[cl-faq] [lispfaq commit] r8 - trunk

[codesite-noreply@google.com]

Author: peter.seibel
Date: Wed Mar  7 21:09:18 2007
New Revision: 8

Modified:
   trunk/faq.txt
   trunk/lispfaq.txt

Log:
Moving MAKE-ARRAY question from lispfaq to faq.

Modified: trunk/faq.txt
==============================================================================
--- trunk/faq.txt	(original)
+++ trunk/faq.txt	Wed Mar  7 21:09:18 2007
@@ -799,6 +799,30 @@
 value as \cl{AND} does.
 
 
+*** I tried to create 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
+something like this:
+
+    (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:
+
+    (make-array 10 :initial-element 0)
+
+To initialize an array with distinct objects, you must iterate over
+the array somehow. One concise idiom is the following:
+
+    (map-into (make-array 10) #'make-foo)
+
+
 ** History
 
 *** Why doesn't Lisp have a Benevolent Dictator like Perl or Python?

Modified: trunk/lispfaq.txt
==============================================================================
--- trunk/lispfaq.txt	(original)
+++ trunk/lispfaq.txt	Wed Mar  7 21:09:18 2007
@@ -27,13 +27,6 @@
 
 Are you in a breakpoint in FOO or in BAR? 
 
-*** I want an array of foos, but  \programlisting{ (make-array 10 :initial-element (make-foo)) }  causes strange bugs. Why? 
-
-Well, the array created above contains 10 pointers to the same foo,
-which will indeed cause strange bugs. The correct way to initialize
-your array is probably \programlisting{ (map-into (make-array 10)
-#'make-foo) }
-
 *** What books should I read to learn more about lisp?
 
 *** Why doesn't Common Lisp have continuations?