Why are delimited continuation primitives named "shift" and "reset"?

Question

I think I understand (in general) what shift and reset mean. However I do not understand why they are named so ? What do shift and reset as Delimited Continuation primitives have to do with "shift" and "reset" words in English?

Daubigny

Answer 1

They're called so because of the way they are implemented (in general).

Quoted from Direct Implementation of Shift and Reset in the MinCaml Compiler

By interpreting a program using the continuation semantics, we can regard the state of the program as a continuation stack. Then, reset can be thought of as marking the continuation stack, and shift capturing the continuation stack up to the nearest mark created by reset. Here is the overview of the implementation:

• When calling reset, set a reset mark to the stack
• When calling shift (fun k -> M), move a part of the stack frames up to the nearest reset mark to the heap
• When calling a continuation k, set a reset mark to the stack and copy the corresponding frames from the heap to the stack top.

A reset mark is inserted when k is called, because captured continuations are executed in an empty continuation.

Norvun

It's worth noting that Scala's delimited continuations aren't implemented in such a direct way - they can't be. The JVM doesn't expose stack manipulation primitives. – Adila

In contrast, Scheme-48 does allow for a direct implementation (Gasbichler/Sperber), as does any language that exposes the call stack, like Squeak Smalltalk. – Interlocutress

Answer 2

Because that's the way Danvy & Filinski called those two operators in the first paper where they exposed that model of continuation-passing style (see also here), and it is what Scala implements.

The implementation in Scala is described in this other paper. The reference therein to Danvy & Filinski is clear:

In this paper, we study the addition of control operators shift and reset to this language framework, which together implement static delimited continuations (Danvy and Filinski 1990, 1992)

Redroot