X-Git-Url: http://git.shadowcat.co.uk/gitweb/gitweb.cgi?a=blobdiff_plain;f=pod%2Fperltrap.pod;h=b5f09351669cd185fc3db4e19d47c380450ac4be;hb=e014eb68559fa27ccb09cba7633eae6ea5936113;hp=6a6a4435684e09870e07a0a55eccfdc7270989ad;hpb=665e98b9987fe678dd115b56e9f0c9eb6bba0f1e;p=p5sagit%2Fp5-mst-13.2.git diff --git a/pod/perltrap.pod b/pod/perltrap.pod index 6a6a443..b5f0935 100644 --- a/pod/perltrap.pod +++ b/pod/perltrap.pod @@ -18,6 +18,11 @@ Accustomed B users should take special note of the following: =item * +A Perl program executes only once, not once for each input line. You can +do an implicit loop with C<-n> or C<-p>. + +=item * + The English module, loaded via use English; @@ -165,8 +170,8 @@ C construct. See L. =item * -There's no switch statement. (But it's easy to build one on the fly, -see L) +The switch statement is called C and only available in +perl 5.10 or newer. See L. =item * @@ -174,7 +179,9 @@ Variables begin with "$", "@" or "%" in Perl. =item * -Comments begin with "#", not "/*". +Comments begin with "#", not "/*" or "//". Perl may interpret C/C++ +comments as division operators, unterminated regular expressions or +the defined-or operator. =item * @@ -206,6 +213,11 @@ Seasoned B programmers should take note of the following: =item * +A Perl program executes only once, not once for each input line. You can +do an implicit loop with C<-n> or C<-p>. + +=item * + Backreferences in substitutions use "$" rather than "\". =item * @@ -255,6 +267,13 @@ The arguments are available via @ARGV, not $1, $2, etc. The environment is not automatically made available as separate scalar variables. +=item * + +The shell's C uses "=", "!=", "<" etc for string comparisons and "-eq", +"-ne", "-lt" etc for numeric comparisons. This is the reverse of Perl, which +uses C, C, C for string comparisons, and C<==>, C C<< < >> etc +for numeric comparisons. + =back =head2 Perl Traps @@ -399,7 +418,7 @@ a bug from perl4. =over 4 -=item * Discontinuance +=item * Symbols starting with "_" no longer forced into main Symbols starting with "_" are no longer forced into package main, except for C<$_> itself (and C<@_>, etc.). @@ -413,7 +432,7 @@ for C<$_> itself (and C<@_>, etc.). # perl4 prints: $_legacy is 1 # perl5 prints: $_legacy is -=item * Deprecation +=item * Double-colon valid package separator in variable name 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. @@ -429,8 +448,8 @@ Given that C<::> 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" ; + $x = 10; + print "x=${'x}\n"; # perl4 prints: x=10 # perl5 prints: Can't find string terminator "'" anywhere before EOF @@ -438,12 +457,12 @@ whether this should be classed as a bug or not. 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" ; + $x = 10; + print "x=${main'x}\n"; Also see precedence traps, for parsing C<$:>. -=item * BugFix +=item * 2nd and 3rd args to C are now in scalar context The second and third arguments of C are now evaluated in scalar context (as the Camel says) rather than list context. @@ -457,7 +476,7 @@ context (as the Camel says) rather than list context. # perl4 prints: a b # perl5 prints: c d e -=item * Discontinuance +=item * Can't do C into a block that is optimized away You can't do a C into a block that is optimized away. Darn. @@ -471,20 +490,20 @@ You can't do a C into a block that is optimized away. Darn. # perl4 prints: Here I is! # perl5 errors: Can't "goto" into the middle of a foreach loop -=item * Discontinuance +=item * Can't use whitespace as variable name or quote delimiter 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. $a = ("foo bar"); - $b = q baz ; + $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 -=item * Discontinuance +=item * C gone The archaic while/if BLOCK BLOCK syntax is no longer supported. @@ -498,7 +517,7 @@ The archaic while/if BLOCK BLOCK syntax is no longer supported. # perl4 prints: True! # perl5 errors: syntax error at test.pl line 1, near "if {" -=item * BugFix +=item * C<**> binds tighter than unary minus The C<**> operator now binds more tightly than unary minus. It was documented to work this way before, but didn't. @@ -508,7 +527,7 @@ It was documented to work this way before, but didn't. # perl4 prints: 16 # perl5 prints: -16 -=item * Discontinuance +=item * C changed when iterating