REPLs are nice but they work well only for reasonably isolated code with few dependencies. It's hard to set up a complex object to pass into a function. It's harder still to set up an elaborate context of dependencies around that function.

Now, not every programmer prefers that kind of development. Some programmers prefer to think of development as a process of designing, planning, making blueprints, and assembling parts on a workbench. There’s nothing wrong with that. Indeed, a multibillion-dollar international industry has been built upon it.

In fact, there’s a style of programming, well known in Lisp and Smalltalk circles, in which you define a toplevel function with calls to other functions that don’t yet exist, and then define those functions as you go in the resulting breakloops. It’s a fast way to implement a procedure when you already know how it should work.

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.