Re^2: Hidden Secrets of PERL

There is no ambiguity, although one has to admit that the case may be confusing. But it's perfectly well documented.

Eh, no... there is ambiguity as the syntax of this piece of code is ambiguous for the grammar.

Only, Perl has a well documented way to disambiguate this case.

Other ambiguities include code block versus anonymous hash, and the opening slash for regexes for optional function arguments:

map { $_ => length } @list   # code block or hash?
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rand /2 /m   # division or regex?
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Occasionally, in particular for the former example, Pelr makes an initial guess and guesses wrong, producing a syntax error further down in the parsing stream — too late to go back and try the other route. It requires a well known disambiguation trick to resolve the problem, early enough:

+{ $_ => length }    # hashref
{ ; $_ => length }   # code block
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Re^3: Hidden Secrets of PERL by Anonymous Monk on Oct 12, 2006 at 07:57 UTC
Only, Perl has a well documented way to disambiguate this case. If it does, it certainly doesn't say so in the documentation of `print`. This is the documentation of `print` (from 5.8.5): print FILEHANDLE LIST print LIST print Prints a string or a list of strings. Returns true if success- ful. FILEHANDLE may be a scalar variable name, in which case the variable contains the name of or a reference to the file- handle, thus introducing one level of indirection. (NOTE: If FILEHANDLE is a variable and the next token is a term, it may be misinterpreted as an operator unless you interpose a "+" or put parentheses around the arguments.) If FILEHANDLE is omit- ted, prints by default to standard output (or to the last selected output channel--see "select"). If LIST is also omit- ted, prints $_ to the currently selected output channel. To set the default output channel to something other than STDOUT use the select operation. The current value of $, (if any) is printed between each LIST item. The current value of "$\" (if any) is printed after the entire LIST has been printed. Because print takes a LIST, anything in the LIST is evaluated in list context, and any subroutine that you call will have one or more of its expressions evaluated in list context. Also be careful not to follow the print keyword with a left parenthesis unless you want the corresponding right parenthesis to termi- nate the arguments to the print--interpose a "+" or put paren- theses around all the arguments. Note that if you're storing FILEHANDLES in an array or other expression, you will have to use a block returning its value instead: print { $files[$i] } "stuff\n"; print { $OK ? STDOUT : STDERR } "stuff\n"; The only part that hints about ambiguaty is the part I boldfaced. But it doesn't explain at all why it parses the statement differently whether there's a space between the third and fourth token. Not that I expect most programmers to know what the phrase next token is a term means.	[reply]

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Re^3: Hidden Secrets of PERL
by Anonymous Monk on Oct 12, 2006 at 07:57 UTC

Only, Perl has a well documented way to disambiguate this case.

If it does, it certainly doesn't say so in the documentation of print. This is the documentation of print (from 5.8.5):

       print FILEHANDLE LIST
       print LIST
       print   Prints a string or a list of strings.  Returns true if success-
               ful.  FILEHANDLE may be a scalar variable name, in which case
               the variable contains the name of or a reference to the file-
               handle, thus introducing one level of indirection.  (NOTE: If
               FILEHANDLE is a variable and the next token is a term, it may
               be misinterpreted as an operator unless you interpose a "+" or
               put parentheses around the arguments.)  If FILEHANDLE is omit-
               ted, prints by default to standard output (or to the last
               selected output channel--see "select").  If LIST is also omit-
               ted, prints $_ to the currently selected output channel.  To
               set the default output channel to something other than STDOUT
               use the select operation.  The current value of $, (if any) is
               printed between each LIST item.  The current value of "$\" (if
               any) is printed after the entire LIST has been printed.
               Because print takes a LIST, anything in the LIST is evaluated
               in list context, and any subroutine that you call will have one
               or more of its expressions evaluated in list context.  Also be
               careful not to follow the print keyword with a left parenthesis
               unless you want the corresponding right parenthesis to termi-
               nate the arguments to the print--interpose a "+" or put paren-
               theses around all the arguments.

               Note that if you're storing FILEHANDLES in an array or other
               expression, you will have to use a block returning its value
               instead:

                   print { $files[$i] } "stuff\n";
                   print { $OK ? STDOUT : STDERR } "stuff\n";

The only part that hints about ambiguaty is the part I boldfaced. But it doesn't explain at all why it parses the statement differently whether there's a space between the third and fourth token. Not that I expect most programmers to know what the phrase next token is a term means.

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