microtrolls” and the insane and their massive egos think they are the norm of reality? … Sorry, your gaslighting mind game doesn’t work anymore. You are the trolls. [EDIT: I’m criticizing the REPL kids.] The tyrants actually. A tiny minority of new kids, who don’t even try to learn why things are a certain way, but aggressively refuse and try to impose their clueless nonsense onto the world. It’s like when Americans travel to other countries and think other people in those other countries are wrong for not adhering to their American culture. 🤣 … We all used to be clueless kids. But nobody before you hat such a level of Dunning-Kruger-effect ego, that they thought of everyone else as wrong and actually fought it! 🤣 We learned the reasons. Grew wise. But today, it seems fashionable to keep every big toddler in their small bubble for ever and all eternity, and never rattle their beliefs, ever; so they can grow a massively overinflated ego while staying big babies. Like Trump. Truly the definition of toxic modern American culture, that even the America of the past rejects as shameful; let alone the rest of the world that fights being poisoned by that mental underdevelopment. [HINT: If you think that is an “insult”, then that’s exactly what I mean. You guys don’t even realize anymore, that being “insulted” is immature pubescent behavior that you are supposed to grow out of, and shamefully hide if you are, in order to be accepted as a grown-up, and not need as a child anymore. Because everyone around you is a child nowadays. Hint 2: Only where you live. The rest of the world still exists and doesn’t let that Sith pass.] — Preceding unsigned comment added by ~2026-29816-1 (talk) 23:36, 16 January 2026 (UTC)

External links are poor

They seem to be two rarely used implementations of REPL. --Paddy (talk) 03:37, 15 January 2008 (UTC)

History

Has anyone got any historical links or information for REPLs? Who wrote the first REPL? — Preceding unsigned comment added by 94.245.127.12 (talk) 09:20, 20 October 2011 (UTC)

Disproportionately large listing of REPLs

One solution would be to link to Rosetta Code's relevant task demonstrative task, where a list and demonstration of REPL environments may be found. Disclaimer: Rosetta Code is my site, so I can't really make that addition with a full appearance of neutrality. However, I think it would help clean up the listing. (Paddy's an active member, too, but I don't think that happened until about mid-2009. Funny seeing him here, though. :)) --Short Circuit (talk) 22:33, 11 January 2010 (UTC)

C# REPL?

One could argue that the immediate window in Visual Studio is an example of a REPL. Does this count? —Preceding unsigned comment added by BrentRockwood (talk • contribs) 19:22, 1 February 2010 (UTC)

Command shells are not REPLs

I think the list is too long for the simple reason that command shells are not, in general, REPLs. The defining characteristics of a REPL are, primarily, that what you type is an expression to be evaluated and what gets printed is the resulting value, and secondarily, that the REPL is an interface to a programming language's execution engine. That's the semantic model. In a command shell, otoh, what you type is a command which the shell executes, and what it prints is whatever output the command produced, if any. A shell is a command intepreter. You don't think of it as an "interface to the shell's language execution engine" (though technically it is that, of course). The distinction is semantic, but significant.

I also feel that the list should be divided to distinguish between "native" REPL environments (such as in Lisp) and those cases where REPL functionality is available via an add-in (such as in C#). —Preceding unsigned comment added by 65.218.84.25 (talk) 13:58, 29 July 2010 (UTC)

A shell, such as bash can easily be thought of as having an "execution engine" that takes your input and decides whether to execute some internal code to produce an output, or, can find an executable on disc and call that. Input 'ls' and the bash internal command will do its thing; input '/bin/ls' and the external binary will be executed to do its thing. I am not sure if it is standard in Tcl, but I have been in a few Tcl REPL's that would also execute external commands if given an internal command that it did not recognize. I would not want to hive-off what you would call command shells as a separate category. --Paddy (talk) 05:56, 30 July 2010 (UTC)

I removed the sentence which included Unix shells as an example.-- era (Talk | History) 04:38, 25 April 2017 (UTC)

Command-line interfaces are precisely interfaces that read, evaluate, and print, in a loop. It evaluates commands. Which is why every CLI with a text interface is also an interpreted scripting language on any system that treats text input as a file (aka UNIX-likes). Stop retroactively making up redefinitions to justify the “validity”, after everybody laughs at you for not having known the term CLI and making up your own, kids. This is pubescent ego-defending behavior, and inappropriate among grown-ups, let alone in a supposed encyclopedia. — Preceding unsigned comment added by ~2026-29816-1 (talk) 23:08, 16 January 2026 (UTC)

Postscript REPL

I believe Postscript interactive executive is a sort of REPL (not merely a shell), maybe more a "REL" (REPL with no Print) : You type a PS construct and the result is pushed on the operand stack, without being displayed. Displaying the result implies popping it with the '=' or '==' operators. The entire stack can be also be displayed, undisturbed ('pstack' or 'stack'). — Preceding unsigned comment added by 151.66.124.73 (talk) 23:12, 7 July 2011 (UTC)

REPLs vs. Shells vs. CLIs vs.

