X-Git-Url: http://git.shadowcat.co.uk/gitweb/gitweb.cgi?a=blobdiff_plain;f=pod%2Fperltrap.pod;h=786dcda6074abc0253494e2f11864f9e233a7d98;hb=84850974f570c6c594cc0df54611ffc5f0b26130;hp=984ba3b5b1d0cae5809102b66ff46d09f94e6116;hpb=5e378fdf771d0a60d95ee1eed3e78057436dc10c;p=p5sagit%2Fp5-mst-13.2.git diff --git a/pod/perltrap.pod b/pod/perltrap.pod index 984ba3b..786dcda 100644 --- a/pod/perltrap.pod +++ b/pod/perltrap.pod @@ -6,7 +6,8 @@ perltrap - Perl traps for the unwary The biggest trap of all is forgetting to use the B<-w> switch; see L. The second biggest trap is not making your entire program -runnable under C. +runnable under C. The third biggest trap is not reading +the list of changes in this version of Perl; see L. =head2 Awk Traps @@ -20,8 +21,8 @@ The English module, loaded via use English; -allows you to refer to special variables (like $RS) as -though they were in B; see L for details. +allows you to refer to special variables (like C<$/>) with names (like +C<$RS>), as though they were in B; see L for details. =item * @@ -47,8 +48,7 @@ You have to decide whether your array has numeric or string indices. =item * -Associative array values do not spring into existence upon mere -reference. +Hash values do not spring into existence upon mere reference. =item * @@ -58,8 +58,8 @@ comparisons. =item * Reading an input line does not split it for you. You get to split it -yourself to an array. And the split() operator has different -arguments. +to an array yourself. And the split() operator has different +arguments than B's. =item * @@ -69,13 +69,13 @@ executed.) See L. =item * -$> does not refer to fields--it refers to substrings matched by -the last match pattern. +$EIE does not refer to fields--it refers to substrings matched +by the last match pattern. =item * The print() statement does not add field and record separators unless -you set C<$,> and C<$.>. You can set $OFS and $ORS if you're using +you set C<$,> and C<$\>. You can set $OFS and $ORS if you're using the English module. =item * @@ -101,9 +101,9 @@ basically incompatible with C.) =item * The concatenation operator is ".", not the null string. (Using the -null string would render C unparsable, since the third slash -would be interpreted as a division operator--the tokener is in fact -slightly context sensitive for operators like "/", "?", and ">". +null string would render C unparsable, because the third slash +would be interpreted as a division operator--the tokenizer is in fact +slightly context sensitive for operators like "/", "?", and "E". And in fact, "." itself can be the beginning of a number.) =item * @@ -158,7 +158,7 @@ You must use C rather than C. =item * -The C and C keywords from C become in +The C and C keywords from C become in Perl C and C, respectively. Unlike in C, these do I work within a C construct. @@ -183,7 +183,7 @@ Comments begin with "#", not "/*". =item * You can't take the address of anything, although a similar operator -in Perl 5 is the backslash, which creates a reference. +in Perl is the backslash, which creates a reference. =item * @@ -241,7 +241,7 @@ The backtick operator does no translation of the return value, unlike B. =item * Shells (especially B) do several levels of substitution on each -command line. Perl does substitution only in certain constructs +command line. Perl does substitution in only certain constructs such as double quotes, backticks, angle brackets, and search patterns. =item * @@ -274,33 +274,33 @@ context than they do in a scalar one. See L for details. =item * -Avoid barewords if you can, especially all lower-case ones. -You can't tell just by looking at it whether a bareword is -a function or a string. By using quotes on strings and -parens on function calls, you won't ever get them confused. +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 +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 B be list operators, never +(User-defined subroutines can be B list operators, never unary ones.) See L. =item * 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 * -The construct is not the name of the filehandle, it is a readline -operation on that handle. The data read is only assigned to $_ if the +The EFHE construct is not the name of the filehandle, it is a readline +operation on that handle. The data read is assigned to $_ only if the file read is the sole condition in a while loop: while () { } - while ($_ = ) { }.. + while (defined($_ = )) { }.. ; # data discarded! =item * @@ -313,14 +313,14 @@ these two constructs are quite different: =item * -The C construct isn't a real loop that you can use +The C construct isn't a real loop that you can use loop control on. =item * -Use C for local variables whenever you can get away with -it (but see L for where