If I'm debugging something I'm writing myself it's usually quite obvious where the problem probably is. With other people's code it takes a bit of digging, but

If I'm debugging something I'm writing myself it's usually quite obvious where the problem probably is. With other people's code it takes a bit of digging, but is usually found without a debugger. Just print enough stuff, with interpreted languages this is usually really fast. Something like Javascript's console.log is actually really nice for this (and browsers have hands down best debugging tools anyway). I'm not sure how REPL helps you out of this. You still have to somehow change the state of the program, but if you "monkeypatch" it using REPL, you now have to keep in your head what the state is. In the end code is just description of how to bring the program to some state. I like to have that description on file so I don't have to keep it in my head.