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    The CL condition system (and similar stuff in Smalltalk) demonstrates how CL is a programming system and not just a programming language. Conditions in a single program tend to have limited usefulness, but in the scope of a larger environment (read: many programs interacting) they can be remarkably useful.

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      Whenever I see CL’s condition system it seems to me that we could achieve the same effect by passing handlers as lambdas to a computation. Or, in the case of an OO language, creating an object for the computation and parameterizing its construction with strategy objects as the handlers.

      Then, I wonder why people don’t do either of those that often.

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        Then, I wonder why people don’t do either of those that often.

        Because it’s a frigging lot of work. And also because eventually a user will want a hook that you didn’t provide, or a hook with a different shape from the one you provided, and then refactoring your code to accomodate their use case will cause you a world of pain.

        The solution as a library author is to say no to inversion of control, and write first-order procedures that do one simple thing at a time. Whenever you would normally use a higher-order procedure (aka “higher-order functions” or “methods taking strategy arguments”), do the following instead:

        • Identify the points at which the higher-order procedure calls a procedure argument.
        • Reify the state of at those points as custom data structures.
        • Instead of calling a procedure argument, return the current state immediately.
        • Write another procedure that resumes the continuation with the result of the procedure argument.

        This gives your users the freedom to handle the continuation however they want, without force-fitting square pegs into round holes. I am firmly convinced now that the only legitimate use case for first-class procedures is implementing custom control flow, e.g., laziness and concurrency.