you can't). -Using C actually gives a local value to a global +Use C for local variables whenever you can get away with +it (but see L for where you can't). +Using C actually gives a local value to a global variable, which leaves you open to unforeseen side-effects of dynamic scoping. @@ -332,9 +332,9 @@ external name is still an alias for the original. =back -=head2 Perl4 to Perl5 Traps +=head2 Perl4 to Perl5 Traps -Practicing Perl4 Programmers should take note of the following +Practicing Perl4 Programmers should take note of the following Perl4-to-Perl5 specific traps. They're crudely ordered according to the following list: @@ -392,17 +392,17 @@ Everything else. =back If you find an example of a conversion trap that is not listed here, -please submit it to Bill Middleton F for inclusion. -Also note that at least some of these can be caught with C<-w>. +please submit it to Bill Middleton > for inclusion. +Also note that at least some of these can be caught with B<-w>. =head2 Discontinuance, Deprecation, and BugFix traps Anything that has been discontinued, deprecated, or fixed as -a bug from perl4. +a bug from perl4. =over 4 -=item * Discontinuance +=item * Discontinuance Symbols starting with "_" are no longer forced into package main, except for C<$_> itself (and C<@_>, etc.). @@ -412,14 +412,14 @@ 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, since the packages don't exist. +behave differently in perl4 vs. perl5, because the packages don't exist. $a=1;$b=2;$c=3;$var=4; print "$a::$b::$c "; @@ -434,11 +434,11 @@ 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 @@ -447,39 +447,39 @@ 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 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 @@ -491,7 +491,7 @@ 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 {" @@ -501,11 +501,11 @@ 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 has changed slightly when it is iterating over a list which is not an array. This used to assign the list to a @@ -519,12 +519,12 @@ 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)){ @@ -549,11 +549,59 @@ behave like C (which does). # perl4 prints: :hi:mom # perl5 prints: hi:mom +=item * BugFix + +Perl 4 would ignore any text which was attached to an B<-e> switch, +always taking the code snippet from the following arg. Additionally, it +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: + # perl5 dies: No code specified for -e. + +=item * Discontinuance + +In Perl 4 the return value of C was undocumented, but it was +actually the last value being pushed onto the target list. In Perl 5 +the return value of C 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 + +=item * Discontinuance + +In Perl 4 (and versions of Perl 5 before 5.004), C<'\r'> characters in +Perl code were silently allowed, although they could cause (mysterious!) +failures in certain constructs, particularly here documents. Now, +C<'\r'> characters cause an immediate fatal error. (Note: In this +example, the notation B<\015> represents the incorrect line +ending. Depending upon your text viewer, it will look different.) + + print "foo";\015 + print "bar"; + + # perl4 prints: foobar + # perl5.003 prints: foobar + # perl5.004 dies: Illegal character \015 (carriage return) + +See L for full details. + =item * Deprecation Some error messages will be different. -=item * Discontinuance +=item * Discontinuance Some bugs may have been inadvertently removed. :-) @@ -571,7 +619,7 @@ Note the space between . and = $string . = "more string"; print $string; - + # perl4 prints: more string # perl5 prints: syntax error at - line 1, near ". =" @@ -582,7 +630,7 @@ Better parsing in perl 5 sub foo {} &foo print("hello, world\n"); - + # perl4 prints: hello, world # perl5 prints: syntax error @@ -592,7 +640,7 @@ Better parsing in perl 5 print ($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 @@ -610,38 +658,39 @@ 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 =item * Numerical -This specific item has been deleted. It demonstrated how the autoincrement +This specific item has been deleted. It demonstrated how the auto-increment operator would not catch when a number went over the signed int limit. Fixed -in 5.003_04. But always be wary when using large ints. If in doubt: +in version 5.003_04. But always be wary when using large integers. +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). Logical tests now return an null, instead of 0 - + $p = ($test == 1); print $p,"\n"; - + # perl4 prints: 0 # perl5 prints: -Also see the L tests for another example -of this new feature... +Also see L<"General Regular Expression Traps using s///, etc."