X-Git-Url: http://lambda.jimpryor.net/git/gitweb.cgi?p=lambda.git;a=blobdiff_plain;f=week11.mdwn;h=b48dd5f410d314554c6fad96293592be0f855d96;hp=f1d9b28d95867179dc26b9bc74b221a6f86671e7;hb=e34cd7e98c3004529c1ba28926e03d6a06bb7f45;hpb=338ce074e719b92e2dbae562d75f96f5b1a67987 diff --git a/week11.mdwn b/week11.mdwn index f1d9b28d..b48dd5f4 100644 --- a/week11.mdwn +++ b/week11.mdwn @@ -919,9 +919,15 @@ describing how to finish building the list. We'll write a new function, `tc` (for task with continuations), that will take an input list (not a zipper!) and a continuation and return a processed list. The structure and the behavior will follow that of `tz` above, with -some small but interesting differences: +some small but interesting differences. We've included the orginal +`tz` to facilitate detailed comparison:
+let rec tz (z:char list_zipper) = + match z with (unzipped, []) -> List.rev(unzipped) (* Done! *) + | (unzipped, 'S'::zipped) -> tz ((List.append unzipped unzipped), zipped) + | (unzipped, target::zipped) -> tz (target::unzipped, zipped);; (* Pull zipper *) + let rec tc (l: char list) (c: (char list) -> (char list)) = match l with [] -> List.rev (c []) | 'S'::zipped -> tc zipped (fun x -> c (c x)) @@ -959,18 +965,23 @@ the recipe to produce the desired result (which is the same list, A good way to test your understanding is to figure out what the continuation function `c` must be at the point in the computation when -`tc` is called with +`tc` is called with the first argument `"Sd"`. Two choices: is it +`fun x -> a::b::x`, or it is `fun x -> b::a::x`? +The way to see if you're right is to execute the following +command and see what happens: + + tc ['S'; 'd'] (fun x -> 'a'::'b'::x);; There are a number of interesting directions we can go with this task. The task was chosen because the computation can be viewed as a simplified picture of a computation using continuations, where `'S'` plays the role of a control operator with some similarities to what is -often called `shift`. &sset; &integral; In the analogy, the list -portrays a string of functional applications, where `[f1; f2; f3; x]` -represents `f1(f2(f3 x))`. The limitation of the analogy is that it -is only possible to represent computations in which the applications -are always right-branching, i.e., the computation `((f1 f2) f3) x` -cannot be directly represented. +often called `shift`. In the analogy, the list portrays a string of +functional applications, where `[f1; f2; f3; x]` represents `f1(f2(f3 +x))`. The limitation of the analogy is that it is only possible to +represent computations in which the applications are always +right-branching, i.e., the computation `((f1 f2) f3) x` cannot be +directly represented. One possibile development is that we could add a special symbol `'#'`, and then the task would be to copy from the target `'S'` only back to