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Diffstat (limited to 'pod/perltrap.pod')
| -rw-r--r-- | pod/perltrap.pod | 262 |
1 files changed, 131 insertions, 131 deletions
diff --git a/pod/perltrap.pod b/pod/perltrap.pod index fd91182d1e..fd41f2ef4d 100644 --- a/pod/perltrap.pod +++ b/pod/perltrap.pod @@ -20,8 +20,8 @@ The English module, loaded via use English; -allows you to refer to special variables (like $RS) as -though they were in B<awk>; see L<perlvar> for details. +allows you to refer to special variables (like C<$/>) with names (like +C<$RS>), as though they were in B<awk>; see L<perlvar> for details. =item * @@ -57,8 +57,8 @@ comparisons. =item * Reading an input line does not split it for you. You get to split it -yourself to an array. And the split() operator has different -arguments. +to an array yourself. And the split() operator has different +arguments than B<awk>'s. =item * @@ -157,7 +157,7 @@ You must use C<elsif> rather than C<else if>. =item * -The C<break> and C<continue> keywords from C become in +The C<break> and C<continue> keywords from C become in Perl C<last> and C<next>, respectively. Unlike in C, these do I<NOT> work within a C<do { } while> construct. @@ -230,18 +230,18 @@ Sharp shell programmers should take note of the following: =item * -The back-tick operator does variable interpolation without regard to +The backtick operator does variable interpolation without regard to the presence of single quotes in the command. =item * -The back-tick operator does no translation of the return value, unlike B<csh>. +The backtick operator does no translation of the return value, unlike B<csh>. =item * Shells (especially B<csh>) do several levels of substitution on each command line. Perl does substitution in only certain constructs -such as double quotes, back-ticks, angle brackets, and search patterns. +such as double quotes, backticks, angle brackets, and search patterns. =item * @@ -274,14 +274,14 @@ context than they do in a scalar one. See L<perldata> for details. =item * Avoid barewords if you can, especially all lowercase ones. -You can't tell by just looking at it whether a bareword is -a function or a string. By using quotes on strings and +You can't tell by just looking at it whether a bareword is +a function or a string. By using quotes on strings and parentheses on function calls, you won't ever get them confused. =item * -You cannot discern from mere inspection which built-ins -are unary operators (like chop() and chdir()) +You cannot discern from mere inspection which builtins +are unary operators (like chop() and chdir()) and which are list operators (like print() and unlink()). (User-defined subroutines can be B<only> list operators, never unary ones.) See L<perlop>. @@ -290,7 +290,7 @@ unary ones.) See L<perlop>. People have a hard time remembering that some functions default to $_, or @ARGV, or whatever, but that others which -you might expect to do not. +you might expect to do not. =item * @@ -299,7 +299,7 @@ operation on that handle. The data read is assigned to $_ only if the file read is the sole condition in a while loop: while (<FH>) { } - while ($_ = <FH>) { }.. + while (defined($_ = <FH>)) { }.. <FH>; # data discarded! =item * @@ -312,14 +312,14 @@ these two constructs are quite different: =item * -The C<do {}> construct isn't a real loop that you can use +The C<do {}> construct isn't a real loop that you can use loop control on. =item * -Use C<my()> for local variables whenever you can get away with -it (but see L<perlform> for where you can't). -Using C<local()> actually gives a local value to a global +Use C<my()> for local variables whenever you can get away with +it (but see L<perlform> for where you can't). +Using C<local()> actually gives a local value to a global variable, which leaves you open to unforeseen side-effects of dynamic scoping. @@ -333,7 +333,7 @@ external name is still an alias for the original. =head2 Perl4 to Perl5 Traps -Practicing Perl4 