Define a function, foo, that calls some other function, bar, that is not yet defined. Now call foo. What happens?

Obviously, the call to foo breaks, because bar is not defined. But what happens when it breaks?

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The answer to that question is the differentiating point of repl-driven programming. In an old-fashioned Lisp or Smalltalk environment, the break in foo drops you into a breakloop.

A breakloop is a full-featured repl, complete with all of the tools of the main repl, but it exists inside the dynamic environment of the broken function. From the breakloop you can roam up and down the suspended call stack, examining all variables that are lexically visible from each stack frame. In fact, you can inspect all live data in the running program.

What’s more, you can edit all live data in the program. If you think that a break was caused by a wrong value in some particular variable or field, you can interactively change it and resume the suspended function.

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Moreover, because the entire language and development system are available, unrestricted, in the repl, you can define the missing function bar, resume foo, and get a sensible result.