Keep a REPL Open

I keep a REPL open at all times whether or not I’m actually doing Lisp development. It’s my assistant that evaluates random expressions for me. I’ll script up little tasks in Common Lisp rather than in Bash. When I need to rapidly prototype something larger, I’ll switch to the REPL and do it there.

At work, my REPL has accumulated a bunch of code for talking to systems like GitHub, CircleCI, and LDAP as well as our in-house tools. These are utilities for my use only. I don’t write mission critical apps in Common Lis. No one else in the company uses it, and it is more important that the code be maintainable by the rest of the team than that it be written in a language I like. So I write the mission critical code in Python, or Golang, or Java, or whatever the rest of the team is using. I keep my Common Lisp to myself. I have, however, used it to protype code that evetually ends up ported to Python or Golang.

On occasion, I’ve wanted to quickly share some functionality before I have taken the time to port it. I’ve found two ways to do this. The first is to slap a web server on it. I use Hunchentoot for this. I translate JSON to Lisp coming in to the web server and Lisp back to JSON going out. This is all you effectively need for a black-box microservice. There have been a couple of transient projects where the whole thing was not expected to be maintained for a long time and by anyone other than me, so I can just throw up a microservice and tell my colleagues to hit it with a curl command.

The second way is to create a docker image that contains the Common Lisp code and all of its dependencies. It can take a bit of work to configure a lisp setup in your environment, so having it hiding inside a docker image allows me to correctly set up the Lisp environment along with the Lisp interpreter and the rest of the code. My colleagues can just pull and run the container and it will work. Again, this is only for small, throwaway projects that no one else is expected to modify or maintain. For anything that is mission critical or is expected to be shared at some point, I write it in Python or Golang or Java, etc.

I could have written these as a series of Bash scripts or Python programs, but when you start connecting a series of these together, you quickly run into the limitations of using a pipe to talk between programs. My Lisp scripts all reside in the same address space, so they can share structured data without any fancy marshaling protocol.