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If your language doesn't have a REPL, your language is less productive. Deal with it. Oh hardly. As someone who's developed software in Perl, Python, C, Java,

If your language doesn't have a REPL, your language is less productive. Deal with it. Oh hardly. As someone who's developed software in Perl, Python, C, Java, C#, Haskell, and a few others besides, a REPL is the one thing I use the least. I've honestly never understood the obsession with it. Is a rapid compile-run-debug cycle important? Absolutely. If I want to test something in isolation, being able to rapidly turn out a UT to prove my code does what I think it does is incredibly important. But a REPL is a nice-to-have. Nothing more. Treating its absence as some fundamental black mark on a language is completely absurd.

sillysaurus3 9y ago
I tend to agree, but being able to have a REPL at all is a symptom that your language is capable of an extremely fast compile-run-debug cycle. In other words, if you can't implement a REPL, that's the definition of "less productive language." It's not about the REPL itself but the capability of implementing one. Think about it like this: On your list of languages, which of them would you say is most productive? It depends on the context, certainly: libraries, familiarity, etc. But let's assume the languages had identical ecosystems, and that you were competent in all of them. Which of them would take the smallest amount of time to write your feature? That'd probably be Python, Haskell, or even Perl. It seems very difficult to argue that Rust would top them. And the reasons why this is might be true are worth examining. Here's another aspect. Is the Rust community claiming that the entire history of computing has lead up to this point, where Rust exists, and now nobody needs to write anything in not-Rust? Are people 200 years from now going to write Rust and nothing else? How about 20? 2? It becomes very difficult to argue that your programming language is the lingua franca of the future. (And that's probably true for every language, not just Rust.)
- zzalpha 9y ago
  Think about it like this: On your list of languages, which of them would you say is most productive? Define productive. Are you measuring raw lines of code generated? Or are we measuring amount of production-ready code? My bet: with an experienced, diligent developer it's a wash. My gut says what you lose in a slow code-execute cycle of static, compiled languages you gain in lower defect rates from compile-time checks. Conversely, what you gain in a dynamic language, you lose in errors caught at run-time. At that point I'd select for languages that are simpler with fewer surprising semantics (crossing Perl and Javascript off the list), with more built-in run-time and/or compiler safety (knocking things like C off the list). I generally also prefer compile-time safety over run-time safety, as I'd rather find problems on my workstation than in the field because I missed testing a codepath.
  - kbenson 9y ago
    > with fewer surprising semantics (crossing Perl and Javascript off the list) Actually, I think Perl has fairly few surprising semantics after you learn the major ones. The real problem is that the major one (or specifically the major one, context) is somewhat hidden in normal C-like usage until it jumps out at you and confuses you. Once you understand what it is, it's not all that surprising (but there are a few annoyances with it that still persist and cause problems). You can write Perl that looks almost exactly like C 95% of the time, but that 5% where it matters makes people that do this very confused. There are, of course, other parts of the language that are poorly chosen, but I maintain there are actually far less of them than many people seem to think. As for JavaScript, I still can't get over all the different ways to check for equality and null or undefined. I understand a good portion of that is because I never have to use JS enough to become truly proficient, and some if it is just poor design.
- tatterdemalion 9y ago
  > Is the Rust community claiming that the entire history of computing has lead up to this point, where Rust exists, and now nobody needs to write anything in not-Rust? Are people 200 years from now going to write Rust and nothing else? Let's be clear - the person who said there'd be no reason to write in anything else is an enthusiastic Rust user, but they don't work on Rust & don't represent "the Rust community." They were speaking from their own experience, based on their own balancing of the trade offs as a user. Presumably a REPL doesn't rate very highly for them. However, the reasons we don't have a REPL are mainly two: the compiler isn't architected for it and code doesn't compile fast. The solutions are: * Re-architect the compiler to support incremental code input. This is pretty similar to what we need to support IDEs (which we're working on), so possibly this will solve the problem. * Get a reimplementation of the backend which is a JIT interpreter instead of LLVM, which takes up most of our compiletime. Work is underway on a project called miri, though it isn't officially supported right now. * Make the typechecking part of the compiler faster. This is underway through a project called chalk. In other words, a REPL is not impossible, it just isn't a top priority right now. There are other lacking tools that we're working on - IDE integration and auto-formatting for example - maybe after those are done it will be the top of the docket.
