Apple

On 08-04-19, at 05:40, Nigel Garvey wrote:

This script is contrived to illustrate something I ran into this morning:

on aHandler(aList)
   script o
     property l : aList
   end script

   item 1 of o's l

end aHandler

try
   repeat with i from 1 to 100
     aHandler(characters of "abcdefghijklm")
   end repeat
on error msg
   display dialog msg & return & "on iteration " & i & "."
end try

Most times, on both my Jaguar and Tiger machines, it displays a message to the effect 'Can't get item 1 of characters of "abcdefghijklm". on iteration 34.' (The iteration number seems to depend on the speed of the machine, the way the script's run, and the length of the string.) When it's run without the 'try' block in Script Editor, the line 'item 1 of o's l' is highlighted by the error. The questions that puzzle me are:

1. Why does it only error sometimes?
2. How does the handler know that the list is derived from the characters of "abcdefghijklm"?

The error never occurs if:

1. The handler's passed an _explicit_ reference.

aHandler(a reference to characters of "abcdefghijklm")

2. The characters are explicitly 'got' beforehand.

aHandler(get characters of "abcdefghijklm")

3. o's l is initialised to an explicitly dereferenced aList.

on aHandler(aList)
   script o
     property l : aList's contents
   end script

4. o's l is _set_ to aList.

on aHandler(aList)
   script o
     property l : missing value
   end script

   set o's l to aList

5. aList is used instead of o's l in the problem line.

item 1 of aList

on SomeHandler(theList)

script internal

property item_list : theList

end script

item 1 of internal's item_list

end SomeHandler

try

set inlist to (characters of "abcdefghijklm")

set lcount to count of inlist

repeat with i from 1 to lcount

SomeHandler(inlist)

end repeat

on error msg

display dialog msg & return & "on iteration " & i & "."

end try

References:
	>Passed lists, references, and occasional errors (From: "Nigel Garvey" <email@hidden>)