Continuation is to be continued

Transcript

1. ©2025 Wantedly, Inc. Continuation is to be continued A revisit of first-class continuations or an invitation to delimited continuations RubyKaigi 2025 Mar 16, 2025 - Masayuki Mizuno
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   NF • Backend engineer at Wantedly, Inc. • Born in Takamatsu City, Kagawa • Programming language lover
3. ©2025 Wantedly, Inc. 1. What is a continuation? 2. Continuations’ use cases: backtracking 3. call/cc’s problem 4. Better alternative: shift/reset 5. Conclusion Agenda
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5. ©2025 Wantedly, Inc. What is a continuation? Computation that receive the evaluation result Or run-time point in program execution
6. ©2025 Wantedly, Inc. Ruby’s built-in continuation operator: call/cc Call/cc captures current continuation Kernel.#callcc / Continuation#call behaves like setjmp / longjmp
7. ©2025 Wantedly, Inc. Calling continuation multiple times Continuation can be invoked arbitrary times However, terminated fiber cannot be called Let's see how call/cc is special, using a DSL as an example
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14. ©2025 Wantedly, Inc. Implementing DSL with call/cc Ruby-level backtracking by call/cc Thanks to one-pass evaluation, most Ruby expressions are also viable in DSL
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16. ©2025 Wantedly, Inc. Problem of call/cc (1/2) Captures unnecessarily large code pieces Need call/cc calls in two different places, one for backtracking and one for conclusion
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     Let's discover the cause for performance degradation
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23. ©2025 Wantedly, Inc. shift/reset: yet another continuation operators Captures delimited code block from shift to reset Unlike call/cc, delimited continuation will go back when reaches reset
24. ©2025 Wantedly, Inc. Implementing “do” syntax like DSL with shift/reset Delimiting continuation leads to simple code Enumerable#flat_map can be used as is
25. ©2025 Wantedly, Inc. Implementing shift/reset with call/cc shift/reset are reproducible by call/cc Thread-local variable is needed to record return addresses
26. ©2025 Wantedly, Inc. Benchmarking “do” syntax like DSL with shift/reset Delimited continuation improves performance Kernel.#callcc’s optimization may be effective for shift/reset implementation
27. ©2025 Wantedly, Inc. Foresight to native shift/reset implementation shift/reset may provide optimization hints Programmers can specify necessary stack range to the interpreter
28. ©2025 Wantedly, Inc. $PODMVTJPO
29. ©2025 Wantedly, Inc. 1. call/cc can manipulate continuations 2. call/cc is sometimes suitable for DSL 3. call/cc has some problems • Hard to use for developers • Poor performance 4. shift/reset probably resolve problems of call/cc • I'm going to introduce the new, performant API Conclusion