  - sillysaurus3 9y ago
    Let's be clear - the person who said there'd be no reason to write in anything else is an enthusiastic Rust user, but they don't work on Rust & don't represent "the Rust community." Respectfully, the community has become increasingly vocal, and it's getting hard to separate the enthusiasm from realism. It's embedded in every HN thread. That's a good problem to have, but it can be a bit repellant to people who aren't Rust converts. Thanks for the breakdown and future roadmap. I hope the compile times can be kept to a minimum in order to avoid C++'s fate.
    - kibwen 9y ago
      The community is in every thread because the people here were HN users before they were Rust users. Look at how old my account is; I haven't quite been shilling Rust since December 2011. :P
- tannhaeuser 9y ago
  > It's not about the REPL itself but the capability of implementing one. That definition excludes any bare-metal language, and use cases where you want to take advantage of cheap low-level memory protection features such as No-Execute bits. For me, a more interesting benchmark would be how well Rust fares in implementing the runtime of a higher-level language.
- derefr 9y ago
  Why must productivity be measured in "smallest amount of time to write your feature"? Why not in "smallest Total Cost of Ownership [in man-hours] of the feature over the project lifecycle"? Some projects are written as explorational prototypes or MVPs. I don't think anyone is saying Rust is much good for those. But (numerous) other projects are written as "the first real Quality implementation of [well-known problem], using a decade's experience with other implementations." Much of the common software we use—{server daemons (HTTP, SSH, DNS...), parsers and encoding libraries (for XML or JSON; JPEG or PNG; MP4 or MKV...), databases, load-balancers, distributed queues, ...}—fits this paradigm. If you're writing Nginx, or Redis, or djbdns, you aren't "adding features" out of some agile user-story kanban; you're carefully implementing a small, curated set of well-known, well-understood features, with much research done to ensure that you arrive at the best and least fraught implementation, the one that will make people prefer your software for its quality and reliability and set-and-forget nature. Rust is for that kind of software. (Which makes sense, given that Mozilla's Servo rendering engine also has the goal of being that kind of software.)
  - nickpsecurity 9y ago
    "Why must productivity be measured in "smallest amount of time to write your feature"? Why not in "smallest Total Cost of Ownership [in man-hours] of the feature over the project lifecycle"?" In the case of startups, their goal is to sell out way before the maintenance phase of the lifecycle. So, it doesn't really matter to them so much as quickly getting out working features. In other cases, esp long-lasting endeavors, then this is a good point to consider.
    - kbenson 9y ago
      > In the case of startups, their goal is to sell out way before the maintenance phase of the lifecycle. Or grow so quickly that replacing most the legacy code from a year or two ago is not a significant hurdle at their new size. In this vein, I wonder if there's actually a third path that might be better than both. Using a low-level but fairly expressive and strongly typed language but very loosely, such that it allows you to iterate fact but with less performance, and you can come back later and replace chunks as needed, but with the same language. I don't have any real experience with that, and I could see it going either way for a number of reasons, but it seems like we have more languages that might fit that criteria now (or at least they are more popular now) than we have previously.
dwaltrip 9y ago
Just because you don't understand the benefits other people get from a REPL doesn't mean they don't exist.
- zzalpha 9y ago
  Just because there are benefits, doesn't make it a deal-breaker.
  - dwaltrip 9y ago
    I didn't say it was a deal breaker? Your post came off as implying REPLs don't have serious benefits for anyone, which felt a bit presumptuous to me.
    - zzalpha 9y ago
      Yup, that's fair. BTW, I actually stand by your interpretation. I challenge anyone to demonstrate significant productivity gains (as measured by number of lines of tested, production code written) that can be attributed to the availability of a REPL (which was the claim of the poster I was replying to). I honestly don't buy it. I believe it's a very nice tool for learning a new language, and maybe for exploring an existing codebase, but I can't envision a workflow where a REPL provides significant productivity gains for an experienced, professional software developer that aren't realized in other ways (e.g. writing unit tests).
