Making sense of the documentation on continuations

Questions & Answers
1

Hi,

The documentation on continuations talks about extending the current continuation. This sounded very mysterious to me, until I realized that this must be what most programming languages refer to as the call stack (I assume that there are reasons for calling it something different). Extending the current continuation, I surmise, is basically like pushing things onto the call stack so that when the current function returns, it "returns" to these other places first. Extending the current continuation is not unique to first class continuations, I think. That happens every time you call a normal function. Doing an abort is like erasing the stack down to the nearest prompt, or whichever prompt applies. And none of this seems to matter unless you return from the function that captured the continuation. If you keep invoking continuations and don't return, then the behavior is the same between call/cc and composable continuations.

Am I on the right track?

Thanks..
Neal

2

On Feb 5, 2026, at 6:03 PM, Neal notifications@racket.discoursemail.com asked whether he was "on the right track” wrt to continuations and extensions.

No.

— Continuations are a concept, a specification if you so wish. The definition says the “rest of the computation from a certain point.”
— The word has a second meaning with many flavors in the docs: a data representation of the concept: abortive (call/cc), imperative (shift, reset), functional (control, prompt).
— Like all kinds of data, it comes with various operations. For example, functional continuations look like functions and act like them. When a program composes two of them (in the sense of f o g from math, or (g (f x))), the underlying implementation “appends” the data structures (because they are not closures).

— A call stack is part of an implementation. It is an old-fashioned word that “old” people w/o understanding of modern PL concepts use, especially when they understand PL ideas only in terms of compilers.

3

As someone without a formal PL background, I found the “Evaluation Model” chapter in the Racket Reference helpful in understanding the concept of continuations:

I also like the box diagrams in many of Matthew's papers and talks. I haven't fully re-watched it now, but I think the talk Delimited and Composable Continuations in PLT Scheme - Microsoft Research and the underlying paper might be help to understand how the zoo of racket/control fits together.

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