> +for another example of this new feature... =back @@ -658,7 +707,7 @@ 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 $a[-2] \n"; - + # perl4 prints: The third element of the array is 4 also expressed as # perl5 prints: The third element of the array is 4 also expressed as 4 @@ -667,13 +716,13 @@ Negative array subscripts now count from the end of the array. Setting C<$#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); $#a =3; print ", Recovered: ",join('',@a),"\n"; - + # perl4 prints: Before: abcde, After: ab, Recovered: abcd # perl5 prints: Before: abcde, After: ab, Recovered: ab @@ -681,11 +730,11 @@ impossible to recover. Hashes get defined before use - local($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 @@ -698,19 +747,19 @@ 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: Literal @fred now requires backslash + # perl5 prints: In string, @fred now must be written as \@fred =item * (Scalar String) @@ -722,7 +771,7 @@ does to auto(magic)increment. print ++$x," : "; print -$x," : "; print ++$x,"\n"; - + # perl4 prints: aab : -0 : 1 # perl5 prints: aab : -aab : aac @@ -740,13 +789,13 @@ perl 4 lets you modify constants: $_[0] = "m"; print " after: $_[0]\n"; } - + # perl4: # before: x after: m # before: a after: m # before: m after: m # before: m after: m - + # Perl5: # before: x after: m # Modification of a read-only value attempted at foo.pl line 12. @@ -757,15 +806,15 @@ perl 4 lets you modify constants: The behavior is slightly different for: print "$x", defined $x - + # perl 4: 1 # perl 5: =item * (Variable Suicide) Variable suicide behavior is more consistent under Perl 5. -Perl5 exhibits the same behavior for associative arrays and scalars, -that perl4 exhibits only for scalars. +Perl5 exhibits the same behavior for hashes and scalars, +that perl4 exhibits for only scalars. $aGlobal{ "aKey" } = "global value"; print "MAIN:", $aGlobal{"aKey"}, "\n"; @@ -775,7 +824,7 @@ that perl4 exhibits only for 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++; @@ -783,14 +832,14 @@ that perl4 exhibits only for 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 @@ -814,19 +863,19 @@ 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 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 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 @@ -838,7 +887,7 @@ 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 @@ -850,15 +899,15 @@ 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 works fine, though: printf STDOUT (@z); - print "\n"; - + print "\n"; + # perl4 prints: foobar # perl5 prints: foobar @@ -893,7 +942,7 @@ 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 $m\n"; - + # perl4 prints: n is 3, m is 6 # perl5 errors and fails to compile @@ -904,7 +953,7 @@ of assignment. Perl 4 mistakenly gave them the precedence of the associated operator. So you now must parenthesize them in expressions like /foo/ ? ($a += 2) : ($a -= 2); - + Otherwise /foo/ ? $a += 2 : $a -= 2 @@ -915,7 +964,7 @@ would be erroneously parsed as On the other hand, - $a += /foo/ ? 1 : 2; + $a += /foo/ ? 1 : 2; now works as a C programmer would expect. @@ -923,11 +972,11 @@ now works as a C programmer would expect. open FOO || die; -is now incorrect. You need parens around the filehandle. -Otherwise, perl5 leaves the statement as it's default precedence: +is now incorrect. You need parentheses around the filehandle. +Otherwise, perl5 leaves the statement as its default precedence: open(FOO || die); - + # perl4 opens or dies # perl5 errors: Precedence problem: open FOO should be open(FOO) @@ -937,16 +986,16 @@ perl4 gives the special variable, C<$:> precedence, where perl5 treats C<$::> as main C $a = "x"; print "$::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 $foo .= "q" - # perl4 prints: no output # perl5 prints: Can't modify -e in concatenation @@ -973,14 +1022,14 @@ All types of RE traps. =item * Regular Expression C now does no interpolation on either side. It used to -interpolate C<$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 $b @@ -995,18 +1044,36 @@ state of the searched string is lost) &doit("blah"); } sub doit{local($_) = shift; print "Got $_ "} - + # perl4 prints: blah blah blah # perl5 prints: infinite loop blah... =item * Regular Expression +Currently, if you use the C qualifier on a regular expression +within an anonymous sub, I closures generated from that anonymous +sub will use the regular expression as it was compiled when it was used +the very first time in any such closure. For instance, if you say + + sub build_match { + my($left,$right) = @_; + return sub { $_[0] =~ /$left stuff $right/o; }; + } + +build_match() will always return a sub which matches the contents of +C<$left> and C<$right> as they were the I time that build_match() +was called, not as they are in the current call. + +This is probably a bug, and may change in future versions of Perl. + +=item * Regular Expression + If no parentheses are used in a match, Perl4 sets C<$+> to the whole match, just like C<$&>. Perl5 does not. "abcdef" =~ /b.