esharris has asked for the wisdom of the Perl Monks concerning the following question:

Why does the following program abort? 
#!/usr/bin/perl -w

use Class::Std;

package Foo;

my %a : ATTR( :default('<p />:('));

print "Hello\n";

[download]
When I run this script, I get "Unterminated attribute parameter in attribute list at ./test14.pl line 7, at end of line syntax error at ./test14.pl line 7, at EOF" I'm trying to understand what is an acceptable value for the :default parameters. After reading the write-up on CPAN and looking at the source, I concluded that the ":default" parameters can be a number, string, array ref, or hash ref literal. Unfortunately, the write-up doesn't provide a grammar and the source code was hard to follow.

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Re: Class::Std :default parameter
by xdg (Monsignor) on Oct 25, 2005 at 18:26 UTC

    First, you want to use Class::Std after your package declaration, not before. Second, I count three left parens and only two right parens.

    -xdg

    Code written by xdg and posted on PerlMonks is public domain. It is provided as is with no warranties, express or implied, of any kind. Posted code may not have been tested. Use of posted code is at your own risk.

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      Moving the use statement after the package declaration doesn't fix the problem. And the 3rd left parenthesis is inside of the string literal. IMHO, the two sets of parentheses match.
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        I apologize -- the parens problem is because it's unbalanced, but, you're right, it's more subtle than what I originally said. The parameter list for an attribute isn't really parsed normally. From attributes:

           An attribute list is a sequence of attribute specifications, separa
+ted
   by whitespace or a colon (with optional whitespace). Each attribute
   specification is a simple name, optionally followed by a parenthesi
+sed
   parameter list. If such a parameter list is present, it is scanned 
+past
   as for the rules for the "q()" operator. (See "Quote and Quote-like
   Operators" in perlop.) The parameter list is passed as it was found
+,
   however, and not as per "q()".

   Some examples of syntactically valid attribute lists:

       switch(10,foo(7,3))  :  expensive
       Ugly('\(") :Bad
       _5x5
       locked method

   Some examples of syntactically invalid attribute lists (with
   annotation):

       switch(10,foo()             # ()-string not balanced
       Ugly('(')                   # ()-string not balanced
       5x5                         # "5x5" not a valid identifier
       Y2::north                   # "Y2::north" not a simple identifi
+er
       foo + bar                   # "+" neither a colon nor whitespac
+e

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        Reading between the lines, the difference in the "Ugly" specs above seems to be the use of the backslash on the "(" in the first example. You might try backspacing your inner "(" and see if that helps.

        If the parens problem is fixed, you will need the use statement after the package statement because Class::Std exports subroutines that the attributes mechanism looks for in the current package when an attribute is detected. See my post, Re: Class::Std : How does MODIFY_HASH_ATTRIBUTES work? for details.

        -xdg

        Code written by xdg and posted on PerlMonks is public domain. It is provided as is with no warranties, express or implied, of any kind. Posted code may not have been tested. Use of posted code is at your own risk.

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Re: Class::Std :default parameter
by Util (Priest) on Oct 26, 2005 at 01:27 UTC

    Escaping the embedded unbalanced paren with a backslash removes the error. 

    package Foo;
use Class::Std;
my %a : ATTR( :default( '<p />:\(' ) );
print "Hello\n";

    [download]

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Re: Class::Std :default parameter
by esharris (Monk) on Oct 25, 2005 at 18:09 UTC
    Here is a follow-up question. :default('') didn't work as expected for me. It initialized the hash bucket to undefined, instead of the null string. (Note: "eval '';" returns normally with the undefined value. To initialize the hash bucket to the null string, I had to do :default("''"). IMHO, this seems like a "gotcha". Is this a bug?
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