StackOverflow2013

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plurals

PO
primarykey
Id
12146147
data
AcceptedAnswerId
0
AnswerCount
0
ClosedDate
CommentCount
0
CommunityOwnedDate
CreationDate
2012-08-27T16:51:43.120
FavoriteCount
0
LastActivityDate
2012-08-27T17:08:50.250
LastEditDate
2012-08-27T17:08:50.250
LastEditorUserId
1628262
OwnerUserId
1628262
ParentId
11283526
PostTypeId
2
Score
1
ViewCount
0
LastEditorDisplayName
text
Body
Why does this expression evaluate to "hi" ? <pre><code>(let ([x (call/cc (lambda (k) k))]) (x (lambda (ignore) "hi"))) </code></pre> The first step is to decide what <code>k</code> looks like: <pre><code>(define k (lambda (value) (let ((x value)) (x (lambda (ignore) "hi"))))) </code></pre> We see immediately that this is the same as <pre><code>(define k (lambda (x) (x (lambda (ignore) "hi")))) </code></pre> But, I failed to mention one little detail. If <code>k</code> is ever invoked, it is as if it were invoked in tail position. So <code>(f (k 3))</code> for all continuations <code>k</code> built by <code>call/cc</code> is the same as <code>(k 3)</code>. That is always a little bit tricky. So, let's use <code>lambda^</code> to mean that the function it introduces is to be invoked as if it were in tail position. <pre><code>(define k (lambda^ (x) (x (lambda (ignore) "hi")))) </code></pre> Now we know what <code>k</code> is, we also need to know that returning out of <code>(call/cc (lambda (k) k))</code> is really using a default. It should have been written correctly as <pre><code>(call/cc (lambda (k) (k k))). </code></pre> There is always an implied invocation of k at the top of the body of the lambda expression passed to <code>call/cc</code>. We know what <code>k</code> is. So, we know that this must be the same as, (for ease of reading let's turn the <code>x</code>'s in the argument position into <code>y</code>'s.) <pre><code>((lambda^ (x) (x (lambda (ignore) "hi"))) (lambda^ (y) (y (lambda (ignore) "hi")))) </code></pre> So, we evaluate both positions to functions. Once we invoke the function in function position, we are done, since it is headed by <code>lambda^</code> So, we need to know that <pre><code>((lambda^ (x) (x (lambda (ignore) "hi"))) (lambda^ (y) (y (lambda (ignore) "hi")))) </code></pre> evaluates to, substituting for <code>x</code> <pre><code>((lambda^ (y) (y (lambda (ignore) "hi"))) (lambda (ignore) "hi")) </code></pre> and one more step, substituting for <code>y</code> leads to <code>((lambda (ignore) "hi") (lambda (ignore) "hi"))</code>, which ignores its argument and returns "hi"
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1. This table or related slice is empty.
PostParentId
1. POhow to understand this continuation?
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PostTypePostTypeId
1. PTAnswer
UserLastEditorUserId
1. USDan
UserOwnerUserId
1. USDan
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1. This table or related slice is empty.
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1. This table or related slice is empty.
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1. VO
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1. This table or related slice is empty.

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