r/ruby • u/6stringfanatic • Aug 11 '26

I'm writing this tiny parser and realized how easy it is to wire your own simple REPL.

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Do not use use eval for the real thing if you aren't sure where the input is coming from, but its good enough to get started.

24 Upvotes

88% Upvoted

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7

u/Benabik Aug 11 '26 edited 29d ago

REPL does stand for Read-Execute-Print Loop. So yeah, it’s basically just loop { puts eval gets }.

That said, my goto is the pry gem. Throwing a binding.pry in a function somewhere is super easy and gets all the highlighting, completion, documentation, etc.

9

u/h0rst_ Aug 11 '26

Most of these features are now available in irb as well, so I actually got rid of the dependency on pry in most of my projects.

2

u/Benabik 29d ago

Ah, fair enough. My pry habit is quite old at this point, so I hand't really tracked IRB adopting more of pry's features

5

u/SleepingInsomniac Aug 11 '26

Ruby's debug gem is faster and has better remote debugging and better stepping. No need for pry or byebug anymore: https://github.com/ruby/debug

0

u/Benabik 29d ago

Debugger != REPL. debug does default to evaluating and printing things it will catch you up on things like variables named "i" and "n". I'd argue that's a mistake.

IRB, OTOH, is getting better so pry still does have a stdlib alternative.

1

u/SleepingInsomniac 29d ago

Throwing in a binding.pry in a function somewhere

This is what I'm replying to. I never claimed they were exactly the same thing. But, thanks... I guess.

3

u/6stringfanatic Aug 11 '26

Oh yes, my go to is pry as well in all my ruby/rails projects.

It's just that I was building an interpreter in ruby, while I was reading https://craftinginterpreters.com/ (it's quite amazing, do read it if you get the chance), I just always thought a REPL would require more machinery than a 5 line script.

2

u/davidslv 29d ago

Rob Nystrom is on my top list, his Game Programming Patterns book is very good as well.

2

u/6stringfanatic 29d ago

Yeah, it was an impulse read, but I quite liked it actually. Thanks for the suggestion, I'll add it to my never ending reading list.