CSE 130 Lecture Notes - Lecture 15: If And Only If, Nested Function, Firstclass

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2 Mar 2018
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Continuations
How continuations do async programming!
Ex:
fs.readFile( blah, callback );
Thread0 : | r | | a | | |
Thread1 : | c | | |
-allows for a form of concurrency
-”continue” doing something else
When you write an explicit callback
- Cooperative multi-threading (in contrast to preemptive multi-threading - stops
processes whether you want to or not)
- We yield() - explicitly tell OS or Runtime the state the save and how to restore
Ex: debugger
Breakpoints: stop execution at a certain point
- To examine the state of the program (memory)
- Want to be able to keep going - can continue running the program (have continuation to
rest of program!)
Continuations are implicit in your code
Consider ( 2*x + 1/y ) * 2
A. multiply 2 and x //previous
B. Divide 1 by y //current
C. Add A and B
D. Multiply C and 2
Rewrite:
let previous = 2 * x
Let cont = curRes => (previous + curRes) * 2
cont( 1 / y ) current computation
Node.js example
const data = fs.readFileSync(‘myFile.txt’)
console.log(data);
processData(data); ← continuation
Explicit continuation (what you have to do today!)
fs.readFile(‘myFile.txt’, callback)
Function callback (err, data) {
console.log(data);
processData(data);
});
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Continuation Passing Style (CPS)
-call/cc (call w/ current continuation) is used to call a function and give it the current
continuation
-why is this powerful?
-we want to bail out! We want to know WHERE to continue if we got an error!
-MOST languages don’t give you this!
-so we have to transform our code to CPS
W/ call/cc:
K <- get Current Continuation
fs.read( function () {
Function () {
… K()
} //need to bail out of here!
CPS
-why do we want to do this?
- makes control flow explicit: NO return!
- order of evaluation - have to specify the order of DOING things
-Why do we care?
- Optimizations:
- we don’t need stack frames! We can just jump to wherever we need to continue!
- threading - iff our runtime allows it!
- Intermediate Representations
To CPS, by example
Function zero() { function zero ( cc ) { //cc is our program continuation!
Return 0; cc(0);
} }
Function fact(n) { function fact( n, cc) {
If (n == 0) { if (n == 0) {
Return 1; cc(1);
} else { } else {
Return n* fact(n-1); fact( n-1, r => cc(r*n))
} }
} }
Note that the return n*fact(n-1) does NOT return immediately
fact(3): CPS fact(3, x => x)
fact(3, id) ->
fact(2, rA => id(3 * rA)) ->
fact(1, rb => (rA => id(3 * rA))(2 * rb)) ->
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