[plt-scheme] Is it important to understand continuations conceptually; not in terms of their implementation?

From: Matthias Felleisen (matthias at ccs.neu.edu)
Date: Sat May 3 12:32:36 EDT 2008

Grant, you're correct in that an understanding of one particular  
implementation technique for a linguistic construct causes huge and  
ubiquitous misunderstandings.

Procedures and procedure calls are the examples that come to mind.  
Those things were explained via a stack-based implementation in the  
1950s and 1960s although [&] abstract explanations in terms of 8th  
grade algebra all the way to Lambda Calculus had been around for, oh,  
a while. [*] As a result, procedure calls had been considered  
expensive and a thing to be avoided. Steele pointed out our  
misunderstanding of this issue AND YET, to this day, people don't  
implement procedures and procedure calls properly and we are still  
suffering from this perception. People still write huge procedures to  
avoid another call, and people still want to see complete stack  
traces in their debuggers for their function calls. So the sentence  
labeled with [*] uses the incorrect tense. It should use "have been  
and are" instead. It is one sad state of affairs. Of course, this  
just refers back to the sentence with [&]: people who design and  
implement programming languages do not wish to study mathematical  
models of PLs, can't and won't. But they sure want credit on all  
fronts. That's why the problem is pervasive.

Continuation objects in Scheme are special-purpose procedures. That  
is, they are procedural representations of the 'rest of the  
computation with respect to some expression evaluation.' So the story  
is related but fortunately (or whatever) doesn't have as much of an  
impact. Continuations aren't as useful as procedure. Yes, there are  
kids out there who think that if you don't implement continuations  
with fast code etc your Scheme implementation isn't worth much. But  
those are just mislead.

Continuations can be implemented with at least four basic techniques  
that I can remember right now. Clinger et al (a nice scientific paper  
from the 80s revised in the 90s) lays out a beautiful and well- 
presented comparison of such techniques. I recommend reading it. And  
of course Dybvig/Hieb's lazy stack copy technique in the original  
paper. Of course in SML/NJ callcc = cons. So that's that.

Continuations can be understood as all kinds of abstract beasts, with  
little more knowledge than 8th grade algebra or Lambda Calculus. But  
that is just an abstract form of 'how'. I have spent a good deal of  
time on this question.

Finally, continuations can be understood from a 'pragmatic'  
perspective ('what are they useful for, and how are they used'). For  
this question, I recommend two books and a paper:
  -- Shriram's PLAI
  -- Friedman and Springers, "Art and Scheme"
  -- Friedman's POPL talk from 1988 on "Applications of Continuations"

Good luck -- Matthias

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