over a list 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 @@ -540,7 +559,7 @@ Otherwise changing $var will clobber the values of @list. (This most often happens when you use C<$_> for the loop variable, and call subroutines in the loop that don't properly localize C<$_>.) -=item * Discontinuance +=item * C with no args behavior changed C with no arguments now behaves like C (which doesn't return an initial null field if $_ starts with whitespace), it used to @@ -552,7 +571,7 @@ behave like C (which does). # perl4 prints: :hi:mom # perl5 prints: hi:mom -=item * BugFix +=item * B<-e> behavior fixed Perl 4 would ignore any text which was attached to an B<-e> switch, always taking the code snippet from the following arg. Additionally, it @@ -569,7 +588,7 @@ these behaviors have been fixed. # perl4 prints: # perl5 dies: No code specified for -e. -=item * Discontinuance +=item * C returns number of elements in resulting list In Perl 4 the return value of C was undocumented, but it was actually the last value being pushed onto the target list. In Perl 5 @@ -582,17 +601,17 @@ number of elements in the resulting list. # perl4 prints: second new # perl5 prints: 3 -=item * Deprecation +=item * Some error messages differ Some error messages will be different. -=item * Discontinuance +=item * C honors subroutine args In Perl 4, if in list context the delimiters to the first argument of C were C, the result would be placed in C<@_> as well as being returned. Perl 5 has more respect for your subroutine arguments. -=item * Discontinuance +=item * Bugs removed Some bugs may have been inadvertently removed. :-) @@ -604,7 +623,7 @@ Perl4-to-Perl5 traps from having to do with parsing. =over 4 -=item * Parsing +=item * Space between . and = triggers syntax error Note the space between . and = @@ -614,7 +633,7 @@ Note the space between . and = # perl4 prints: more string # perl5 prints: syntax error at - line 1, near ". =" -=item * Parsing +=item * Better parsing in perl 5 Better parsing in perl 5 @@ -625,7 +644,7 @@ Better parsing in perl 5 # perl4 prints: hello, world # perl5 prints: syntax error -=item * Parsing +=item * Function parsing "if it looks like a function, it is a function" rule. @@ -635,7 +654,7 @@ Better parsing in perl 5 # perl4 prints: is zero # perl5 warns: "Useless use of a constant in void context" if using -w -=item * Parsing +=item * String interpolation of C<$#array> differs String interpolation of the C<$#array> construct differs when braces are to used around the name. @@ -652,7 +671,7 @@ are to used around the name. # perl4 prints: {a} # perl5 prints: 2 -=item * Parsing +=item * Perl guesses on C, C followed by C<{> if it starts BLOCK or hash ref When perl sees C (or C), it has to guess whether the C<{> starts a BLOCK or a hash reference. If it guesses wrong, it will report @@ -671,7 +690,7 @@ operands, or output from same. =over 5 -=item * Numerical +=item * Formatted output and significant digits Formatted output and significant digits. In general, Perl 5 tries to be more precise. For example, on a Solaris Sparc: @@ -692,7 +711,7 @@ Notice how the first result looks better in Perl 5. Your results may vary, since your floating point formatting routines and even floating point format may be slightly different. -=item * Numerical +=item * Auto-increment operator over signed int limit deleted This specific item has been deleted. It demonstrated how the auto-increment operator would not catch when a number went over the signed int limit. Fixed @@ -701,7 +720,7 @@ If in doubt: use Math::BigInt; -=item * Numerical +=item * Assignment of return values from numeric equality tests doesn't work Assignment of return values from numeric equality tests does not work in perl5 when the test evaluates to false (0). @@ -749,7 +768,7 @@ within certain expressions and/or context. =over 5 -=item * (Arrays) +=item * Negative array subscripts now count from the end of array Negative array subscripts now count from the end of the array. @@ -759,7 +778,7 @@ Negative array subscripts now count from the end of the array. # perl4 prints: The third element of the array is 4 also expressed as # perl5 prints: The third element of the array is 4 also expressed as 4 -=item * (Arrays) +=item * Setting C<$#array> lower now discards array