Comments on: A dollop of Lisp http://4.flowsnake.org/archives/602 A Pythoneer's adventures with Scheme, etc. Mon, 18 Jul 2011 14:37:45 +0000 hourly 1 http://wordpress.org/?v=3.3 By: Paul hollingsworth http://4.flowsnake.org/archives/602/comment-page-1#comment-526 Paul hollingsworth Tue, 16 Jun 2009 04:45:58 +0000 http://4.flowsnake.org/?p=602#comment-526 You can avoid mutating state by keeping two stacks: 1. the stack of "things still to be done" as you already have. 2. a stack of return values that have been calculated. Your main loop repeatedly pops items off 1 until it is empty. The final result can then be popped off 2. When evaluating a compound form you expand it by pushing a "collapse" task, and then "expand" tasks for each of the sub-expressions. By combining the two stacks into a single immutable data structure (is use two linked lists) you can efficiently implement continuations. You can avoid mutating state by keeping two stacks:
1. the stack of “things still to be done” as you already have.
2. a stack of return values that have been calculated.

Your main loop repeatedly pops items off 1 until it is empty. The final result can then be popped off 2.

When evaluating a compound form you expand it by pushing a “collapse” task, and then “expand” tasks for each of the sub-expressions.

By combining the two stacks into a single immutable data structure (is use two linked lists) you can efficiently implement continuations.

]]> By: Hans Nowak http://4.flowsnake.org/archives/602/comment-page-1#comment-525 Hans Nowak Mon, 15 Jun 2009 16:02:00 +0000 http://4.flowsnake.org/?p=602#comment-525 Good... I'm doing *something* right then... :-) Good… I’m doing *something* right then… :-)

]]> By: John Cowan http://4.flowsnake.org/archives/602/comment-page-1#comment-524 John Cowan Mon, 15 Jun 2009 14:17:08 +0000 http://4.flowsnake.org/?p=602#comment-524 This, of course, is exactly how the original Scheme interpreter worked: it was embedded in Maclisp, which (like Python) provided neither tail recursion nor call/cc. This, of course, is exactly how the original Scheme interpreter worked: it was embedded in Maclisp, which (like Python) provided neither tail recursion nor call/cc.

]]>