cont Module Predicates Need Better Names

[dougransom]

The cont module predicates, and perhaps the module itself, need better names.

#2537 (comment)

[triska]

As mentioned in #2537 (comment), there may be better names entirely for this interface!

[dougransom]

You may experience this if you misunderstand the predicates. #3237

[triska]

Yes indeed. What do you think would best motivate a good discussion about these predicates, and someone to post their opinions? Having documentation, or not having it?

[dougransom]

I didn't know a good discussion was required until now. I was just trying to learn cont out of curiosity, and to see if I could generate a lazy list.

[dougransom]

This was from the original text of the issue:

The Scryer github readme states 'Delimited continuations based on reset/3, shift/1 (documented in "Delimited Continuations for Prolog").`

While that is true, it is also true that the interface is somewhat different in cont, and that can be a source of confusion for someone learning about these tools, since we are directed to the paper.

I think the predicates in cont should be documented in a little more detail, and a simple example of using shift and reset included in the module documentation.

I changed the issue description based on the comments so far.

[dougransom]

I asked copilot "in prolog, in discussions, what have been suggested improved predicate names for shift and reset?"

Suggested Improved Predicate Names for shift and reset in Prolog

Community discussions often note that shift/1 and reset/3 are technically correct but not very intuitive. Over the years, several alternative naming schemes have been proposed.

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🌱 Names emphasizing delimited control

Current	Suggested	Rationale
reset/3	delimit/3, prompt/3	Makes the boundary semantics explicit.
shift/1	control/1, signal/1, yield/1	Emphasizes suspension and continuation transfer.

---

🎛 Names inspired by effect handlers

Current	Suggested	Rationale
reset/3	handle/3, with_handler/3	Treats the reset point as an effect handler.
shift/1	perform/1, invoke/1	Treats a shift as performing an effect.

---

🔁 Names emphasizing coroutine‑like behavior

Current	Suggested	Rationale
reset/3	run/3, start/3	Marks the entry point of a coroutine‑like computation.
shift/1	suspend/1, pause/1, await/1	Highlights suspension and later resumption.

---

🧩 Names used in actual alternative interfaces

Library / Proposal	Replacement Names	Notes
Multiprompt delimited continuations (delimcc.pl)	p_reset/3, p_shift/2, pr_reset/3	Adds explicit prompts and handler‑style semantics.
Functional‑style interfaces	call_with_prompt/3, take_continuation/1	Mirrors Scheme’s terminology.

---

🧭 Why no consensus?

Each naming scheme reflects a different mental model:

• Delimited continuations → prompt, signal
• Effect handlers → perform, handle
• Coroutines → suspend, resume
• Control operators → control, delimit

Prolog’s built‑in names come from the original control‑operator literature, but many developers prefer names that better communicate intent to newcomers.

What do other languages do?
Here you go — a clean, copy‑ready Markdown block that lists what other programming languages call the same concepts that Prolog exposes as shift/1 and reset/3.

No extra commentary, no formatting tricks — just pure Markdown you can paste anywhere.

Names Used in Other Languages for Delimited Continuations

Delimited continuations appear in many languages, but the naming conventions differ widely.
Prolog uses shift and reset, which come from the Danvy–Filinski tradition.

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1. Languages that use shift / reset

These follow the original operators from the delimited‑continuation literature.

Language	Operators	Notes
Scheme (various research dialects)	shift, reset	The original names from Danvy & Filinski.
OCaml (via libraries like delimcc)	shift, reset	Common in Oleg Kiselyov’s libraries.
Haskell (research libraries)	shift, reset	Provided in experimental continuation libraries.
Racket (packages)	shift, reset	Available in control‑operator libraries.

---

2. Languages that use control / prompt

Another major family of delimited‑control operators.

Language	Operators	Notes
Racket	control, prompt	Felleisen’s operators; widely used in research.
OCaml (multi‑prompt libraries)	control, prompt	Used in multi‑prompt delimited‑continuation systems.

---

3. Languages that use call/cc (undelimited continuations)

Not delimited, but historically related.

Language	Operators	Notes
Scheme	call/cc	Captures the entire continuation.
Racket	call/cc	Still supported.

---

4. Languages that use algebraic effects instead

These languages avoid exposing shift/reset directly and instead use effect handlers.

Language	Operators	Notes
Eff	perform, handler	Canonical algebraic‑effects language.
Koka	perform, handler	Uses delimited continuations under the hood.
OCaml 5	perform, effect, continue	OCaml’s effect system is built on delimited continuations.

---

5. Other operator names found in the literature

Operator	Notes
cupto	Another delimited‑control operator family.
fcontrol	Variant operator described in research papers.
prompt0, prompt1, etc.	Multi‑prompt systems.

---

Summary

Concept	Names in Other Languages
Delimited continuation boundary	reset, prompt, delimit, handler
Capture continuation up to boundary	shift, control, cupto, perform
Undelimited continuation	call/cc

Prolog’s shift and reset align with the Danvy–Filinski naming tradition, which is common in research languages but not always in production languages.

[dougransom]

Also relevant https://groups.google.com/g/swi-prolog/c/I2Xx0t3YmKo/m/pVzOlw2MDAAJ

https://www.swi-prolog.org/download/publications/iclp2013.pdf

[triska]

Interesting terminology suggestions! I note that suspend and resume appear verbatim in the paper:

Subroutines that can be suspended and resumed at certain locations.

[triska]

@samer--, since you are the greatest Prolog-related specialist I know regarding delimited continuations, I would greatly appreciate if you could take a look at this issue: Do you have any suggestions regarding naming improvements of the library(cont) interface? Now would be a great time to make any changes. Thank you a lot!