      - goatlover 9y ago
        Lispers might be able to meet that challenge.
        nickpsecurity 9y ago
        I could probably have countered it, too, in the same language during the challenge back when I used it. I found the incremental, per-function compilation feature reduced compilation time of most changes to a fraction of a second. Most of time in programming is spent thinking, tweaking, debugging, dealing with 3rd-party libs, etc. I don't think a difference in the source-to-execution part of under 1 second is going to make a big deal. It's high-speed iteration of code one is exploring that makes both REPL's and incremental compilation superior for productivity to full compilation.
zambal 9y ago
The usefulness of a REPL depends a lot on the language/interpreter/vm. In Erlang and Elixir the REPL is more than nice to have. Being able to connect to a production node and peeking around what is happening can be very valuable. I guess it's one of those things that's imho hard to appreciate if you never experienced it.
- zzalpha 9y ago
  The usefulness of a REPL depends a lot on the language/interpreter/vm. In Erlang and Elixir the REPL is more than nice to have. Being able to connect to a production node and peeking around what is happening can be very valuable. That's called a remote debugger, and it's hardly unique to languages supporting a REPL.
zambal 9y ago
The usefulness of a REPL depends a lot on the language/interpreter/vm. In Erlang and Elixir the REPL is imo more than a nice to have. Being able to connect to a production node and peeking around what is happening can be very valuable. I guess it's one of those things that's imho hard to appreciate if you never experienced it.
derefr 9y ago
> I've honestly never understood the obsession with it. The true key to understanding REPLs is their role in consuming third-party libraries. While languages both with and without REPLs will have well-used packages in their package ecosystems that are well-documented; languages with REPLs will also manage to have well-used, poorly-documented packages: packages whose "documentation" consists only of the de-facto ability to reflect on the API in a REPL, doing the moral equivalent of exploring a filesystem using cd(1) and cat(1), until you figure out what functions—and what arguments—combine to produce an acceptable result for your use-case. That may sound like a bad thing, but it's not! Those packages are packages that wouldn't exist otherwise; they're marginal packages, packages which people wouldn't have had time to write if they were required to also document them properly enough to make use of them without a REPL. They're needs being satisfied, that would otherwise—given the stricter requirement of useful API documentation—be going un-met. If you look at e.g. Ruby's gem ecosystem, there are tons of packages that have more than a million downloads, and either no docs or awful docs. How? Because Ruby is extremely amenable to exploring APIs through a REPL. You take a module or class, ask it what its children are, what its class methods are, what its instance methods are. Find a class with instance methods that sound right, attempt to instantiate it. Figure out you can't do that, look around the class methods for something that sounds like a factory method. What arguments does said factory-function take? Well, feed it some at random and see what error you get. Etc.
- oblio 9y ago
  Many languages have good IDE support, which is basically REPL on steroids.
  - derefr 9y ago
    True, though "good IDE support" in the sense you're talking about (i.e. being able to discover an API through the IDE) mostly relies on strong typing in the language. (Non-inferred) type declarations are their own form of documentation, with a similar effect on likelihood of a "marginal" ecosystem package getting written.
    - deleted 9y ago
pjmlp 9y ago
I love using a REPL, but I kind of agree with you. I happen to use more often the IDE's graphical debugger than the REPL. Still Visual Studio has had immediate mode (aka REPL) since VB days, followed by .NET support.
sametmax 9y ago
As a Python dev with 12 years of experience, I still use the REPL every day. When I wonder how something works or if I need my memory to be refreshed, I just fire up jupyter console and test quickly the code. I don't open the doc or search on the net. I have the live result immediatly. When I want to debug my web server, I drop a REPL in the server and play with the code. It's more efficient that just debugging, because I can actually modify the code live and see what happens. When I need to understand a format and how to manipulate / tranform data, I use the REPL (or jupyter notebook) because it's the most natural way to do it. I don't know how much Python you have done, but you have been missing one of the greatest tool of the ecosystem. It's like saying you don't see that much the benefits of an IDE for refactoring in Java. "yeah it's cool but how much do you use it hu ?".

