X-Git-Url: http://lambda.jimpryor.net/git/gitweb.cgi?a=blobdiff_plain;f=damn.mdwn;h=9524086ab9dfb1b3c0ec56a4b732a8680a38359f;hb=2664d5fe30e878c4c6fb6f6ef7549dea0bf03582;hp=cd011344e1e9a9f1247a8bf8f408934a00788c5e;hpb=57e0ef044fcf3252575126e6ffbc2ac49445d0fc;p=lambda.git

diff --git a/damn.mdwn b/damn.mdwn
index cd011344..9524086a 100644
--- a/damn.mdwn
+++ b/damn.mdwn
@@ -248,7 +248,7 @@ It's not immediately clear how to do it with "undelimited" continuations, of the
 	(define damn (lambda () (call/cc (lambda (k) (cons (cons 'side-effect 'bad) (k 'id))))))
 
 
-The idea here is we capture the continuation that `(damn)` has when it gets evaluated. This continuation is bound to the variable `k`. We supply `'id` as an argument to that continuation. When the main, at-issue tree is all built, then we return a pair `'((side-effect bad) AT-ISSUE-TREE)`.
+The idea here is we capture the continuation that `(damn)` has when it gets evaluated. This continuation is bound to the variable `k`. We supply `'id` as an argument to that continuation. When the main, at-issue tree is all built, then we return a pair `'((side-effect . bad) AT-ISSUE-TREE)`.
 
 However, this doesn't work. The reason is that an undelimited continuation represents the future of the evaluation of `(damn)` *until the end of the computation*. So when `'id` is supplied to `k`, we go back to building the at-issue tree until we're finished *and that's the end of the computation*. We never get to go back and evaluate the application of `(cons (cons 'side-effect 'bad) <>)` to anything.
 
@@ -276,6 +276,8 @@ If a block `...` never invokes a shift, then `(reset ...)` will evaluate just th
 
 Going back to the beginning, then. We start with:
 
+	(require racket/control) ; this tells Scheme to let us use shift and reset
+
 	(define damn (lambda () 'id))
 
 We evaluate:
@@ -295,20 +297,19 @@ Evaluating that gives us:
 
 Now to pair that with an affective side-issue content, we'd instead define `damn` as:
 
-	(require racket/control) ; this tells Scheme to let us use shift and reset
 	(define damn (lambda () (shift k (cons (cons 'side-effect 'bad) (k 'id)))))
 
 And voila:
 
-(reset (cons (cons 'the 'man) 
-      (cons 'read
-            (cons 'the
-                  (cons (damn) 
-                        'book)))))
+	(reset (cons (cons 'the 'man) 
+		  (cons 'read
+				(cons 'the
+					  (cons (damn) 
+							'book)))))
 
 evaluates to:
 
-	'((side-effect bad) ((the . man) . (read . (the . (id . book)))))
+	'((side-effect . bad) ((the . man) . (read . (the . (id . book)))))
 
 So that's the straightforward way of repairing the strategy we used in class, without using `print`. We also have to switch to using delimited continuations.
 
@@ -318,17 +319,17 @@ Ken's proposal
 
 Ken Shan pointed out a lovely way to get to the same end-point still using only undelimited continuations (`call/cc`).
 
-(let ((pragma
-       ; An ordered pair whose first component is the assertion
-       ; operator, a unary function, and whose second component
-       ; is the meaning of "damn", a thunk.
-       (call/cc (lambda (k)
-          (cons (lambda (p) p)
-                (lambda () (k (cons (lambda (p) (cons (cons 'side-effect 'bad) p))
-                                    (lambda () 'id)))))))))
-  (let ((assert (car pragma)) ; this binds assert to the first element of the pair pragma
-        (damn   (cdr pragma))) ; this binds damn to the second element of the pair pragma
-    (assert (cons (cons 'the 'student) (cons 'read (cons 'the (cons (damn) 'book)))))))
+	(let ((pragma
+		   ; An ordered pair whose first component is the assertion
+		   ; operator, a unary function, and whose second component
+		   ; is the meaning of "damn", a thunk.
+		   (call/cc (lambda (k)
+			  (cons (lambda (p) p)
+					(lambda () (k (cons (lambda (p) (cons (cons 'side-effect 'bad) p))
+										(lambda () 'id)))))))))
+	  (let ((assert (car pragma)) ; this binds assert to the first element of the pair pragma
+			(damn   (cdr pragma))) ; this binds damn to the second element of the pair pragma
+		(assert (cons (cons 'the 'student) (cons 'read (cons 'the (cons (damn) 'book)))))))
 
 We won't do much to explain this. We'll just leave it for you to chew on.