Author: peter.seibel
Date: Wed Mar 7 21:23:14 2007
New Revision: 9
Modified:
trunk/faq.txt
trunk/lispfaq.txt
Log:
Moving continuation question from lispfaq to faq.
Modified: trunk/faq.txt
==============================================================================
--- trunk/faq.txt (original)
+++ trunk/faq.txt Wed Mar 7 21:23:14 2007
@@ -799,7 +799,7 @@
value as \cl{AND} does.
-*** I tried to create an array of objects with \cl{MAKE-ARRAY} and am seeing
strange bugs. Why?
+*** I created an array of objects with \cl{MAKE-ARRAY} and am seeing strange
bugs. Why?
Because \cl{MAKE-ARRAY} is a regular function, it's arguments are all
evaluated before it is called. This can trip people up when they say
@@ -807,13 +807,13 @@
(make-array 10 :initial-element (make-foo))
-when they mean to make an array of ten distinct \code{foo} objects.
-When this expression is evaluated, \code{(make-foo)} is evaluated once
-and the resulting object is passed to \cl{MAKE-ARRAY} which makes it
-the value of all ten elements of the array. If the object is later
-modified, the change will be visible via any and all elements of the
-array. Of course if the object is immutable that probably wouldn't
-matter which is why it's okay to say things like:
+intending to make an array of ten distinct objects. When this
+expression is evaluated, \code{(make-foo)} is evaluated once and the
+resulting object is passed to \cl{MAKE-ARRAY} which makes it the value
+of all ten elements of the array. If the object is later modified, the
+change will be visible via any and all elements of the array. Of
+course if the object is immutable that probably wouldn't matter which
+is why it's okay to say things like:
(make-array 10 :initial-element 0)
@@ -822,6 +822,25 @@
(map-into (make-array 10) #'make-foo)
+
+** Why doesn't Lisp ...
+
+*** Why doesn't Common Lisp have continuations?
+
+Continuations are a great theoretical tool; if a language has
+first-class, multiply-invocable continuations then one can build
+threads, exceptions, coroutines, and the kitchen sink on top.
+
+However, such continuations present a a heavy burden for a Lisp
+implementor and may preclude or at least complicate other desirable
+optimizations. The ANSI standardizing committee decided that it would
+be better to specify the user-level control structure (\cl{CATCH},
+\cl{UNWIND-PROTECT}, and so on) and let implementors choose whether to
+build those on top of continuations or not.
+
+If you need to play with continuations, they are standard in Scheme
+though make sure you pick a Scheme implementation that actually
+supports them because they don't all.
** History
Modified: trunk/lispfaq.txt
==============================================================================
--- trunk/lispfaq.txt (original)
+++ trunk/lispfaq.txt Wed Mar 7 21:23:14 2007
@@ -27,24 +27,6 @@
Are you in a breakpoint in FOO or in BAR?
-*** What books should I read to learn more about lisp?
-
-*** Why doesn't Common Lisp have continuations?
-
-Continuations are a great theoretical tool; if a language has
-first-class, multiply invocable continuations then one can build
-threads, exceptions, coroutines, and the kitchen sink on top.
-
-However, there is an implementation burden with continuations;
-supporting first-class, multiply invocable continuations complicates
-things tremendously for the Lisp implementor. The ANSI standardizing
-committee J13, mindful of this, took the view that it would be better
-to specify the user-level control structure (CATCH, UNWIND-PROTECT,
-and so on) and let implementors choose whether to build those on top
-of continuations or not.
-
-If you need to play with continuations, you should use a Scheme
implementation.
-
*** Why isn't there a DEEP-COPY function in the language?
Copying an arbitrary structure or object needs context to determine
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