sillysaurus3 9y ago

I tend to agree, but being able to have a REPL at all is a symptom that your language is capable of an extremely fast compile-run-debug cycle. In other words, if you can't implement a REPL, that's the definition of "less productive language." It's not about the REPL itself but the capability of implementing one. Think about it like this: On your list of languages, which of them would you say is most productive? It depends on the context, certainly: libraries, familiarity, etc. But let's assume the languages had identical ecosystems, and that you were competent in all of them. Which of them would take the smallest amount of time to write your feature? That'd probably be Python, Haskell, or even Perl. It seems very difficult to argue that Rust would top them. And the reasons why this is might be true are worth examining. Here's another aspect. Is the Rust community claiming that the entire history of computing has lead up to this point, where Rust exists, and now nobody needs to write anything in not-Rust? Are people 200 years from now going to write Rust and nothing else? How about 20? 2? It becomes very difficult to argue that your programming language is the lingua franca of the future. (And that's probably true for every language, not just Rust.)

zzalpha 9y ago
Think about it like this: On your list of languages, which of them would you say is most productive? Define productive. Are you measuring raw lines of code generated? Or are we measuring amount of production-ready code? My bet: with an experienced, diligent developer it's a wash. My gut says what you lose in a slow code-execute cycle of static, compiled languages you gain in lower defect rates from compile-time checks. Conversely, what you gain in a dynamic language, you lose in errors caught at run-time. At that point I'd select for languages that are simpler with fewer surprising semantics (crossing Perl and Javascript off the list), with more built-in run-time and/or compiler safety (knocking things like C off the list). I generally also prefer compile-time safety over run-time safety, as I'd rather find problems on my workstation than in the field because I missed testing a codepath.
- kbenson 9y ago
  > with fewer surprising semantics (crossing Perl and Javascript off the list) Actually, I think Perl has fairly few surprising semantics after you learn the major ones. The real problem is that the major one (or specifically the major one, context) is somewhat hidden in normal C-like usage until it jumps out at you and confuses you. Once you understand what it is, it's not all that surprising (but there are a few annoyances with it that still persist and cause problems). You can write Perl that looks almost exactly like C 95% of the time, but that 5% where it matters makes people that do this very confused. There are, of course, other parts of the language that are poorly chosen, but I maintain there are actually far less of them than many people seem to think. As for JavaScript, I still can't get over all the different ways to check for equality and null or undefined. I understand a good portion of that is because I never have to use JS enough to become truly proficient, and some if it is just poor design.
tatterdemalion 9y ago
> Is the Rust community claiming that the entire history of computing has lead up to this point, where Rust exists, and now nobody needs to write anything in not-Rust? Are people 200 years from now going to write Rust and nothing else? Let's be clear - the person who said there'd be no reason to write in anything else is an enthusiastic Rust user, but they don't work on Rust & don't represent "the Rust community." They were speaking from their own experience, based on their own balancing of the trade offs as a user. Presumably a REPL doesn't rate very highly for them. However, the reasons we don't have a REPL are mainly two: the compiler isn't architected for it and code doesn't compile fast. The solutions are: * Re-architect the compiler to support incremental code input. This is pretty similar to what we need to support IDEs (which we're working on), so possibly this will solve the problem. * Get a reimplementation of the backend which is a JIT interpreter instead of LLVM, which takes up most of our compiletime. Work is underway on a project called miri, though it isn't officially supported right now. * Make the typechecking part of the compiler faster. This is underway through a project called chalk. In other words, a REPL is not impossible, it just isn't a top priority right now. There are other lacking tools that we're working on - IDE integration and auto-formatting for example - maybe after those are done it will be the top of the docket.
- sillysaurus3 9y ago
  Let's be clear - the person who said there'd be no reason to write in anything else is an enthusiastic Rust user, but they don't work on Rust & don't represent "the Rust community." Respectfully, the community has become increasingly vocal, and it's getting hard to separate the enthusiasm from realism. It's embedded in every HN thread. That's a good problem to have, but it can be a bit repellant to people who aren't Rust converts. Thanks for the breakdown and future roadmap. I hope the compile times can be kept to a minimum in order to avoid C++'s fate.
