style

[lambda.git] / topics / week13_coroutines_exceptions_and_aborts.mdwn

diff --git a/topics/week13_coroutines_exceptions_and_aborts.mdwn b/topics/week13_coroutines_exceptions_and_aborts.mdwn
index df9c134..0246663 100644 (file)
--- a/topics/week13_coroutines_exceptions_and_aborts.mdwn
+++ b/topics/week13_coroutines_exceptions_and_aborts.mdwn
@@ -4,7 +4,7 @@
 
 Recall [[the recent homework assignment|/exercises/assignment12]] where you solved the same-fringe problem with a `make_fringe_enumerator` function, or in the Scheme version using streams instead of zippers, with a `lazy-flatten` function.
 
 
 Recall [[the recent homework assignment|/exercises/assignment12]] where you solved the same-fringe problem with a `make_fringe_enumerator` function, or in the Scheme version using streams instead of zippers, with a `lazy-flatten` function.
 

-The technique illustrated in those solutions is a powerful and important one. It's an example of what's sometimes called **cooperative threading**. A "thread" is a subprogram that the main computation spawns off. Threads are called "cooperative" when the code of the main computation and the thread fixes when control passes back and forth between them. (When the code doesn't control this---for example, it's determined by the operating system or the hardware in ways that the programmer can't predict---that's called "preemptive threading.") Cooperative threads are also sometimes called *coroutines* or *generators*.
+The technique illustrated in those solutions is a powerful and important one. It's an example of what's sometimes called **cooperative threading**. A "thread" is a subprogram that the main computation spawns off. Threads are called "cooperative" when the code of the main computation and the thread fixes when control passes back and forth between them. (When the code doesn't control this --- for example, it's determined by the operating system or the hardware in ways that the programmer can't predict --- that's called "preemptive threading.") Cooperative threads are also sometimes called *coroutines* or *generators*.

 
 With cooperative threads, one typically yields control to the thread, and then back again to the main program, multiple times. Here's the pattern in which that happens in our `same_fringe` function:
 
 
 With cooperative threads, one typically yields control to the thread, and then back again to the main program, multiple times. Here's the pattern in which that happens in our `same_fringe` function:
 


@@ -13,7 +13,7 @@ With cooperative threads, one typically yields control to the thread, and then b
     start next1
     (paused)            starting
     (paused)            calculate first leaf
     start next1
     (paused)            starting
     (paused)            calculate first leaf

-    (paused)            <--- return it
+    (paused)            <-- return it

     start next2         (paused)            starting
     (paused)            (paused)            calculate first leaf
     (paused)            (paused)            <-- return it
     start next2         (paused)            starting
     (paused)            (paused)            calculate first leaf
     (paused)            (paused)            <-- return it


@@ -78,7 +78,7 @@ Some languages have a native syntax for coroutines. Here's how we'd write the sa
         {left = {left = {leaf=1}, right = {leaf=2}}, right = {leaf=3}} )
     true
 
         {left = {left = {leaf=1}, right = {leaf=2}}, right = {leaf=3}} )
     true
 

-We're going to think about the underlying principles to this execution pattern, and instead learn how to implement it from scratch---without necessarily having zippers or dedicated native syntax to rely on.
+We're going to think about the underlying principles to this execution pattern, and instead learn how to implement it from scratch --- without necessarily having zippers or dedicated native syntax to rely on.

 
 
 ##Exceptions and Aborts##
 
 
 ##Exceptions and Aborts##


@@ -327,7 +327,7 @@ Well, that's when we use the `outer_snapshot` code in an unusual way. If we enco
         ) + 100
     in outer_snapshot foo_applied_to_x;;
 
         ) + 100
     in outer_snapshot foo_applied_to_x;;
 