Programmers should take note of the following +Practicing Perl4 Programmers should take note of the following Perl4-to-Perl5 specific traps. They're crudely ordered according to the following list: @@ -397,11 +397,11 @@ Also note that at least some of these can be caught with B<-w>. =head2 Discontinuance, Deprecation, and BugFix traps Anything that has been discontinued, deprecated, or fixed as -a bug from perl4. +a bug from perl4. =over 4 -=item * Discontinuance +=item * Discontinuance Symbols starting with "_" are no longer forced into package main, except for C<$_> itself (and C<@_>, etc.). @@ -411,11 +411,11 @@ for C<$_> itself (and C<@_>, etc.). package main; print "\$_legacy is ",$_legacy,"\n"; - + # perl4 prints: $_legacy is 1 # perl5 prints: $_legacy is -=item * Deprecation +=item * Deprecation Double-colon is now a valid package separator in a variable name. Thus these behave differently in perl4 vs. perl5, because the packages don't exist. @@ -433,11 +433,11 @@ whether this should be classed as a bug or not. $x = 10 ; print "x=${'x}\n" ; - + # perl4 prints: x=10 # perl5 prints: Can't find string terminator "'" anywhere before EOF -Also see precedence traps, for parsing C<$:>. +Also see precedence traps, for parsing C<$:>. =item * BugFix @@ -446,39 +446,39 @@ context (as the Camel says) rather than list context. sub sub1{return(0,2) } # return a 2-elem array sub sub2{ return(1,2,3)} # return a 3-elem array - @a1 = ("a","b","c","d","e"); + @a1 = ("a","b","c","d","e"); @a2 = splice(@a1,&sub1,&sub2); print join(' ',@a2),"\n"; - + # perl4 prints: a b - # perl5 prints: c d e + # perl5 prints: c d e -=item * Discontinuance +=item * Discontinuance You can't do a C<goto> into a block that is optimized away. Darn. goto marker1; - for(1){ + for(1){ marker1: print "Here I is!\n"; - } - + } + # perl4 prints: Here I is! # perl5 dumps core (SEGV) -=item * Discontinuance +=item * Discontinuance It is no longer syntactically legal to use whitespace as the name of a variable, or as a delimiter for any kind of quote construct. -Double darn. +Double darn. $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: Bare word found where operator expected + # perl5 errors: Bareword found where operator expected =item * Discontinuance @@ -490,7 +490,7 @@ The archaic while/if BLOCK BLOCK syntax is no longer supported. else { print "False!"; } - + # perl4 prints: True! # perl5 errors: syntax error at test.pl line 1, near "if {" @@ -500,11 +500,11 @@ The C<**> 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 -=item * Discontinuance +=item * Discontinuance The meaning of C<foreach{}> has changed slightly when it is iterating over a list which is not an array. This used to assign the list to a @@ -518,12 +518,12 @@ values. $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 +explicitly to a temporary array and then iterate over that. For example, you might need to change foreach $var (grep(/ab/,@list)){ @@ -556,10 +556,10 @@ would silently accept an B<-e> switch without a following arg. Both of these behaviors have been fixed. perl -e'print "attached to -e"' 'print "separate arg"' - + # perl4 prints: separate arg # perl5 prints: attached to -e - + perl -e # perl4 prints: @@ -574,7 +574,7 @@ number of elements in the resulting list. @x = ('existing'); print push(@x, 'first new', 'second new'); - + # perl4 prints: second new # perl5 prints: 3 @@ -600,7 +600,7 @@ See L<perldiag> for full details. Some error messages will be different. -=item * Discontinuance +=item * Discontinuance Some bugs may have been inadvertently removed. :-) @@ -618,7 +618,7 @@ Note the space between . and = $string . = "more string"; print $string; - + # perl4 prints: more string # perl5 prints: syntax error at - line 1, near ". =" @@ -629,7 +629,7 @@ Better parsing in perl 5 sub foo {} &foo print("hello, world\n"); - + # perl4 prints: hello, world # perl5 prints: syntax error @@ -639,7 +639,7 @@ Better parsing in perl 5 print ($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 @@ -657,12 +657,12 @@ operands, or output from same. Formatted output and significant digits print 7.373504 - 0, "\n"; - printf "%20.18f\n", 7.373504 - 0; - + printf "%20.18f\n", 7.373504 - 0; + # Perl4 prints: 7.375039999999996141 7.37503999999999614 - + # Perl5 prints: 7.373504 7.37503999999999614 @@ -676,7 +676,7 @@ If in doubt: use Math::BigInt; -=item * Numerical +=item * Numerical Assignment of return values from numeric equality tests does not work in perl5 when the test evaluates to false (0). @@ -706,7 +706,7 @@ Negative array subscripts now count from the end of the array. @a = (1, 2, 3, 4, 5); print "The third element of the array is $a[3] also expressed as $a[-2] \n"; - + # perl4 prints: The third element of the array is 4 also expressed as # perl5 prints: The third element of the array is 4 also expressed as 4 @@ -715,13 +715,13 @@ Negative array subscripts now count from the end of the array. Setting C<$#array> lower now discards array elements, and makes them impossible to recover. - @a = (a,b,c,d,e); + @a = (a,b,c,d,e); print "Before: ",join('',@a); - $#a =1; + $#a =1; print ", After: ",join('',@a); $#a =3; print ", Recovered: ",join('',@a),"\n"; - + # perl4 prints: Before: abcde, After: ab, Recovered: abcd # perl5 prints: Before: abcde, After: ab, Recovered: ab @@ -729,11 +729,11 @@ impossible to recover. Hashes get defined before use - local($s,@a,%h); + 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 @@ -746,17 +746,17 @@ variable is localized subsequent to the assignment *b = *a; local(@a); print @b,"\n"; - + # perl4 prints: This is Perl 4 # perl5 prints: - + # Another example - + *fred = *barney; # fred is aliased to barney @barney = (1, 2, 4); # @fred; print "@fred"; # should print "1, 2, 4" - + # perl4 prints: 1 2 4 # perl5 prints: In string, @fred now must be written as \@fred @@ -770,7 +770,7 @@ does to auto(magic)increment. print ++$x," : "; print -$x," : "; print ++$x,"\n"; - + # perl4 prints: aab : -0 : 1 # perl5 prints: aab : -aab : aac @@ -788,13 +788,13 @@ perl 4 lets you modify constants: $_[0] = "m"; print " after: $_[0]\n"; } - + # perl4: # before: x after: m # before: a after: m # before: m after: m # before: m after: m - + # Perl5: # before: x after: m # Modification of a read-only value attempted at foo.pl line 12. @@ -805,7 +805,7 @@ perl 4 lets you modify constants: The behavior is slightly different for: print "$x", defined $x - + # perl 4: 1 # perl 5: <no output, $x is not called into existence> @@ -823,7 +823,7 @@ that perl4 exhibits for only scalars. sub test { local( *theArgument ) = @_; local( %aNewLocal ); # perl 4 != 5.001l,m - $aNewLocal{"aKey"} = "this should never appear"; + $aNewLocal{"aKey"} = "this should never appear"; print "SUB: ", $theArgument{"aKey"}, "\n"; $aNewLocal{"aKey"} = "level $GlobalLevel"; # what should print $GlobalLevel++; @@ -831,14 +831,14 @@ that perl4 exhibits for only scalars. &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 @@ -862,19 +862,19 @@ context. This means you can interpolate list values now. @<<<<< @||||| @>>>>> @fmt; . - write; - + write; + # perl4 errors: Please use commas to separate fields in file # perl5 prints: foo bar baz =item * (scalar context) -The C<caller()> function now returns a false value in a scalar context -if there is no caller. This lets library files determine if they're +The C<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 @@ -886,7 +886,7 @@ scalar context to its arguments. @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 @@ -898,15 +898,15 @@ This test could be added to t/op/sprintf.t @z = ('%s%s', 'foo', 'bar'); $x = sprintf(@z); if ($x eq 'foobar') {print "ok 2\n";} else {print "not ok 2 '$x'\n";} - + # perl4 prints: ok 2 # perl5 prints: not ok 2 C<printf()> works fine, though: printf STDOUT (@z); - print "\n"; - + print "\n"; + # perl4 prints: foobar # perl5 prints: foobar @@ -941,7 +941,7 @@ These are now semantic errors because of precedence: 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 @@ -963,7 +963,7 @@ would be erroneously parsed as On the other hand, - $a += /foo/ ? 