问题
I see I can do something like this:
print STDOUT (split /\./, 'www.stackoverflow.com')[1];
and "stackoverflow" is printed. However, this:
print +(split /\./, 'www.stackoverflow.com')[1];
does the same, and this:
print (split /\./, 'www.stackoverflow.com')[1];
is a syntax error. So what exactly is going on here? I've always understood the unary plus sign to do nothing whatsoever in any context. And if "print FILEHANDLE EXPR" works, I would have imagined that "print EXPR" would always work equally well. Any insights?
回答1:
You do not have warnings enabled. In the print(...)[1] case, the set of parentheses are regarded as part of the function syntax.
print (...) interpreted as function at C:\Temp\t.pl line 4.
From, perldoc -f print:
Also be careful not to follow the print keyword with a left parenthesis unless you want the corresponding right parenthesis to terminate the arguments to the print—interpose a
+or put parentheses around all the arguments.
See also Why aren't newlines being printed in this Perl code?
回答2:
perldoc for print includes this nugget:
Also be careful not to follow the print keyword with a left parenthesis unless you want the corresponding right parenthesis to terminate the arguments to the print--interpose a "+" or put parentheses around all the arguments.
print always evaluates its arguments in LIST context.
To say
print (split /\./, 'www.stackoverflow.com')
is ok. But when you say
print (split /\./, 'www.stackoverflow.com')[0]
the parser expects a LIST after it sees the first (, and considers the LIST to be complete when it sees the closing ). The [0] is not interpreted as operating on anything, so you get a syntax error.
print "abc","def"; # prints "abcdef"
print ("abc","def"); # prints "abcdef"
print ("abc"), "def"; # prints "abc"
Other Perl functions that can take a LIST as the first argument behave the same way:
warn ($message),"\n" # \n not passed to warn, line # info not suppressed
system ("echo"),"x" # not same as system("echo","x") or system "echo","x"
# or system(("echo"),"x")
来源：https://stackoverflow.com/questions/1683158/why-cant-i-have-a-literal-list-slice-right-after-a-print-in-perl