  - kibwen 9y ago
    The community is in every thread because the people here were HN users before they were Rust users. Look at how old my account is; I haven't quite been shilling Rust since December 2011. :P
tannhaeuser 9y ago
> It's not about the REPL itself but the capability of implementing one. That definition excludes any bare-metal language, and use cases where you want to take advantage of cheap low-level memory protection features such as No-Execute bits. For me, a more interesting benchmark would be how well Rust fares in implementing the runtime of a higher-level language.
derefr 9y ago
Why must productivity be measured in "smallest amount of time to write your feature"? Why not in "smallest Total Cost of Ownership [in man-hours] of the feature over the project lifecycle"? Some projects are written as explorational prototypes or MVPs. I don't think anyone is saying Rust is much good for those. But (numerous) other projects are written as "the first real Quality implementation of [well-known problem], using a decade's experience with other implementations." Much of the common software we use—{server daemons (HTTP, SSH, DNS...), parsers and encoding libraries (for XML or JSON; JPEG or PNG; MP4 or MKV...), databases, load-balancers, distributed queues, ...}—fits this paradigm. If you're writing Nginx, or Redis, or djbdns, you aren't "adding features" out of some agile user-story kanban; you're carefully implementing a small, curated set of well-known, well-understood features, with much research done to ensure that you arrive at the best and least fraught implementation, the one that will make people prefer your software for its quality and reliability and set-and-forget nature. Rust is for that kind of software. (Which makes sense, given that Mozilla's Servo rendering engine also has the goal of being that kind of software.)
- nickpsecurity 9y ago
  "Why must productivity be measured in "smallest amount of time to write your feature"? Why not in "smallest Total Cost of Ownership [in man-hours] of the feature over the project lifecycle"?" In the case of startups, their goal is to sell out way before the maintenance phase of the lifecycle. So, it doesn't really matter to them so much as quickly getting out working features. In other cases, esp long-lasting endeavors, then this is a good point to consider.
  - kbenson 9y ago
    > In the case of startups, their goal is to sell out way before the maintenance phase of the lifecycle. Or grow so quickly that replacing most the legacy code from a year or two ago is not a significant hurdle at their new size. In this vein, I wonder if there's actually a third path that might be better than both. Using a low-level but fairly expressive and strongly typed language but very loosely, such that it allows you to iterate fact but with less performance, and you can come back later and replace chunks as needed, but with the same language. I don't have any real experience with that, and I could see it going either way for a number of reasons, but it seems like we have more languages that might fit that criteria now (or at least they are more popular now) than we have previously.

dwaltrip 9y ago

Just because you don't understand the benefits other people get from a REPL doesn't mean they don't exist.

zzalpha 9y ago
Just because there are benefits, doesn't make it a deal-breaker.
- dwaltrip 9y ago
  I didn't say it was a deal breaker? Your post came off as implying REPLs don't have serious benefits for anyone, which felt a bit presumptuous to me.
  - zzalpha 9y ago
    Yup, that's fair. BTW, I actually stand by your interpretation. I challenge anyone to demonstrate significant productivity gains (as measured by number of lines of tested, production code written) that can be attributed to the availability of a REPL (which was the claim of the poster I was replying to). I honestly don't buy it. I believe it's a very nice tool for learning a new language, and maybe for exploring an existing codebase, but I can't envision a workflow where a REPL provides significant productivity gains for an experienced, professional software developer that aren't realized in other ways (e.g. writing unit tests).
    - goatlover 9y ago
      Lispers might be able to meet that challenge.
      - nickpsecurity 9y ago
        I could probably have countered it, too, in the same language during the challenge back when I used it. I found the incremental, per-function compilation feature reduced compilation time of most changes to a fraction of a second. Most of time in programming is spent thinking, tweaking, debugging, dealing with 3rd-party libs, etc. I don't think a difference in the source-to-execution part of under 1 second is going to make a big deal. It's high-speed iteration of code one is exploring that makes both REPL's and incremental compilation superior for productivity to full compilation.