1 : 2; + $a += /foo/ ? 1 : 2; now works as a C programmer would expect. @@ -975,7 +975,7 @@ is now incorrect. You need parentheses around the filehandle. Otherwise, perl5 leaves the statement as its default precedence: open(FOO || die); - + # perl4 opens or dies # perl5 errors: Precedence problem: open FOO should be open(FOO) @@ -985,16 +985,16 @@ perl4 gives the special variable, C<$:> precedence, where perl5 treats C<$::> as main C<package> $a = "x"; print "$::a"; - + # perl 4 prints: -:a # perl 5 prints: x =item * Precedence -concatenation precedence over filetest operator? +concatenation precedence over filetest operator? + + -e $foo .= "q" - -e $foo .= "q" - # perl4 prints: no output # perl5 prints: Can't modify -e in concatenation @@ -1021,14 +1021,14 @@ All types of RE traps. =item * Regular Expression C<s'$lhs'$rhs'> now does no interpolation on either side. It used to -interpolate C<$lhs> but not C<$rhs>. (And still does not match a literal +interpolate C<$lhs> but not C<$rhs>. (And still does not match a literal '$' 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 $b @@ -1043,7 +1043,7 @@ state of the searched string is lost) &doit("blah"); } sub doit{local($_) = shift; print "Got $_ "} - + # perl4 prints: blah blah blah # perl5 prints: infinite loop blah... @@ -1072,7 +1072,7 @@ the whole match, just like C<$&>. Perl5 does not. "abcdef" =~ /b.*e/; print "\$+ = $+\n"; - + # perl4 prints: bcde # perl5 prints: @@ -1083,7 +1083,7 @@ substitution now returns the null string if it fails $string = "test"; $value = ($string =~ s/foo//); print $value, "\n"; - + # perl4 prints: 0 # perl5 prints: @@ -1091,13 +1091,13 @@ Also see L<Numerical Traps> for another example of this new feature. =item * Regular Expression -C<s`lhs`rhs`> (using back-ticks) is now a normal substitution, with no -back-tick expansion +C<s`lhs`rhs`> (using backticks) is now a normal substitution, with no +backtick expansion $string = ""; $string =~ s`^`hostname`; print $string, "\n"; - + # perl4 prints: <the local hostname> # perl5 prints: hostname @@ -1106,7 +1106,7 @@ back-tick expansion Stricter parsing of variables used in regular expressions s/^([^$grpc]*$grpc[$opt$plus$rep]?)//o; - + # perl4: compiles w/o error # perl5: with Scalar found where operator expected ..., near "$opt$plus" @@ -1114,12 +1114,12 @@ an added component of this example, apparently from the same script, is the actual value of the s'd string after the substitution. C<[$opt]> is a character class in perl4 and an array subscript in perl5 - $grpc = 'a'; + $grpc = 'a'; $opt = 'r'; $_ = 'bar'; s/^([^$grpc]*$grpc[$opt]?)/foo/; print ; - + # perl4 prints: foo # perl5 prints: foobar @@ -1134,11 +1134,11 @@ repeatedly, like C</x/> or C<m!x!>. if( &match() ) { # m?x? matches more then once print "perl4\n"; - } else { + } else { # m?x? matches only once - print "perl5\n"; + print "perl5\n"; } - + # perl4 prints: perl4 # perl5 prints: perl5 @@ -1156,7 +1156,7 @@ found using the C<pos()> function--see L<perlfunc/pos>). print $1 while ($test =~ /(o)/g); # pos $test = 0; # to get old behavior } - + # perl4 prints: oooooo # perl5.004 prints: oo @@ -1181,7 +1181,7 @@ calls if a subroutine by that name is defined before the compiler sees them. sub SeeYa { warn"Hasta la vista, baby!" } $SIG{'TERM'} = SeeYa; print "SIGTERM is now $SIG{'TERM'}\n"; - + # perl4 prints: SIGTERM is main'SeeYa # perl5 prints: SIGTERM is now main::1 @@ -1192,10 +1192,10 @@ Use B<-w> to catch this one reverse is no longer allowed as the name of a sort subroutine. sub reverse{ print "yup "; $a <=> $b } - print sort reverse a,b,c; - + print sort reverse a,b,c; + # perl4 prints: yup yup yup yup abc - # perl5 prints: abc + # perl5 prints: abc =item * warn() won't let you specify a filehandle. @@ -1205,7 +1205,7 @@ filehandle in perl4. With perl5 it does not. warn STDERR "Foo!"; # perl4 prints: