Coroutines in Lua Coroutines An unconventional, but quite powerful - - PowerPoint PPT Presentation

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Coroutines in Lua Coroutines An unconventional, but quite powerful - - PowerPoint PPT Presentation

Jun 26, 2023 418 likes •630 views

Coroutines in Lua Coroutines An unconventional, but quite powerful control mechanism Well known as an abstract concept, but with several variations Variations with big differences 2 Kinds of Coroutines Symmetric or asymmetric

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SLIDE 1

Coroutines in Lua

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SLIDE 2

2

Coroutines

  • An unconventional, but quite powerful control

mechanism

  • Well known as an abstract concept, but with

several variations

  • Variations with big differences
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SLIDE 3

3

Kinds of Coroutines

  • Symmetric or asymmetric
  • Stackful
  • First-class values
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Symmetric and Asymmetric Coroutines

  • Symmetric coroutines: one primitive for

transfering control

  • Typically called transfer
  • Asymmetric coroutines: two primitives for

transferring control

  • Typically called resume and yield

resume/yield ↔ call/return transfer ↔ goto

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Stackful Coroutines

  • Non-stackful coroutines can be suspended only

inside the body of the original function

  • Original concept (co-routine x sub-routine)
  • Stackful coroutines can be suspended while

calling other functions

  • As implemented in Modula
  • Similar to cooperative multithreading
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First-class Coroutines

  • Coroutines can be represented by first-class

values

  • can be resumed anywhere in a program
  • Restricted forms of coroutines are not first class
  • e.g., generators in CLU and other languages
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Full Coroutines

  • A full coroutine is a stackful, first-class

coroutine

  • For full coroutines, symmetric and asymmetric

control are equivalent

  • you can implement one with the other
  • just like goto x call/return
  • Full coroutines are equivalent to one-shot

continuations

  • you can implement call/1cc with them
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Coroutines in Lua

  • Full, asymmetric coroutines
  • Full coroutines present one-shot continuations

in a format that is more familiar to conventional programmers

  • similar to multithreading
  • Full coroutines allow a simple and efficient

implementation

  • as compared with one-shot continuations
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Asymmetric coroutines

  • Asymmetric and symmetric coroutines are

equivalent

  • Not when there are different kinds of contexts
  • integration with C
  • How to do a transfer with C activation

records in the stack?

  • resume fits naturally in the C API
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Coroutines: First Example

co = coroutine.wrap(function (x) print(x) coroutine.yield() print(2*x) end) co(20) --> 20 co() --> 40 co()

  • -> error: cannot resume dead coroutine
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Coroutines: exchanging values

co = coroutine.wrap(function (x) x = coroutine.yield(2*x) return 3*x end) print(co(20)) --> 40 print(co(2)) --> 6 co()

  • -> error: cannot resume dead coroutine
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Producer - Consumer

function consume () while true do local x = receive() print(x) end end function produce () while true do local x = io.read() send(x) end end send = coroutine.yield receive = coroutine.wrap(produce) consume()

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Coroutines and Iterators

function permgen (a, n, f) if n <= 1 then f(a) else for i = 1, n do a[n], a[i] = a[i], a[n] permgen(a, n - 1, f) a[n], a[i] = a[i], a[n] end end end

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Coroutines and Iterators

function permutations (a) return coroutine.wrap(function () permgen(a, #a, coroutine.yield) end) end for a in permutations({1,2,3,4}) do printPerm(a) end

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Who is the Main Program

How to turn a complex interactive application into a library?

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Who is the Main Program

/* huge and complex application */ int main (int argc, char **argv) { ... } void readCommand (char *buff) { printf("enter command:\n"); fgets(buff, MAX, stdin); }

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Who is the Main Program

/* huge and complex application */ int main (int argc, char **argv) { /* create coroutine with Lua script */ ... } void readCommand (char *buff) { lua_resume(...); /* pass result to buffer */ ... }

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Who is the Main Program

  • - Lua script

emitCommand = coroutine.yield emitCommand("doCommand1") ... emitCommand("doCommand2") ... emitCommand("doCommand3")

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Coroutines x continuations

  • Most uses of continuations can be coded with

coroutines

  • coroutines ☺
  • “who has the main loop” problem

– Producer-consumer – extending x embedding

  • iterators x generators

– the same-fringe problem

  • collaborative multithreading
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Coroutines x continuations

  • Multi-shot continuations are more expressive

than coroutines

  • Some techniques need code reorganization to

be solved with coroutines or one-shot continuations

  • e.g., oracle functions