elements Setting C<$#array> lower now discards array elements, and makes them impossible to recover. @@ -774,7 +793,7 @@ impossible to recover. # perl4 prints: Before: abcde, After: ab, Recovered: abcd # perl5 prints: Before: abcde, After: ab, Recovered: ab -=item * (Hashes) +=item * Hashes get defined before use Hashes get defined before use @@ -789,7 +808,7 @@ Hashes get defined before use Perl will now generate a warning when it sees defined(@a) and defined(%h). -=item * (Globs) +=item * Glob assignment from localized variable to variable glob assignment from variable to variable will fail if the assigned variable is localized subsequent to the assignment @@ -802,7 +821,7 @@ variable is localized subsequent to the assignment # perl4 prints: This is Perl 4 # perl5 prints: -=item * (Globs) +=item * Assigning C to glob Assigning C to a glob has no effect in Perl 5. In Perl 4 it undefines the associated scalar (but may have other side effects @@ -820,7 +839,7 @@ has quite a few effects. # perl5 prints: bar # perl5 warns: "Undefined value assigned to typeglob" if using -w -=item * (Scalar String) +=item * Changes in unary negation (of strings) Changes in unary negation (of strings) This change effects both the return value and what it @@ -834,7 +853,7 @@ does to auto(magic)increment. # perl4 prints: aab : -0 : 1 # perl5 prints: aab : -aab : aac -=item * (Constants) +=item * Modifying of constants prohibited perl 4 lets you modify constants: @@ -860,7 +879,7 @@ perl 4 lets you modify constants: # Modification of a read-only value attempted at foo.pl line 12. # before: a -=item * (Scalars) +=item * C behavior changed The behavior is slightly different for: @@ -869,7 +888,7 @@ The behavior is slightly different for: # perl 4: 1 # perl 5: -=item * (Variable Suicide) +=item * Variable Suicide Variable suicide behavior is more consistent under Perl 5. Perl5 exhibits the same behavior for hashes and scalars, @@ -912,7 +931,7 @@ that perl4 exhibits for only scalars. =over 5 -=item * (list context) +=item * Elements of argument lists for formats evaluated in list context The elements of argument lists for formats are now evaluated in list context. This means you can interpolate list values now. @@ -927,7 +946,7 @@ context. This means you can interpolate list values now. # perl4 errors: Please use commas to separate fields in file # perl5 prints: foo bar baz -=item * (scalar context) +=item * C returns false value in scalar context if no caller present 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 @@ -938,7 +957,7 @@ being required. # perl4 errors: There is no caller # perl5 prints: Got a 0 -=item * (scalar context) +=item * Comma operator in scalar context gives scalar context to args The comma operator in a scalar context is now guaranteed to give a scalar context to its arguments. @@ -950,7 +969,7 @@ scalar context to its arguments. # Perl4 prints: x = c # Thinks list context interpolates list # Perl5 prints: x = 3 # Knows scalar uses length of list -=item * (list, builtin) +=item * C prototyped as C<($;@)> C is prototyped as ($;@), so its first argument is given scalar context. Thus, if passed an array, it will probably not do what you want, @@ -983,7 +1002,7 @@ inconsistencies that made the behavior differ from what was documented. =over 5 -=item * Precedence +=item * LHS vs. RHS of any assignment operator LHS vs. RHS of any assignment operator. LHS is evaluated first in perl4, second in perl5; this can affect the relationship @@ -996,7 +1015,7 @@ between side-effects in sub-expressions. # perl4 prints: left # perl5 prints: right -=item * Precedence +=item * Semantic errors introduced due to precedence These are now semantic errors because of precedence: @@ -1010,7 +1029,7 @@ These are now semantic errors because of precedence: # perl4 prints: n is 3, m is 6 # perl5 errors and fails to compile -=item * Precedence +=item * Precedence of assignment operators same as the precedence of assignment The precedence of assignment operators is now the same as the precedence of assignment. Perl 4 mistakenly gave them the precedence of the associated @@ -1032,7 +1051,7 @@ On the other hand, now works as a C programmer would expect. -=item * Precedence +=item * C requires parentheses around filehandle open FOO || die; @@ -1044,7 +1063,7 @@ Otherwise, perl5 leaves the statement as its default precedence: # perl4 opens or dies # perl5 opens FOO, dying only if 'FOO' is false, i.e. never -=item * Precedence +=item * C<$:> precedence over C<$::> gone perl4 gives the special variable, C<$:> precedence, where perl5 treats C<$::> as main C @@ -1054,7 +1073,7 @@ treats C<$::> as main C # perl 4 prints: -:a # perl 5 prints: x -=item * Precedence +=item * Precedence of file test operators documented perl4 had buggy precedence for the file test operators vis-a-vis the assignment operators. Thus, although the precedence table @@ -1067,7 +1086,7 @@ In perl5, the precedence is as documented. # perl4 prints: no output # perl5 prints: Can't modify -e in concatenation -=item * Precedence +=item * C, C, C are regular named unary operators 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 @@ -1092,7 +1111,7 @@ All types of RE traps. =over 5 -=item * Regular Expression +=item * C interpolates on either side C now does no interpolation on either side. It used to interpolate $lhs but not $rhs. (And still does not match a literal @@ -1106,7 +1125,7 @@ interpolate $lhs but not $rhs. (And still does not match a literal # perl4 prints: $b 2 $a $b # perl5 prints: 1 2 $a $b -=item * Regular Expression +=item * C attaches its state to the searched string C 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 @@ -1121,7 +1140,7 @@ state of the searched string is lost) # perl4 prints: Got blah Got blah Got blah Got blah # perl5 prints: infinite loop blah... -=item * Regular Expression +=item * C used within an anonymous sub Currently, if you use the C qualifier on a regular expression within an anonymous sub, I closures generated from that anonymous @@ -1147,7 +1166,7 @@ build_match() will always return a sub which matches the contents of $left and $right as they were the I time that build_match() was called, not as they are in the current call. -=item * Regular Expression +=item * C<$+> isn't set to whole match If no parentheses are used in a match, Perl4 sets C<$+> to the whole match, just like C<$&>. Perl5 does not. @@ -1158,7 +1177,7 @@ the whole match, just like C<$&>. Perl5 does not. # perl4 prints: bcde # perl5 prints: -=item * Regular Expression +=item * Substitution now returns null string if it fails substitution now returns the null string if it fails @@ -1171,7 +1190,7 @@ substitution now returns the null string if it fails Also see L for another example of this new feature. -=item * Regular Expression +=item * C is now a normal substitution C (using backticks) is now a normal substitution, with no backtick expansion @@ -1183,7 +1202,7 @@ backtick expansion # perl4 prints: # perl5 prints: hostname -=item * Regular Expression +=item * Stricter parsing of variables in regular expressions Stricter parsing of variables used in regular expressions @@ -1200,12 +1219,12 @@ C<[$opt]> is a character class in perl4 and an array subscript in perl5 $opt = 'r'; $_ = 'bar'; s/^([^$grpc]*$grpc[$opt]?)/foo/; - print ; + print; # perl4 prints: foo # perl5 prints: foobar -=item * Regular Expression +=item * C matches only once Under perl5, C matches only once, like C. Under perl4, it matched repeatedly, like C or C. @@ -1224,7 +1243,7 @@ repeatedly, like C or C. # perl4 prints: perl4 # perl5 prints: perl5 -=item * Regular Expression +=item * Failed matches don't reset the match variables Unlike in Ruby, failed matches in Perl do not reset the match variables ($1, $2, ..., C<$`>, ...). @@ -1239,7 +1258,7 @@ general subroutine traps. Includes some OS-Specific traps. =over 5 -=item * (Signals) +=item * Barewords that used to look like strings look like subroutine calls Barewords that used to look like strings to Perl will now look like subroutine calls if a subroutine by that name is defined before the compiler sees them. @@ -1253,7 +1272,7 @@ calls if a subroutine by that name is defined before the compiler sees them. Use B<-w> to catch this one -=item * (Sort Subroutine) +=item * Reverse is no longer allowed as the name of a sort subroutine reverse is no longer allowed as the name of a sort subroutine. @@ -1264,7 +1283,7 @@ reverse is no longer allowed as the name of a sort subroutine. # perl5 prints: 123 # perl5 warns (if using -w): Ambiguous call resolved as