|
perltrap - Perl traps for the unwary |
perltrap - Perl traps for the unwary
The biggest trap of all is forgetting to use warnings or use the -w
switch; see the perllexwarn manpage and the perlrun manpage. The second biggest trap is not
making your entire program runnable under use strict. The third biggest
trap is not reading the list of changes in this version of Perl; see
the perldelta manpage.
Accustomed awk users should take special note of the following:
use English;
allows you to refer to special variables (like $/) with names (like
$RS), as though they were in awk; see the perlvar manpage for details.
ifs and whiles.
substr() and
index().
split() operator has different
arguments than awk's.
print() statement does not add field and record separators unless
you set $, and $\. You can set $OFS and $ORS if you're using
the English module.
/pat/ /pat/ unparsable, because the third slash
would be interpreted as a division operator--the tokenizer is in fact
slightly context sensitive for operators like ``/'', ``?'', and ``>''.
And in fact, ``.'' itself can be the beginning of a number.)
next, exit, and continue keywords work differently.
Awk Perl
ARGC scalar @ARGV (compare with $#ARGV)
ARGV[0] $0
FILENAME $ARGV
FNR $. - something
FS (whatever you like)
NF $#Fld, or some such
NR $.
OFMT $#
OFS $,
ORS $\
RLENGTH length($&)
RS $/
RSTART length($`)
SUBSEP $;
Cerebral C programmers should take note of the following:
if's and while's.
elsif rather than else if.
break and continue keywords from C become in
Perl last and next, respectively.
Unlike in C, these do not work within a do { } while construct.
ARGV must be capitalized. $ARGV[0] is C's argv[1], and argv[0]
ends up in $0.
kill -l
to find their names on your system.
Seasoned sed programmers should take note of the following:
..., rather than comma.
Sharp shell programmers should take note of the following:
BEGIN blocks, which
execute at compile time).
Practicing Perl Programmers should take note of the following:
chop() and chdir())
and which are list operators (like print() and unlink()).
(Unless prototyped, user-defined subroutines can only be list
operators, never unary ones.) See the perlop manpage and the perlsub manpage.
while (<FH>) { }
while (defined($_ = <FH>)) { }..
<FH>; # data discarded!
= when you need =~;
these two constructs are quite different:
$x = /foo/;
$x =~ /foo/;
do {} construct isn't a real loop that you can use
loop control on.
my() for local variables whenever you can get away with
it (but see the perlform manpage for where you can't).
Using local() actually gives a local value to a global
variable, which leaves you open to unforeseen side-effects
of dynamic scoping.
Practicing Perl4 Programmers should take note of the following Perl4-to-Perl5 specific traps.
They're crudely ordered according to the following list:
dbmopen(), and specific dbm implementations.
If you find an example of a conversion trap that is not listed here,
please submit it to <perlbug@perl.org> for inclusion.
Also note that at least some of these can be caught with the
use warnings pragma or the -w switch.
Anything that has been discontinued, deprecated, or fixed as a bug from perl4.
$_ itself (and @_, etc.).
package test;
$_legacy = 1;
package main;
print "\$_legacy is ",$_legacy,"\n";
# perl4 prints: $_legacy is 1
# perl5 prints: $_legacy is
$a=1;$b=2;$c=3;$var=4;
print "$a::$b::$c ";
print "$var::abc::xyz\n";
# perl4 prints: 1::2::3 4::abc::xyz
# perl5 prints: 3
Given that :: is now the preferred package delimiter, it is debatable
whether this should be classed as a bug or not.
