lists-to-contin tweaks

[lambda.git] / coroutines_and_aborts.mdwn

diff --git a/coroutines_and_aborts.mdwn b/coroutines_and_aborts.mdwn
index 8973c42..8cca9e3 100644 (file)
--- a/coroutines_and_aborts.mdwn
+++ b/coroutines_and_aborts.mdwn
@@ -561,7 +561,7 @@ These snapshots are called **continuations** because they represent how the comp
 
 You can think of them as functions that represent "how the rest of the computation proposes to continue." Except that, once we're able to get our hands on those functions, we can do exotic and unwholesome things with them. Like use them to suspend and resume a thread. Or to abort from deep inside a sub-computation: one function might pass the command to abort *it* to a subfunction, so that the subfunction has the power to jump directly to the outside caller. Or a function might *return* its continuation function to the outside caller, giving *the outside caller* the ability to "abort" the function (the function that has already returned its value---so what should happen then?) Or we may call the same continuation function *multiple times* (what should happen then?). All of these weird and wonderful possibilities await us.
 
 
 You can think of them as functions that represent "how the rest of the computation proposes to continue." Except that, once we're able to get our hands on those functions, we can do exotic and unwholesome things with them. Like use them to suspend and resume a thread. Or to abort from deep inside a sub-computation: one function might pass the command to abort *it* to a subfunction, so that the subfunction has the power to jump directly to the outside caller. Or a function might *return* its continuation function to the outside caller, giving *the outside caller* the ability to "abort" the function (the function that has already returned its value---so what should happen then?) Or we may call the same continuation function *multiple times* (what should happen then?). All of these weird and wonderful possibilities await us.
 

-The key idea behind working with continuations is that we're *inverting control*. In the fragment above, the code `(if x = 1 then ... else snapshot 20) + 100`---which is written so as to supply a value to the outside context that we snapshotted---itself *makes non-trivial use of* that snapshot. So it has to be able to refer to that snapshot; the snapshot has to somehow be available to our inside-the-box code as an *argument* or bound variable. That is: the cde that is *written* like it's supplying an argument to the outside context is instead *getting that context as its own argument*. He who is written as value-supplying slave is instead become the outer context's master.
+The key idea behind working with continuations is that we're *inverting control*. In the fragment above, the code `(if x = 1 then ... else snapshot 20) + 100`---which is written so as to supply a value to the outside context that we snapshotted---itself *makes non-trivial use of* that snapshot. So it has to be able to refer to that snapshot; the snapshot has to somehow be available to our inside-the-box code as an *argument* or bound variable. That is: the code that is *written* like it's supplying an argument to the outside context is instead *getting that context as its own argument*. He who is written as value-supplying slave is instead become the outer context's master.

 
 In fact you've already seen this several times this semester---recall how in our implementation of pairs in the untyped lambda-calculus, the handler who wanted to use the pair's components had *in the first place to be supplied to the pair as an argument*. So the exotica from the end of the seminar was already on the scene in some of our earliest steps. Recall also what we did with v2 and v5 lists. Version 5 lists were the ones that let us abort a fold early: 
 go back and re-read the material on "Aborting a Search Through a List" in [[Week4]].
 
 In fact you've already seen this several times this semester---recall how in our implementation of pairs in the untyped lambda-calculus, the handler who wanted to use the pair's components had *in the first place to be supplied to the pair as an argument*. So the exotica from the end of the seminar was already on the scene in some of our earliest steps. Recall also what we did with v2 and v5 lists. Version 5 lists were the ones that let us abort a fold early: 
 go back and re-read the material on "Aborting a Search Through a List" in [[Week4]].


@@ -583,7 +583,8 @@ When working with continuations, it's easiest in the first place to write them o
        let foo x =
            try begin
                (if x = 1 then 10
        let foo x =
            try begin
                (if x = 1 then 10

-               else abort 20) + 100
+               else abort 20
+                       ) + 100

            end
        in (foo 2) + 1;;
 
            end
        in (foo 2) + 1;;
 


@@ -593,11 +594,11 @@ into this:
        in let snapshot = fun box ->
                let foo_result = box
                in (foo_result) + 1000
        in let snapshot = fun box ->
                let foo_result = box
                in (foo_result) + 1000

-       in let finish_value = fun start ->
-               let value = start + 100
+       in let continue_normally = fun from_value ->
+               let value = from_value + 100

                in snapshot value
        in 
                in snapshot value
        in 

-               if x = 1 then finish_value 10
+               if x = 1 then continue_normally 10

                else snapshot 20;;
 
 Code written in the latter form is said to be written in **explicit continuation-passing style** or CPS. Later we'll talk about algorithms that mechanically convert an entire program into CPS.
                else snapshot 20;;
 
 Code written in the latter form is said to be written in **explicit continuation-passing style** or CPS. Later we'll talk about algorithms that mechanically convert an entire program into CPS.