CORE::reverse() -=item * warn() won't let you specify a filehandle. +=item * C won't let you specify a filehandle. Although it _always_ printed to STDERR, warn() would let you specify a filehandle in perl4. With perl5 it does not. @@ -1280,7 +1299,7 @@ filehandle in perl4. With perl5 it does not. =over 5 -=item * (SysV) +=item * SysV resets signal handler correctly 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 @@ -1307,7 +1326,7 @@ Since version 5.002, Perl uses sigaction() under SysV. # perl4 (HPUX) prints: Got INT... # perl5 (HPUX) prints: Got INT... Got INT... -=item * (SysV) +=item * SysV C appends correctly Under SysV OSes, C on a file opened to append C<<< >> >>> now does the right thing w.r.t. the fopen() manpage. e.g., - When a file is opened @@ -1315,11 +1334,11 @@ 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 ; + $end = tell TEST; seek(TEST,$start,0); print TEST "18 characters here"; @@ -1337,7 +1356,7 @@ within certain expressions, statements, contexts, or whatever. =over 5 -=item * Interpolation +=item * C<@> always interpolates an array in double-quotish strings @ now always interpolates an array in double-quotish strings. @@ -1347,7 +1366,7 @@ within certain expressions, statements, contexts, or whatever. # 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 -=item * Interpolation +=item * Double-quoted strings may no longer end with an unescaped $ Double-quoted strings may no longer end with an unescaped $. @@ -1359,7 +1378,7 @@ Double-quoted strings may no longer end with an unescaped $. Note: perl5 DOES NOT error on the terminating @ in $bar -=item * Interpolation +=item * Arbitrary expressions are evaluated inside braces within double quotes Perl now sometimes evaluates arbitrary expressions inside braces that occur within double quotes (usually when the opening brace is preceded by C<$> @@ -1376,7 +1395,7 @@ or C<@>). Note that you can C to ward off such trappiness under perl5. -=item * Interpolation +=item * C<$$x> now tries to dereference $x The construct "this is $$x" used to interpolate the pid at that point, but now tries to dereference $x. C<$$> by itself still works fine, however. @@ -1388,7 +1407,7 @@ now tries to dereference $x. C<$$> by itself still works fine, however. # perl4 prints: this is XXXx (XXX is the current pid) # perl5 prints: this is a reference -=item * Interpolation +=item * Creation of hashes on the fly with C requires protection Creation of hashes on the fly with C now requires either both C<$>'s to be protected in the specification of the hash name, or both curlies @@ -1429,7 +1448,7 @@ causes the following result: # and is compatible for both versions -=item * Interpolation +=item * Bugs in earlier perl versions perl4 programs which unconsciously rely on the bugs in earlier perl versions. @@ -1438,7 +1457,7 @@ perl4 programs which unconsciously rely on the bugs in earlier perl versions. # perl4 prints: This is not perl5 # perl5 prints: This is perl5 -=item * Interpolation +=item * Array and hash brackets during interpolation You also have to be careful about array and hash brackets during interpolation. @@ -1460,9 +1479,9 @@ to mimic the behavior of Perl 4, you must escape the bracket like so. print "$foo\["; print "$foo\{"; -=item * Interpolation +=item * Interpolation of C<\$$foo{bar}> -Similarly, watch out for: +Similarly, watch out for: C<\$$foo{bar}> $foo = "baz"; print "\$$foo{bar}\n"; @@ -1474,7 +1493,7 @@ Perl 5 is looking for C<$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 C's. -=item * Interpolation +=item * C string passed to C will not find string terminator C string passed to C @@ -1495,7 +1514,7 @@ General DBM traps. =over 5 -=item * DBM +=item * Perl5 must have been linked with same dbm/ndbm as the default for C Existing dbm databases created under perl4 (or any other dbm/ndbm tool) may cause the same script, run under perl5, to fail. The build of perl5 @@ -1509,7 +1528,7 @@ to function properly without C'ing to an extension dbm implementation. # perl5 prints: ok (IFF linked with -ldbm or -lndbm) -=item * DBM +=item * DBM exceeding limit on the key/value size will cause perl5 to exit immediately Existing dbm databases created under perl4 (or any other dbm/ndbm tool) may cause the same script, run under perl5, to fail. The error generated