[brundolf]

Maybe I've just never given it a chance, but I've never understood the appeal of being able to modify code in-memory while it's running.

I like a REPL for testing things out, or for doing quick one-off computations, but that's it. I would never want to, say, redefine a function in memory "while the code is running". Not just because of ergonomics, but because if I decide to keep that change, I now have to track down the code I typed in and manually copy it back over into my source files (assuming I can still find it at all). And if I make a series of changes over a session, the environment potentially gets more and more diverged from what's in sourced if I forget to copy any changes over. So I'd often want to re-load from scratch anyway, at least before I commit.

Am I missing something? Am I misunderstanding what people mean when they talk about coding from a REPL?

[Jach]

You can get an approximate taste of what it's like in plain old Java + JRebel, it's seriously about the same as trying to do it with Python + something like Flask. Start up a big application server in debug mode with Eclipse/IntelliJ, be annoyed that it takes 2+ minutes to start after everything's compiled. Now you want to work on a story, or some bug. You can make changes, save, it recompiles just that file (incremental compilation is a godsend in itself), and hotswaps it into the running application memory. No need to restart anything (with JRebel, for most common kinds of changes; without JRebel, only for some changes). It's particularly useful when you're debugging, you find the problem, change the code, and re-execute immediately to verify to yourself it's fixed.

You also can get an experience like PHP, where you just have to change and save some files, and your subsequent requests will use the new code. This is so much better than shutting down everything and restarting and is a large part of why CGI workflows dominated the web.

Common Lisp takes these experiences and dials them to 11, the whole language is built to reinforce the development style of dynamic changes, rather than an after-thought that requires a huge IDE+proprietary java agent. It's still best to use some sort of editor or IDE, and then you don't have any worry about source de-syncs -- frequently you'll make multiple changes across multiple files and then just reload the whole module and any files that changed with one function call, which you might bind to an editor shortcut, but crucially like debugging is not centrally a feature of the editor but the language; the language's plain REPL by itself is just a lot more supportive of interactive development than Python/JS/Ruby's. Clojure, and I personally think even Java with the appropriate tools, are between Python and CL for niceness of interactive development, but Clojure tends to be better than the Java IDE experience because of its other focus on immutability.

[NightMKoder]

As others have mentioned - you’re probably talking about something like a python or node repl. Lisp repl development is not like python or node - you _work_ in the repl. The closer comparison might be between bash+node as a repl - up+enter and the like to rerun tests has an equivalent in clojure+IntelliJ. There’s no copy pasting but there are different key bindings.

One of the best parts about lisp style repl development is that you end up doing TDD automatically. You just redefine a function until it does what you want from sample data you pass in - without changing files or remembering how your test framework works. You can save the output of some http call in a top level variable and iterate on the code to process it into something useful. The code you evaluate lives in the file that will eventually house it anyway so it’s pretty common to just eval the entire file instead of just one function.

Since you don’t ever shut the repl down, developing huge apps is also quite pleasant. You only reload the code that you’re changing - not the rest of the infra so things like “memoize” can work in local development. That’s why it’s a bit closer to your bash shell in other languages.

If you’ve never tried it, I highly recommend trying the Clojure reloaded workflow [1] to build a web app with a db connection. You can really get into a flow building stuff instead of waiting for your migrations to run on every test boot.

[1] https://cognitect.com/blog/2013/06/04/clojure-workflow-reloa...

[shaunxcode]

Yes, what you are missing is that usually you are using an editor like emacs where you can modify a specific form and send it to the repl. That way there is no chasing back through repl history for a form to paste back into a file.

[sooheon]

Instead of typing things into a repl and copying back into source, imagine stepping through your source in a debugger.