(The older package delimiter, ' ,is used here)
$x = 10 ;
print "x=${'x}\n" ;
# perl4 prints: x=10
# perl5 prints: Can't find string terminator "'" anywhere before EOF
You can avoid this problem, and remain compatible with perl4, if you always explicitly include the package name:
$x = 10 ;
print "x=${main'x}\n" ;
Also see precedence traps, for parsing $:.
splice() are now evaluated in scalar
context (as the Camel says) rather than list context.
sub sub1{return(0,2) } # return a 2-element list
sub sub2{ return(1,2,3)} # return a 3-element list
@a1 = ("a","b","c","d","e");
@a2 = splice(@a1,&sub1,&sub2);
print join(' ',@a2),"\n";
# perl4 prints: a b
# perl5 prints: c d e
goto into a block that is optimized away. Darn.
goto marker1;
for(1){
marker1:
print "Here I is!\n";
}
# perl4 prints: Here I is!
# perl5 errors: Can't "goto" into the middle of a foreach loop
$a = ("foo bar");
$b = q baz ;
print "a is $a, b is $b\n";
# perl4 prints: a is foo bar, b is baz
# perl5 errors: Bareword found where operator expected
if { 1 } {
print "True!";
}
else {
print "False!";
}
# perl4 prints: True!
# perl5 errors: syntax error at test.pl line 1, near "if {"
** operator now binds more tightly than unary minus.
It was documented to work this way before, but didn't.
print -4**2,"\n";
# perl4 prints: 16
# perl5 prints: -16
foreach{} has changed slightly when it is iterating over a
list which is not an array. This used to assign the list to a
temporary array, but no longer does so (for efficiency). This means
that you'll now be iterating over the actual values, not over copies of
the values. Modifications to the loop variable can change the original
values.
@list = ('ab','abc','bcd','def');
foreach $var (grep(/ab/,@list)){
$var = 1;
}
print (join(':',@list));
# perl4 prints: ab:abc:bcd:def
# perl5 prints: 1:1:bcd:def
To retain Perl4 semantics you need to assign your list explicitly to a temporary array and then iterate over that. For example, you might need to change
foreach $var (grep(/ab/,@list)){
to
foreach $var (@tmp = grep(/ab/,@list)){
Otherwise changing $var will clobber the values of @list. (This most often
happens when you use $_ for the loop variable, and call subroutines in
the loop that don't properly localize $_.)
split with no arguments now behaves like split ' ' (which doesn't
return an initial null field if $_ starts with whitespace), it used to
behave like split /\s+/ (which does).
$_ = ' hi mom';
print join(':', split);
# perl4 prints: :hi:mom
# perl5 prints: hi:mom
perl -e'print "attached to -e"' 'print "separate arg"'
# perl4 prints: separate arg
# perl5 prints: attached to -e
perl -e
# perl4 prints:
# perl5 dies: No code specified for -e.
push was undocumented, but it was
actually the last value being pushed onto the target list. In Perl 5
the return value of push is documented, but has changed, it is the
number of elements in the resulting list.
@x = ('existing');
print push(@x, 'first new', 'second new');
# perl4 prints: second new
# perl5 prints: 3
split() were ??, the result would be placed in @_ as well as
being returned. Perl 5 has more respect for your subroutine arguments.
Perl4-to-Perl5 traps from having to do with parsing.
$string . = "more string";
print $string;
# perl4 prints: more string
# perl5 prints: syntax error at - line 1, near ". ="
sub foo {}
&foo
print("hello, world\n");
# perl4 prints: hello, world
# perl5 prints: syntax error
print
($foo == 1) ? "is one\n" : "is zero\n";
# perl4 prints: is zero
# perl5 warns: "Useless use of a constant in void context" if using -w
$#array construct differs when braces
are to used around the name.
@a = (1..3);
print "${#a}";
# perl4 prints: 2
# perl5 fails with syntax error
@ = (1..3);
print "$#{a}";
# perl4 prints: {a}
# perl5 prints: 2
Perl4-to-Perl5 traps having to do with numerical operators, operands, or output from same.
print 7.373504 - 0, "\n";
printf "%20.18f\n", 7.373504 - 0;
# Perl4 prints:
7.375039999999996141
7.37503999999999614
# Perl5 prints:
7.373504
7.37503999999999614
use Math::BigInt;
$p = ($test == 1);
print $p,"\n";
# perl4 prints: 0
# perl5 prints:
Also see General Regular Expression Traps using s///, etc. for another example of this new feature...
& | ^ ~) are given only strings as arguments, perl4 would
treat the operands as bitstrings so long as the program contained a call
to the vec() function. perl5 treats the string operands as bitstrings.
