perltrap.pod

来自「MSYS在windows下模拟了一个类unix的终端」· POD 代码 · 共 1,539 行 · 第 1/3 页

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 $var will clobber the values of @list.  (This most often
happens when you use C<$_> for the loop variable, and call subroutines in
the loop that don't properly localize C<$_>.)

=item * Discontinuance

C<split> with no arguments now behaves like C<split ' '> (which doesn't
return an initial null field if $_ starts with whitespace), it used to
behave like C<split /\s+/> (which does).

    $_ = ' hi mom';
    print join(':', split);

    # 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<push> was undocumented, but it was
actually the last value being pushed onto the target list.  In Perl 5
the return value of C<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

=item * Deprecation

Some error messages will be different.

=item * Discontinuance

In Perl 4, if in list context the delimiters to the first argument of
C<split()> 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

Some bugs may have been inadvertently removed.  :-)

=back

=head2 Parsing Traps

Perl4-to-Perl5 traps from having to do with parsing.

=over 4

=item * Parsing

Note the space between . and =

    $string . = "more string";
    print $string;

    # perl4 prints: more string
    # perl5 prints: syntax error at - line 1, near ". ="

=item * Parsing

Better parsing in perl 5

    sub foo {}
    &foo
    print("hello, world\n");

    # perl4 prints: hello, world
    # perl5 prints: syntax error

=item * Parsing

"if it looks like a function, it is a function" rule.

  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

=item * Parsing

String interpolation of the C<$#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

=back

=head2 Numerical Traps

Perl4-to-Perl5 traps having to do with numerical operators,
operands, or output from same.

=over 5

=item * Numerical

Formatted output and significant digits

    print 7.373504 - 0, "\n";
    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 auto-increment
operator would not catch when a number went over the signed int limit.  Fixed
in version 5.003_04.  But always be wary when using large integers.
If in doubt:

   use Math::BigInt;

=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 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<vec()> function. perl5 treats the string operands as bitstrings.
(See L<perlop/Bitwise String Operators> 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

Perl4-to-Perl5 traps involving most data-types, and their usage
within certain expressions and/or context.

=over 5

=item * (Arrays)

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

=item * (Arrays)

Setting C<$#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

=item * (Hashes)

Hashes get defined before use

    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).

=item * (Globs)

glob assignment from variable to variable will fail if the assigned
variable is localized subsequent to the assignment

    @a = ("This is Perl 4");
    *b = *a;
    local(@a);
    print @b,"\n";

    # perl4 prints: This is Perl 4
    # perl5 prints:

=item * (Globs)

Assigning C<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 C<undef> is assigned to a
typeglob. (Note that assigning C<undef> to a typeglob is different
than calling the C<undef> function on a typeglob (C<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

=item * (Scalar String)

Changes in unary negation (of strings)
This change effects both the return value and what it
does to auto(magic)increment.

    $x = "aaa";
    print ++$x," : ";
    print -$x," : ";
    print ++$x,"\n";

    # perl4 prints: aab : -0 : 1
    # perl5 prints: aab : -aab : aac

=item * (Constants)

perl 4 lets you modify constants:

    $foo = "x";
    &mod($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

=item * (Scalars)

The behavior is slightly different for:

    print "$x", defined $x

    # perl 4: 1
    # perl 5: <no output, $x is not called into existence>

=item * (Variable Suicide)

Variable suicide behavior is more consistent under Perl 5.
Perl5 exhibits the same behavior for hashes and scalars,
that perl4 exhibits for only scalars.

    $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

=back

=head2 Context Traps - scalar, list contexts

=over 5

=item * (list context)

The elements of argument lists for formats are now evaluated in list
context.  This means you can interpolate list values now.

    @fmt = ("foo","bar","baz");
    format STDOUT=
    @<<<<< @||||| @>>>>>
    @fmt;
    .
    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
being required.

    caller() ? (print "You rang?\n") : (print "Got a 0\n");

    # perl4 errors: There is no caller
    # perl5 prints: Got a 0

=item * (scalar context)

The comma operator in a scalar context is now guaranteed to give a
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

=item * (list, builtin)

C<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

C<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

=back

=head2 Precedence Traps

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

LHS vs. RHS of any assignment operator.  LHS is evaluated first
in perl4, second in perl5; this can affect the relationship
between side-effects in sub-expressions.

    @arr = ( 'left', 'right' );
    $a{shift @arr} = shift @arr;
    print join( ' ', keys %a );

    # perl4 prints: left
    # perl5 prints: right

=item * Precedence

These are now semantic errors because of precedence:

    @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

=item * Precedence

The precedence of assignment operators is now the same as the precedence
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

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.

=item * Precedence

    open FOO || die;

is now incorrect.  You need parentheses around the filehandle.
Otherwise, perl5 leaves the statement as its default precedence:

    open(FOO || die);