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I worked with a 60K LOC thing* that talked to multiple services and had complex configuration. Ran fine on my laptop pointed at the company's dev env.

The REPL let me test my changes while inside the thing as it ran. No problems. Someone wrote a nice *recording* debugger too which helped immensely -- no more "oops, I'm past the interesting part and have to start over"

* in prod we usually give it a small number of large instances

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Forgot to add: multiple threads ... multiple DBs ... multiple topics on more than one kind of message queue ... hitting and serving REST endpoints -- all no problem for the REPL and the debugger[0].

If the thing was in Java, each fix attempt would mean waiting for startup and state re-creation. And each successful debug could have meant multiple sessions (vs visiting any mix of spots in a single recording)

[0] https://www.flow-storm.org/

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