(See Bitwise String Operators in the perlop manpage for more details.)
$fred = "10";
$barney = "12";
$betty = $fred & $barney;
print "$betty\n";
# Uncomment the next line to change perl4's behavior
# ($dummy) = vec("dummy", 0, 0);
# Perl4 prints:
8
# Perl5 prints:
10
# If vec() is used anywhere in the program, both print:
10
Perl4-to-Perl5 traps involving most data-types, and their usage within certain expressions and/or context.
@a = (1, 2, 3, 4, 5);
print "The third element of the array is $a[3] also expressed as $a[-2] \n";
# perl4 prints: The third element of the array is 4 also expressed as
# perl5 prints: The third element of the array is 4 also expressed as 4
$#array lower now discards array elements, and makes them
impossible to recover.
@a = (a,b,c,d,e);
print "Before: ",join('',@a);
$#a =1;
print ", After: ",join('',@a);
$#a =3;
print ", Recovered: ",join('',@a),"\n";
# perl4 prints: Before: abcde, After: ab, Recovered: abcd
# perl5 prints: Before: abcde, After: ab, Recovered: ab
local($s,@a,%h);
die "scalar \$s defined" if defined($s);
die "array \@a defined" if defined(@a);
die "hash \%h defined" if defined(%h);
# perl4 prints:
# perl5 dies: hash %h defined
Perl will now generate a warning when it sees defined(@a) and
defined(%h).
@a = ("This is Perl 4");
*b = *a;
local(@a);
print @b,"\n";
# perl4 prints: This is Perl 4
# perl5 prints:
undef to a glob has no effect in Perl 5. In Perl 4
it undefines the associated scalar (but may have other side effects
including SEGVs). Perl 5 will also warn if undef is assigned to a
typeglob. (Note that assigning undef to a typeglob is different
than calling the undef function on a typeglob (undef *foo), which
has quite a few effects.
$foo = "bar";
*foo = undef;
print $foo;
# perl4 prints:
# perl4 warns: "Use of uninitialized variable" if using -w
# perl5 prints: bar
# perl5 warns: "Undefined value assigned to typeglob" if using -w
$x = "aaa";
print ++$x," : ";
print -$x," : ";
print ++$x,"\n";
# perl4 prints: aab : -0 : 1
# perl5 prints: aab : -aab : aac
$foo = "x";
&mod($foo);
for ($x = 0; $x < 3; $x++) {
&mod("a");
}
sub mod {
print "before: $_[0]";
$_[0] = "m";
print " after: $_[0]\n";
}
# perl4:
# before: x after: m
# before: a after: m
# before: m after: m
# before: m after: m
# Perl5:
# before: x after: m
# Modification of a read-only value attempted at foo.pl line 12.
# before: a
print "$x", defined $x
# perl 4: 1
# perl 5: <no output, $x is not called into existence>
$aGlobal{ "aKey" } = "global value";
print "MAIN:", $aGlobal{"aKey"}, "\n";
$GlobalLevel = 0;
&test( *aGlobal );
sub test {
local( *theArgument ) = @_;
local( %aNewLocal ); # perl 4 != 5.001l,m
$aNewLocal{"aKey"} = "this should never appear";
print "SUB: ", $theArgument{"aKey"}, "\n";
$aNewLocal{"aKey"} = "level $GlobalLevel"; # what should print
$GlobalLevel++;
if( $GlobalLevel<4 ) {
&test( *aNewLocal );
}
}
# Perl4:
# MAIN:global value
# SUB: global value
# SUB: level 0
# SUB: level 1
# SUB: level 2
# Perl5:
# MAIN:global value
# SUB: global value
# SUB: this should never appear
# SUB: this should never appear
# SUB: this should never appear
@fmt = ("foo","bar","baz");
format STDOUT=
@<<<<< @||||| @>>>>>
@fmt;
.
write;
# perl4 errors: Please use commas to separate fields in file
# perl5 prints: foo bar baz
caller() function now returns a false value in a scalar context
if there is no caller. This lets library files determine if they're
being required.