zambal 9y ago

The usefulness of a REPL depends a lot on the language/interpreter/vm. In Erlang and Elixir the REPL is more than nice to have. Being able to connect to a production node and peeking around what is happening can be very valuable. I guess it's one of those things that's imho hard to appreciate if you never experienced it.

zzalpha 9y ago
The usefulness of a REPL depends a lot on the language/interpreter/vm. In Erlang and Elixir the REPL is more than nice to have. Being able to connect to a production node and peeking around what is happening can be very valuable. That's called a remote debugger, and it's hardly unique to languages supporting a REPL.

zambal 9y ago

The usefulness of a REPL depends a lot on the language/interpreter/vm. In Erlang and Elixir the REPL is imo more than a nice to have. Being able to connect to a production node and peeking around what is happening can be very valuable. I guess it's one of those things that's imho hard to appreciate if you never experienced it.

derefr 9y ago

> I've honestly never understood the obsession with it. The true key to understanding REPLs is their role in consuming third-party libraries. While languages both with and without REPLs will have well-used packages in their package ecosystems that are well-documented; languages with REPLs will also manage to have well-used, poorly-documented packages: packages whose "documentation" consists only of the de-facto ability to reflect on the API in a REPL, doing the moral equivalent of exploring a filesystem using cd(1) and cat(1), until you figure out what functions—and what arguments—combine to produce an acceptable result for your use-case. That may sound like a bad thing, but it's not! Those packages are packages that wouldn't exist otherwise; they're marginal packages, packages which people wouldn't have had time to write if they were required to also document them properly enough to make use of them without a REPL. They're needs being satisfied, that would otherwise—given the stricter requirement of useful API documentation—be going un-met. If you look at e.g. Ruby's gem ecosystem, there are tons of packages that have more than a million downloads, and either no docs or awful docs. How? Because Ruby is extremely amenable to exploring APIs through a REPL. You take a module or class, ask it what its children are, what its class methods are, what its instance methods are. Find a class with instance methods that sound right, attempt to instantiate it. Figure out you can't do that, look around the class methods for something that sounds like a factory method. What arguments does said factory-function take? Well, feed it some at random and see what error you get. Etc.

oblio 9y ago
Many languages have good IDE support, which is basically REPL on steroids.
- derefr 9y ago
  True, though "good IDE support" in the sense you're talking about (i.e. being able to discover an API through the IDE) mostly relies on strong typing in the language. (Non-inferred) type declarations are their own form of documentation, with a similar effect on likelihood of a "marginal" ecosystem package getting written.
  - deleted 9y ago

pjmlp 9y ago

I love using a REPL, but I kind of agree with you. I happen to use more often the IDE's graphical debugger than the REPL. Still Visual Studio has had immediate mode (aka REPL) since VB days, followed by .NET support.

sametmax 9y ago

As a Python dev with 12 years of experience, I still use the REPL every day. When I wonder how something works or if I need my memory to be refreshed, I just fire up jupyter console and test quickly the code. I don't open the doc or search on the net. I have the live result immediatly. When I want to debug my web server, I drop a REPL in the server and play with the code. It's more efficient that just debugging, because I can actually modify the code live and see what happens. When I need to understand a format and how to manipulate / tranform data, I use the REPL (or jupyter notebook) because it's the most natural way to do it. I don't know how much Python you have done, but you have been missing one of the greatest tool of the ecosystem. It's like saying you don't see that much the benefits of an IDE for refactoring in Java. "yeah it's cool but how much do you use it hu ?".