Foo! - # perl5 prints: String found where operator expected + # perl5 prints: String found where operator expected =back @@ -1215,47 +1215,47 @@ filehandle in perl4. With perl5 it does not. =item * (SysV) -Under HPUX, and some other SysV OS's, one had to reset any signal handler, -within the signal handler function, each time a signal was handled with -perl4. With perl5, the reset is now done correctly. Any code relying +Under HPUX, and some other SysV OSes, one had to reset any signal handler, +within the signal handler function, each time a signal was handled with +perl4. With perl5, the reset is now done correctly. Any code relying on the handler _not_ being reset will have to be reworked. Since version 5.002, Perl uses sigaction() under SysV. sub gotit { - print "Got @_... "; - } + print "Got @_... "; + } $SIG{'INT'} = 'gotit'; - + $| = 1; $pid = fork; if ($pid) { kill('INT', $pid); sleep(1); kill('INT', $pid); - } else { + } else { while (1) {sleep(10);} - } - + } + # perl4 (HPUX) prints: Got INT... # perl5 (HPUX) prints: Got INT... Got INT... =item * (SysV) -Under SysV OS's, C<seek()> on a file opened to append C<E<gt>E<gt>> now does -the right thing w.r.t. the fopen() man page. e.g., - When a file is opened +Under SysV OSes, C<seek()> on a file opened to append C<E<gt>E<gt>> 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 ; + $start = tell TEST ; foreach(1 .. 9){ print TEST "$_ "; } $end = tell TEST ; seek(TEST,$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 @@ -1274,8 +1274,8 @@ within certain expressions, statements, contexts, or whatever. @ now always interpolates an array in double-quotish strings. - print "To: someone@somewhere.com\n"; - + print "To: someone@somewhere.com\n"; + # perl4 prints: To:someone@somewhere.com # perl5 errors : In string, @somewhere now must be written as \@somewhere @@ -1286,7 +1286,7 @@ Double-quoted strings may no longer end with an unescaped $ or @. $foo = "foo$"; $bar = "bar@"; print "foo is $foo, bar is $bar\n"; - + # perl4 prints: foo is foo$, bar is bar@ # perl5 errors: Final $ should be \$ or $name @@ -1322,8 +1322,8 @@ works fine, however. =item * Interpolation -Creation of hashes on the fly with C<eval "EXPR"> now requires either both -C<$>'s to be protected in the specification of the hash name, or both curlies +Creation of hashes on the fly with C<eval "EXPR"> now requires either both +C<$>'s to be protected in the specification of the hash name, or both curlies to be protected. If both curlies are protected, the result will be compatible with perl4 and perl5. This is a very common practice, and should be changed to use the block form of C<eval{}> if possible. @@ -1366,13 +1366,13 @@ causes the following result: perl4 programs which unconsciously rely on the bugs in earlier perl versions. perl -e '$bar=q/not/; print "This is $foo{$bar} perl5"' - + # perl4 prints: This is not perl5 # perl5 prints: This is perl5 =item * Interpolation -You also have to be careful about array references. +You also have to be careful about array references. print "$foo{" @@ -1385,7 +1385,7 @@ Similarly, watch out for: $foo = "array"; print "\$$foo{bar}\n"; - + # perl4 prints: $array{bar} # perl5 prints: $ @@ -1402,9 +1402,9 @@ C<qq()> string passed to C<eval> \$count++; } ); - + # perl4 runs this ok - # perl5 prints: Can't find string terminator ")" + # perl5 prints: Can't find string terminator ")" =back @@ -1463,7 +1463,7 @@ If the file doit.pl has: sub foo { $rc = do "./do.pl"; return 8; - } + } print &foo, "\n"; And the do.pl file has the following single line: @@ -1473,12 +1473,12 @@ And the do.pl file has the following single line: Running doit.pl gives the following: # perl 4 prints: 3 (aborts the subroutine early) - # perl 5 prints: 8 + # perl 5 prints: 8 Same behavior if you replace C<do> with C<require>. =back -As always, if any of these are ever officially declared as bugs, +As always, if any of these are ever officially declared as bugs, they'll be fixed and removed. |