caller() ? (print "You rang?\n") : (print "Got a 0\n");
# perl4 errors: There is no caller
# perl5 prints: Got a 0
@y= ('a','b','c');
$x = (1, 2, @y);
print "x = $x\n";
# Perl4 prints: x = c # Thinks list context interpolates list
# Perl5 prints: x = 3 # Knows scalar uses length of list
sprintf() is prototyped as ($;@), so its first argument is given scalar
context. Thus, if passed an array, it will probably not do what you want,
unlike Perl 4:
@z = ('%s%s', 'foo', 'bar');
$x = sprintf(@z);
print $x;
# perl4 prints: foobar
# perl5 prints: 3
printf() works the same as it did in Perl 4, though:
@z = ('%s%s', 'foo', 'bar');
printf STDOUT (@z);
# perl4 prints: foobar
# perl5 prints: foobar
Perl4-to-Perl5 traps involving precedence order.
Perl 4 has almost the same precedence rules as Perl 5 for the operators that they both have. Perl 4 however, seems to have had some inconsistencies that made the behavior differ from what was documented.
@arr = ( 'left', 'right' );
$a{shift @arr} = shift @arr;
print join( ' ', keys %a );
# perl4 prints: left
# perl5 prints: right
@list = (1,2,3,4,5);
%map = ("a",1,"b",2,"c",3,"d",4);
$n = shift @list + 2; # first item in list plus 2
print "n is $n, ";
$m = keys %map + 2; # number of items in hash plus 2
print "m is $m\n";
# perl4 prints: n is 3, m is 6
# perl5 errors and fails to compile
/foo/ ? ($a += 2) : ($a -= 2);
Otherwise
/foo/ ? $a += 2 : $a -= 2
would be erroneously parsed as
(/foo/ ? $a += 2 : $a) -= 2;
On the other hand,
$a += /foo/ ? 1 : 2;
now works as a C programmer would expect.
open FOO || die;
is now incorrect. You need parentheses around the filehandle. Otherwise, perl5 leaves the statement as its default precedence:
open(FOO || die);
# perl4 opens or dies
# perl5 opens FOO, dying only if 'FOO' is false, i.e. never
$: precedence, where perl5
treats $:: as main package
$a = "x"; print "$::a";
# perl 4 prints: -:a
# perl 5 prints: x
-e $foo .= "q" should parse as
((-e $foo) .= "q"), it actually parses as (-e ($foo .= "q")).
In perl5, the precedence is as documented.
-e $foo .= "q"
# perl4 prints: no output
# perl5 prints: Can't modify -e in concatenation
each() and values() were special high-precedence operators
that operated on a single hash, but in perl5, they are regular named unary
operators. As documented, named unary operators have lower precedence
than the arithmetic and concatenation operators + - ., but the perl4
variants of these operators actually bind tighter than + - ..
Thus, for:
%foo = 1..10;
print keys %foo - 1
# perl4 prints: 4
# perl5 prints: Type of arg 1 to keys must be hash (not subtraction)
The perl4 behavior was probably more useful, if less consistent.
All types of RE traps.
s'$lhs'$rhs' now does no interpolation on either side. It used to
interpolate $lhs but not $rhs. (And still does not match a literal
'$' in string)
$a=1;$b=2;
$string = '1 2 $a $b';
$string =~ s'$a'$b';
print $string,"\n";
# perl4 prints: $b 2 $a $b
# perl5 prints: 1 2 $a $b
m//g now attaches its state to the searched string rather than the
regular expression. (Once the scope of a block is left for the sub, the
state of the searched string is lost)
$_ = "ababab";
while(m/ab/g){
&doit("blah");
}
sub doit{local($_) = shift; print "Got $_ "}
# perl4 prints: Got blah Got blah Got blah Got blah
# perl5 prints: infinite loop blah...
m//o qualifier on a regular expression
within an anonymous sub, all closures generated from that anonymous
sub will use the regular expression as it was compiled when it was used
the very first time in any such closure. For instance, if you say
sub build_match {
my($left,$right) = @_;
return sub { $_[0] =~ /$left stuff $right/o; };
}
$good = build_match('foo','bar');
$bad = build_match('baz','blarch');
print $good->('foo stuff bar') ? "ok\n" : "not ok\n";
print $bad->('baz stuff blarch') ? "ok\n" : "not ok\n";
print $bad->('foo stuff bar') ? "not ok\n" : "ok\n";
For most builds of Perl5, this will print: ok not ok not ok
build_match() will always return a sub which matches the contents of
$left and $right as they were the first time that build_match()
was called, not as they are in the current call.