*e/; print "\$+ = $+\n"; - + # perl4 prints: bcde # perl5 prints: @@ -1017,7 +1084,7 @@ substitution now returns the null string if it fails $string = "test"; $value = ($string =~ s/foo//); print $value, "\n"; - + # perl4 prints: 0 # perl5 prints: @@ -1025,13 +1092,13 @@ Also see L for another example of this new feature. =item * Regular Expression -C (using backticks) is now a normal substitution, with no +C (using backticks) is now a normal substitution, with no backtick expansion $string = ""; $string =~ s`^`hostname`; print $string, "\n"; - + # perl4 prints: # perl5 prints: hostname @@ -1040,7 +1107,7 @@ backtick expansion Stricter parsing of variables used in regular expressions s/^([^$grpc]*$grpc[$opt$plus$rep]?)//o; - + # perl4: compiles w/o error # perl5: with Scalar found where operator expected ..., near "$opt$plus" @@ -1048,12 +1115,12 @@ 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 @@ -1068,15 +1135,35 @@ repeatedly, like C or C. 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 +=item * Regular Expression + +Under perl4 and upto version 5.003, a failed C match used to +reset the internal iterator, so that subsequent C match attempts +began from the beginning of the string. In perl version 5.004 and later, +failed C matches do not reset the iterator position (which can be +found using the C function--see L). + + $test = "foop"; + for (1..3) { + print $1 while ($test =~ /(o)/g); + # pos $test = 0; # to get old behavior + } + + # perl4 prints: oooooo + # perl5.004 prints: oo + +You may always reset the iterator yourself as shown in the commented line +to get the old behavior. + =back =head2 Subroutine, Signal, Sorting Traps @@ -1095,7 +1182,7 @@ 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 @@ -1106,17 +1193,20 @@ 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. -=item * warn() specifically implies STDERR +Although it _always_ printed to STDERR, 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 + # perl5 prints: String found where operator expected =back @@ -1126,47 +1216,47 @@ reverse is no longer allowed as the name of a sort subroutine. =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. -5.002 and beyond uses sigaction() under SysV +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 on a file opened to append CE> now does -the right thing w.r.t. the fopen() man page. e.g. - When a file is opened +Under SysV OSes, C on a file opened to append CE> 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 @@ -1176,28 +1266,28 @@ the file. =head2 Interpolation Traps +Perl4-to-Perl5 traps having to do with how things get interpolated +within certain expressions, statements, contexts, or whatever. + =over 5 =item * Interpolation @ 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 : Literal @somewhere now requires backslash + # perl5 errors : In string, @somewhere now must be written as \@somewhere =item * Interpolation -Perl4-to-Perl5 traps having to do with how things get interpolated -within certain expressions, statements, contexts, or whatever. - 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 @@ -1205,6 +1295,23 @@ Note: perl5 DOES NOT error on the terminating @ in $bar =item * Interpolation +Perl now sometimes evaluates arbitrary expressions inside braces that occur +within double quotes (usually when the opening brace is preceded by C<$> +or C<@>). + + @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 C to ward off such trappiness under perl5. + +=item * Interpolation + The construct "this is $$x" used to interpolate the pid at that point, but now apparently tries to dereference C<$x>. C<$$> by itself still works fine, however. @@ -1216,8 +1323,8 @@ works fine, however. =item * Interpolation -Creation of hashes on the fly with C now requires either both -C<$>'s to be protected in the specification of the hash name, or both curlies +Creation of hashes on the fly with C now requires either both +C<$>'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 if possible. @@ -1260,13 +1367,13 @@ 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{" @@ -1279,7 +1386,7 @@ Similarly, watch out for: $foo = "array"; print "\$$foo{bar}\n"; - + # perl4 prints: $array{bar} # perl5 prints: $ @@ -1296,9 +1403,9 @@ C string passed to C \$count++; } ); - + # perl4 runs this ok - # perl5 prints: Can't find string terminator ")" + # perl5 prints: Can't find string terminator ")" =back @@ -1357,7 +1464,7 @@ 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: @@ -1367,12 +1474,12 @@ 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 with C. =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.