I was writing a new language the other day and I thought, “this puppy needs a REPL”!

But before I could write one, I had to decide how it would look and behave. I mean, I knew the basics: take something in, execute it, then display the result. But how do you open the help? How do you handle multi-line input? Can I use terminal colors? What does the prompt look like?

To answer that last one, I took a quick survey of my favorite languages - turns out they’ve all coalesced to the > prompt, but there are some fun variations:


> 2 + 3;;
val it : int = 5

F# is my favorite language, but the REPL is a bit busy for me. First the language dictates this weird crying emoji (;;) in input and the result is always encumbered by val it : noise. But I still <3 you F#.


csharp> 2 + 3

Much cleaner! No semicolons, and just the answer. Well C# does show its vanity a bit with its name announcement on each line - but heh, it deserves it.


>>> 2 + 3

Elegant and bold at the same time. The >>> means Python, but you don’t actually say “Python”. So hipster, so cool. I’m sure I could copy >>> as remixing is hip these days; but no, I’d be trying too hard.


irb(main):009:0> 2 + 3
=> 5

OK, I get what you’re going for here Ruby. Part of me even likes it. But no. Too much. I would expect this kind of complexity and technical jargon when logging into my refrigerator - but my dev environments should have a little more refinement.

On the plus side - Ruby outputs the answer in yellow. I’m totally stealing that.

Oh and geeze, they use the same symbol as Calca - so how could I not love that?


> 2 + 3

Look at you. Simple, reasonable, well thought out. It’s like staring at an oil on canvas painting containing only the lowercase Helvetica a.


While we have standardized on > as the one prompt to rule them all, there is a fair amount of diversity as to what comes before it.

I’m a fan of simplicity and in the end, I went with C#’s vanity> prompt. Cause, like, I’m vain.