$+ to
the whole match, just like $&. Perl5 does not.
"abcdef" =~ /b.*e/;
print "\$+ = $+\n";
# perl4 prints: bcde
# perl5 prints:
$string = "test";
$value = ($string =~ s/foo//);
print $value, "\n";
# perl4 prints: 0
# perl5 prints:
Also see Numerical Traps for another example of this new feature.
s`lhs`rhs` (using backticks) is now a normal substitution, with no
backtick expansion
$string = "";
$string =~ s`^`hostname`;
print $string, "\n";
# perl4 prints: <the local hostname>
# perl5 prints: hostname
s/^([^$grpc]*$grpc[$opt$plus$rep]?)//o;
# perl4: compiles w/o error
# perl5: with Scalar found where operator expected ..., near "$opt$plus"
an added component of this example, apparently from the same script, is
the actual value of the s'd string after the substitution.
[$opt] is a character class in perl4 and an array subscript in perl5
$grpc = 'a';
$opt = 'r';
$_ = 'bar';
s/^([^$grpc]*$grpc[$opt]?)/foo/;
print ;
# perl4 prints: foo
# perl5 prints: foobar
m?x? matches only once, like ?x?. Under perl4, it matched
repeatedly, like /x/ or m!x!.
$test = "once";
sub match { $test =~ m?once?; }
&match();
if( &match() ) {
# m?x? matches more then once
print "perl4\n";
} else {
# m?x? matches only once
print "perl5\n";
}
# perl4 prints: perl4
# perl5 prints: perl5
The general group of Perl4-to-Perl5 traps having to do with Signals, Sorting, and their related subroutines, as well as general subroutine traps. Includes some OS-Specific traps.
sub SeeYa { warn"Hasta la vista, baby!" }
$SIG{'TERM'} = SeeYa;
print "SIGTERM is now $SIG{'TERM'}\n";
# perl4 prints: SIGTERM is now main'SeeYa
# perl5 prints: SIGTERM is now main::1 (and warns "Hasta la vista, baby!")
Use -w to catch this one
sub reverse{ print "yup "; $a <=> $b }
print sort reverse (2,1,3);
# perl4 prints: yup yup 123
# perl5 prints: 123
# perl5 warns (if using -w): Ambiguous call resolved as CORE::reverse()
warn() won't let you specify a filehandle.warn() would let you specify a
filehandle in perl4. With perl5 it does not.
warn STDERR "Foo!";
# perl4 prints: Foo!
# perl5 prints: String found where operator expected
Since version 5.002, Perl uses sigaction() under SysV.
sub gotit {
print "Got @_... ";
}
$SIG{'INT'} = 'gotit';
$| = 1;
$pid = fork;
if ($pid) {
kill('INT', $pid);
sleep(1);
kill('INT', $pid);
} else {
while (1) {sleep(10);}
}
# perl4 (HPUX) prints: Got INT...
# perl5 (HPUX) prints: Got INT... Got INT...
seek() on a file opened to append >> now does
the right thing w.r.t. the fopen() manpage. e.g., - When a file is opened
for append, it is impossible to overwrite information already in
the file.
open(TEST,">>seek.test");
$start = tell TEST ;
foreach(1 .. 9){
print TEST "$_ ";
}
$end = tell TEST ;
seek(TEST,$start,0);
print TEST "18 characters here";
# perl4 (solaris) seek.test has: 18 characters here
# perl5 (solaris) seek.test has: 1 2 3 4 5 6 7 8 9 18 characters here
Perl4-to-Perl5 traps having to do with how things get interpolated within certain expressions, statements, contexts, or whatever.