A REPL is a special type of shell, a form of an interactive toplevel. It is usually not line oriented, thus it is not a command line interface. Lisp is not line oriented. S-expressions are are not delimited by lines, but by a start and end parenthesis. The typical Lisp REPL does not read lines from input, but it reads s-expressions from input.

Let's look at the first paragraph:

A read–eval–print loop (REPL), also known as an interactive toplevel or language shell, is a simple, interactive computer programming environment that takes single user inputs (i.e. single expressions), evaluates them, and returns the result to the user; a program written in a REPL environment is executed piecewise. The term is most usually used to refer to programming interfaces similar to the classic Lisp machine interactive environment. Common examples include command line shells and similar environments for programming languages, and is particularly characteristic of scripting languages.[1]

A REPL does not need to be interactive (!). The first Lisp from McCarthy took expressions from punch cards as a batch job, read every expression, evaluated it, printed it to I/O and did that again until end of input. That's basically what every modern Lisp does, when it loads a Lisp file.

A REPL also does not need to be simple. Many Lisp systems have sophisticated REPLs with error handling, integrated debugger with multi-level debugger REPLs.

The term REPL in the Lisp world also does not usually refer to a classic Lisp Machine environment. Many Lisp systems had a typical REPL before and after Lisp Machines existed - Interlisp, Maclisp, ..., CLISP, SBCL, ...

Command line shells are usually not a REPL. A REPL is also not a characteristic of scripting language. Scripting languages often have interactive toplevels, but since most of them are not homoiconic, they don't use Read/Eval/Print like Lisp, where the REPL/toplevel reads a data structure and prints a datastructure.

Joswig (talk) 17:05, 12 October 2015 (UTC)

You do realize that having no actual arguments, merely stating things as “being“ that way, and compensating with loudness (“(!)”), only strengthens the existing observation of your belief being invalid? A CLI, by definition, is an interface, where you input a command, which gets read, evaluated, and its results printed, in a loop. Which is why every shell with a CLI is also a scripting language. “REPL” was only abused as a term by children who grew up, having never used a CLI, and when their scripting language got a CLI, looked for a term for it, and stumbled onto “REPL”, because CLIs were so foreign to them. … And now that everybody laughs at them, they desperately try to come up with a justification and conveniently try to redefine those terms to be “different”, but at the same time need them to still be the same, … which leads to funny trainwrecks like this attempt. 😁 … Just chill. You’re not an idiot. You could not have known what you never came across. It’s OK. But please stop acting like pubescent children / Americans with unstable egos who link everything with their ego and think they need to “defend” ind and double down on it (“doing a Trump”). … I was a clueless kid too. And if I didn’t learn from my ancestors and acted like that, I still would be. :) [Go watch a few videos on how Unix and especially Plan 9 works, and why, and about Bell Labs. Like I did. It’s fun and offers much wisdom.] — Preceding unsigned comment added by ~2026-29816-1 (talk) 23:18, 16 January 2026 (UTC)

This article should not exist

It’s extremely obvious that this term was invented by “computer-illiterate techie” kids who have never heard of the term “command-line interface”, decided to make up their own, and when laughed at for it, in a failed attempt to justify it and thereby their ego, came up with a definition that is intentionally vague in a way that it both means exactly the same thing but can mean a merely 99% overlapping but less useful set too. 😁

It’s a CLI. Obviously. All CLIs read, evaluate and print in a loop. There is no need for a new term. Just like with Apple religious jewelry, its users are the only ones who believe it makes them superior and that they are “the times” to get with. Everyone else laughs and watches them drive, not forward, not backward, but sideways, off the cliff.

Despite the certainty of certain loud permanently pubescent egos being certainly rustled [😁], let’s merge it as a

A command-line interface (CLI), sometimes called a command-line shell, or a read–eval–print loop (REPL) by some people, …

into the CLI article.

— ~2026-29816-1 (talk) 23:01, 16 January 2026 (UTC)

See Read–eval–print loop § History for early uses of "READ-EVAL-PRINT cycle" and "read-eval-print loop"; the former dates back to a LISP implementation in 1964, and the latter dates back at least to another LISP implementation in 1974. [[L. Peter Deutsch[[ is currently 79, Edmund Berkeley would, if still alive, be 117, and David A. Moon is probably in his 70's as well.

If you're complaining that only "computer-illiterate techie kids" would call a CLI a REPL, that's another matter. Guy Harris (talk) 23:32, 16 January 2026 (UTC)