X-Git-Url: http://git.shadowcat.co.uk/gitweb/gitweb.cgi?a=blobdiff_plain;f=pod%2Fperltrap.pod;h=f278fa0929b6f27942a0a37eb37ce4174392f84a;hb=73659bf1a819ac7d9f9fcae022ed8755e46824eb;hp=415977714686df5cdd1bb7536bd53febf0f67e77;hpb=1d2dff63f533f62282a700198c67c41dcb6ad6df;p=p5sagit%2Fp5-mst-13.2.git diff --git a/pod/perltrap.pod b/pod/perltrap.pod index 4159777..f278fa0 100644 --- a/pod/perltrap.pod +++ b/pod/perltrap.pod @@ -22,7 +22,7 @@ The English module, loaded via use English; allows you to refer to special variables (like C<$/>) with names (like -C<$RS>), as though they were in B; see L for details. +$RS), as though they were in B; see L for details. =item * @@ -160,7 +160,7 @@ You must use C rather than C. 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. +Unlike in C, these do I work within a C construct. =item * @@ -305,7 +305,7 @@ file read is the sole condition in a while loop: =item * -Remember not to use "C<=>" when you need "C<=~>"; +Remember not to use C<=> when you need C<=~>; these two constructs are quite different: $x = /foo/; @@ -585,24 +585,6 @@ number of elements in the resulting list. # perl4 prints: second new # perl5 prints: 3 -=item * Discontinuance - -In Perl 4 (and versions of Perl 5 before 5.004), C<'\r'> characters in -Perl code were silently allowed, although they could cause (mysterious!) -failures in certain constructs, particularly here documents. Now, -C<'\r'> characters cause an immediate fatal error. (Note: In this -example, the notation B<\015> represents the incorrect line -ending. Depending upon your text viewer, it will look different.) - - print "foo";\015 - print "bar"; - - # perl4 prints: foobar - # perl5.003 prints: foobar - # perl5.004 dies: Illegal character \015 (carriage return) - -See L for full details. - =item * Deprecation Some error messages will be different. @@ -650,6 +632,23 @@ Better parsing in perl 5 # perl4 prints: is zero # perl5 warns: "Useless use of a constant in void context" if using -w +=item * Parsing + +String interpolation of the C<$#array> construct differs when braces +are to used around the name. + + @ = (1..3); + print "${#a}"; + + # perl4 prints: 2 + # perl5 fails with syntax error + + @ = (1..3); + print "$#{a}"; + + # perl4 prints: {a} + # perl5 prints: 2 + =back =head2 Numerical Traps @@ -698,6 +697,30 @@ Logical tests now return an null, instead of 0 Also see L<"General Regular Expression Traps using s///, etc."> for another example of this new feature... +=item * Bitwise string ops + +When bitwise operators which can operate upon either numbers or +strings (C<& | ^ ~>) are given only strings as arguments, perl4 would +treat the operands as bitstrings so long as the program contained a call +to the C function. perl5 treats the string operands as bitstrings. +(See L for more details.) + + $fred = "10"; + $barney = "12"; + $betty = $fred & $barney; + print "$betty\n"; + # Uncomment the next line to change perl4's behavior + # ($dummy) = vec("dummy", 0, 0); + + # Perl4 prints: + 8 + + # Perl5 prints: + 10 + + # If vec() is used anywhere in the program, both print: + 10 + =back =head2 General data type traps @@ -744,6 +767,9 @@ Hashes get defined before use # perl4 prints: # perl5 dies: hash %h defined +Perl will now generate a warning when it sees defined(@a) and +defined(%h). + =item * (Globs) glob assignment from variable to variable will fail if the assigned @@ -921,6 +947,10 @@ Probably a bug. Perl4-to-Perl5 traps involving precedence order. +Perl 4 has almost the same precedence rules as Perl 5 for the operators +that they both have. Perl 4 however, seems to have had some +inconsistencies that made the behavior differ from what was documented. + =over 5 =item * Precedence @@ -996,13 +1026,34 @@ treats C<$::> as main C =item * Precedence -concatenation precedence over filetest operator? +perl4 had buggy precedence for the file test operators vis-a-vis +the assignment operators. Thus, although the precedence table +for perl4 leads one to believe C<-e $foo .= "q"> should parse as +C<((-e $foo) .= "q")>, it actually parses as C<(-e ($foo .= "q"))>. +In perl5, the precedence is as documented. -e $foo .= "q" # perl4 prints: no output # perl5 prints: Can't modify -e in concatenation +=item * Precedence + +In perl4, keys(), each() and values() were special high-precedence operators +that operated on a single hash, but in perl5, they are regular named unary +operators. As documented, named unary operators have lower precedence +than the arithmetic and concatenation operators C<+ - .>, but the perl4 +variants of these operators actually bind tighter than C<+ - .>. +Thus, for: + + %foo = 1..10; + print keys %foo - 1 + + # perl4 prints: 4 + # perl5 prints: Type of arg 1 to keys must be hash (not subtraction) + +The perl4 behavior was probably more useful, if less consistent. + =back =head2 General Regular Expression Traps using s///, etc. @@ -1014,7 +1065,7 @@ All types of RE traps. =item * Regular Expression C now does no interpolation on either side. It used to -interpolate C<$lhs> but not C<$rhs>. (And still does not match a literal +interpolate $lhs but not $rhs. (And still does not match a literal '$' in string) $a=1;$b=2; @@ -1053,7 +1104,7 @@ the very first time in any such closure. For instance, if you say } 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() +$left and $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. @@ -1136,26 +1187,6 @@ repeatedly, like C or C. # 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 @@ -1305,7 +1336,7 @@ 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 +point, but now apparently tries to dereference $x. C<$$> by itself still works fine, however. print "this is $$x\n";