print "To: someone@somewhere.com\n";
# perl4 prints: To:someone@somewhere.com
# perl < 5.6.1, error : In string, @somewhere now must be written as \@somewhere
# perl >= 5.6.1, warning : Possible unintended interpolation of @somewhere in string
$foo = "foo$";
$bar = "bar@";
print "foo is $foo, bar is $bar\n";
# perl4 prints: foo is foo$, bar is bar@
# perl5 errors: Final $ should be \$ or $name
Note: perl5 DOES NOT error on the terminating @ in $bar
$
or @).
@www = "buz";
$foo = "foo";
$bar = "bar";
sub foo { return "bar" };
print "|@{w.w.w}|${main'foo}|";
# perl4 prints: |@{w.w.w}|foo|
# perl5 prints: |buz|bar|
Note that you can use strict; to ward off such trappiness under perl5.
$$ by itself still works fine, however.
$s = "a reference";
$x = *s;
print "this is $$x\n";
# perl4 prints: this is XXXx (XXX is the current pid)
# perl5 prints: this is a reference
eval "EXPR" now requires either both
$'s to be protected in the specification of the hash name, or both curlies
to be protected. If both curlies are protected, the result will be compatible
with perl4 and perl5. This is a very common practice, and should be changed
to use the block form of eval{} if possible.
$hashname = "foobar";
$key = "baz";
$value = 1234;
eval "\$$hashname{'$key'} = q|$value|";
(defined($foobar{'baz'})) ? (print "Yup") : (print "Nope");
# perl4 prints: Yup
# perl5 prints: Nope
Changing
eval "\$$hashname{'$key'} = q|$value|";
to
eval "\$\$hashname{'$key'} = q|$value|";
causes the following result:
# perl4 prints: Nope
# perl5 prints: Yup
or, changing to
eval "\$$hashname\{'$key'\} = q|$value|";
causes the following result:
# perl4 prints: Yup
# perl5 prints: Yup
# and is compatible for both versions
perl -e '$bar=q/not/; print "This is $foo{$bar} perl5"'
# perl4 prints: This is not perl5
# perl5 prints: This is perl5
print "$foo{"
perl 4 prints: {
perl 5 prints: syntax error
$foo = "baz";
print "\$$foo{bar}\n";
# perl4 prints: $baz{bar}
# perl5 prints: $
Perl 5 is looking for $foo{bar} which doesn't exist, but perl 4 is
happy just to expand $foo to ``baz'' by itself. Watch out for this
especially in eval's.
qq() string passed to eval
eval qq(
foreach \$y (keys %\$x\) {
\$count++;
}
);
# perl4 runs this ok
# perl5 prints: Can't find string terminator ")"
General DBM traps.
dbmopen()
to function properly without tie'ing to an extension dbm implementation.
dbmopen (%dbm, "file", undef);
print "ok\n";
# perl4 prints: ok
# perl5 prints: ok (IFF linked with -ldbm or -lndbm)
dbmopen(DB, "testdb",0600) || die "couldn't open db! $!";
$DB{'trap'} = "x" x 1024; # value too large for most dbm/ndbm
print "YUP\n";
# perl4 prints:
dbm store returned -1, errno 28, key "trap" at - line 3.
YUP
# perl5 prints:
dbm store returned -1, errno 28, key "trap" at - line 3.
Everything else.
require/do trap using returned value
sub foo {
$rc = do "./do.pl";
return 8;
}
print &foo, "\n";
And the do.pl file has the following single line:
return 3;
Running doit.pl gives the following:
# perl 4 prints: 3 (aborts the subroutine early)
# perl 5 prints: 8
split on empty string with LIMIT specified
$string = '';
@list = split(/foo/, $string, 2)
Perl4 returns a one element list containing the empty string but Perl5 returns an empty list.
As always, if any of these are ever officially declared as bugs, they'll be fixed and removed.
|
perltrap - Perl traps for the unwary |