-Except that isn't quite right, yet---in this fragment, after the `outer_snapshot 20` code is finished, we'd pick up again inside `let foo_applied_to_x = (...) + 100 in outer_snapshot foo_applied_to_x`. That's not what we want. We don't want to pick up again there. We want instead to do this:
+Except that isn't quite right, yet --- in this fragment, after the `outer_snapshot 20` code is finished, we'd pick up again inside `let foo_applied_to_x = (...) + 100 in outer_snapshot foo_applied_to_x`. That's not what we want. We don't want to pick up again there. We want instead to do this:

 
     let x = 2
     in let outer_snapshot = fun box ->
 
     let x = 2
     in let outer_snapshot = fun box ->


@@ -370,11 +370,11 @@ A similar kind of "snapshotting" lets coroutines keep track of where they left o
 
 These snapshots are called **continuations** because they represent how the computation will "continue" once some target code (in our example, the code in the box) delivers up a value.
 
 
 These snapshots are called **continuations** because they represent how the computation will "continue" once some target code (in our example, the code in the box) delivers up a value.
 

-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 outer_snapshot 20) + 100`---which is written as if it were 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.
+The key idea behind working with continuations is that we're *inverting control*. In the fragment above, the code `(if x = 1 then ... else outer_snapshot 20) + 100` --- which is written as if it were 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 our [[abortable list traversals|/topics/week12_abortable_traversals]]. (The `outer_snapshot` corresponds to the "done" handler in those traversals; and the `continue_foo_snapshot` to the "keep_going" handler.)
+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 our [[abortable list traversals|/topics/week12_abortable_traversals]]. (The `outer_snapshot` corresponds to the "done" handler in those traversals; and the `continue_foo_snapshot` to the "keep_going" handler.)

 
 This inversion of control should also remind you of Montague's treatment of determiner phrases in ["The Proper Treatment of Quantification in Ordinary English"](http://www.blackwellpublishing.com/content/BPL_Images/Content_store/Sample_chapter/0631215417%5CPortner.pdf) (PTQ).
 
 
 This inversion of control should also remind you of Montague's treatment of determiner phrases in ["The Proper Treatment of Quantification in Ordinary English"](http://www.blackwellpublishing.com/content/BPL_Images/Content_store/Sample_chapter/0631215417%5CPortner.pdf) (PTQ).
 


@@ -466,9 +466,9 @@ There are also different kinds of "syntactic sugar" we can use to hide the conti
 -->
 
 
 -->
 
 

-Various of the tools we've been introducing over the past weeks are inter-related. We saw coroutines implemented first with zippers; here we've talked in the abstract about their being implemented with continuations. Oleg says that "Zipper can be viewed as a delimited continuation reified as a data structure." Ken expresses the same idea in terms of a zipper being a "defunctionalized" continuation---that is, take something implemented as a function (a continuation) and implement the same thing as an inert data structure (a zipper).
+Various of the tools we've been introducing over the past weeks are inter-related. We saw coroutines implemented first with zippers; here we've talked in the abstract about their being implemented with continuations. Oleg says that "Zipper can be viewed as a delimited continuation reified as a data structure." Ken expresses the same idea in terms of a zipper being a "defunctionalized" continuation --- that is, take something implemented as a function (a continuation) and implement the same thing as an inert data structure (a zipper).

 
 Mutation, delimited continuations, and monads can also be defined in terms of each other in various ways. We find these connections fascinating but the seminar won't be able to explore them very far.
 
 
 Mutation, delimited continuations, and monads can also be defined in terms of each other in various ways. We find these connections fascinating but the seminar won't be able to explore them very far.
 

-We recommend reading [the Yet Another Haskell Tutorial on Continuation Passing Style](http://en.wikibooks.org/wiki/Haskell/YAHT/Type_basics#Continuation_Passing_Style)---though the target language is Haskell, this discussion is especially close to material we're discussing in the seminar.
+We recommend reading [the Yet Another Haskell Tutorial on Continuation Passing Style](http://en.wikibooks.org/wiki/Haskell/YAHT/Type_basics#Continuation_Passing_Style) --- though the target language is Haskell, this discussion is especially close to material we're discussing in the seminar.