Updated.

[p5sagit/p5-mst-13.2.git] / pod / perltrap.pod

=head1 NAME

perltrap - Perl traps for the unwary

=head1 DESCRIPTION

The biggest trap of all is forgetting to use the B<-w> switch; see
L<perlrun>.  The second biggest trap is not making your entire program
runnable under C<use strict>.

=head2 Awk Traps

Accustomed B<awk> users should take special note of the following:

=over 4

=item *

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.

=item *

Semicolons are required after all simple statements in Perl (except
at the end of a block).  Newline is not a statement delimiter.

=item *

Curly brackets are required on C<if>s and C<while>s.

=item *

Variables begin with "$" or "@" in Perl.

=item *

Arrays index from 0.  Likewise string positions in substr() and
index().

=item *

You have to decide whether your array has numeric or string indices.

=item *

Associative array values do not spring into existence upon mere
reference.

=item *

You have to decide whether you want to use string or numeric
comparisons.

=item *

Reading an input line does not split it for you.  You get to split it
yourself to an array.  And split() operator has different
arguments.

=item *

The current input line is normally in $_, not $0.  It generally does
not have the newline stripped.  ($0 is the name of the program
executed.)  See L<perlvar>.

=item *

$<I<digit>> 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
the English module.

=item *

You must open your files before you print to them.

=item *

The range operator is "..", not comma.  The comma operator works as in
C.

=item *

The match operator is "=~", not "~".  ("~" is the one's complement
operator, as in C.)

=item *

The exponentiation operator is "**", not "^".  "^" is the XOR
operator, as in C.  (You know, one could get the feeling that B<awk> is
basically incompatible with C.)

=item *

The concatenation operator is ".", not the null string.  (Using the
null string would render C</pat/ /pat/> unparsable, since the third slash
would be interpreted as a division operator--the tokener is in fact
slightly context sensitive for operators like "/", "?", and ">".
And in fact, "." itself can be the beginning of a number.)

=item *

The C<next>, C<exit>, and C<continue> keywords work differently.

=item *


The following variables work differently:

      Awk	Perl
      ARGC	$#ARGV or scalar @ARGV
      ARGV[0]	$0
      FILENAME	$ARGV
      FNR	$. - something
      FS	(whatever you like)
      NF	$#Fld, or some such
      NR	$.
      OFMT	$#
      OFS	$,
      ORS	$\
      RLENGTH	length($&)
      RS	$/
      RSTART	length($`)
      SUBSEP	$;

=item *

You cannot set $RS to a pattern, only a string.

=item *

When in doubt, run the B<awk> construct through B<a2p> and see what it
gives you.

=back

=head2 C Traps

Cerebral C programmers should take note of the following:

=over 4

=item *

Curly brackets are required on C<if>'s and C<while>'s.

=item *

You must use C<elsif> rather than C<else if>.

=item *

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.

=item *

There's no switch statement.  (But it's easy to build one on the fly.)

=item *

Variables begin with "$" or "@" in Perl.

=item *

printf() does not implement the "*" format for interpolating
field widths, but it's trivial to use interpolation of double-quoted
strings to achieve the same effect.

=item *

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.

=item *

C<ARGV> must be capitalized.  C<$ARGV[0]> is C's C<argv[1]>, and C<argv[0]>
ends up in C<$0>.

=item *

System calls such as link(), unlink(), rename(), etc. return nonzero for
success, not 0.

=item *

Signal handlers deal with signal names, not numbers.  Use C<kill -l>
to find their names on your system.

=back

=head2 Sed Traps

Seasoned B<sed> programmers should take note of the following:

=over 4

=item *

Backreferences in substitutions use "$" rather than "\".

=item *

The pattern matching metacharacters "(", ")", and "|" do not have backslashes
in front.

=item *

The range operator is C<...>, rather than comma.

=back

=head2 Shell Traps

Sharp shell programmers should take note of the following:

=over 4

=item *

The backtick operator does variable interpretation without regard to
the presence of single quotes in the command.

=item *

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 only in certain constructs
such as double quotes, backticks, angle brackets, and search patterns.

=item *

Shells interpret scripts a little bit at a time.  Perl compiles the
entire program before executing it (except for C<BEGIN> blocks, which
execute at compile time).

=item *

The arguments are available via @ARGV, not $1, $2, etc.

=item *

The environment is not automatically made available as separate scalar
variables.

=back

=head2 Perl Traps

Practicing Perl Programmers should take note of the following:

=over 4

=item *

Remember that many operations behave differently in a list
context than they do in a scalar one.  See L<perldata> 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.

=item *

You cannot discern from mere inspection which built-ins
are unary operators (like chop() and chdir()) 
and which are list operators (like print() and unlink()).
(User-defined subroutines can B<only> be list operators, never
unary ones.)  See L<perlop>.

=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.  

=item * 

The <FH> 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
file read is the sole condition in a while loop:

    while (<FH>)      { }
    while ($_ = <FH>) { }..
    <FH>;  # data discarded!

=item * 

Remember not to use "C<=>" when you need "C<=~>";
these two constructs are quite different:

    $x =  /foo/;
    $x =~ /foo/;

=item *

The C<do {}> construct isn't a real loop that you can use 
loop control on.

=item *

Use my() for local variables whenever you can get away with 
it (but see L<perlform> for where you can't).  
Using local() actually gives a local value to a global 
variable, which leaves you open to unforeseen side-effects
of dynamic scoping.

=back

=head2 Perl4 Traps

Penitent Perl 4 Programmers should take note of the following
incompatible changes that occurred between release 4 and release 5:

=over 4

=item *

C<@> now always interpolates an array in double-quotish strings.  Some programs
may now need to use backslash to protect any C<@> that shouldn't interpolate.

=item *

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.
For example:

    sub SeeYa { die "Hasta la vista, baby!" }
    $SIG{'QUIT'} = SeeYa;

In Perl 4, that set the signal handler; in Perl 5, it actually calls the
function!  You may use the B<-w> switch to find such places.

=item *

Symbols starting with C<_> are no longer forced into package C<main>, except
for $_ itself (and @_, etc.).

=item *

Double-colon is now a valid package separator in an identifier.  Thus these
behave differently in perl4 vs. perl5:

    print "$a::$b::$c\n";
    print "$var::abc::xyz\n";

=item *

C<s'$lhs'$rhs'> now does no interpolation on either side.  It used to
interpolate C<$lhs> but not C<$rhs>.

=item *

The second and third arguments of splice() are now evaluated in scalar
context (as the book says) rather than list context.

=item *

These are now semantic errors because of precedence:

    shift @list + 20;	
    $n = keys %map + 20; 

Because if that were to work, then this couldn't:

    sleep $dormancy + 20;

=item *

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 *

C<open FOO || die> is now incorrect.  You need parens around the filehandle.
While temporarily supported, using such a construct will 
generate a non-fatal (but non-suppressible) warning.

=item *

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

=item *

You can't do a C<goto> into a block that is optimized away.  Darn.

=item *

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.

=item *

The 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.

=item *

C<m//g> now attaches its state to the searched string rather than the
regular expression.

=item *

C<reverse> is no longer allowed as the name of a sort subroutine.

=item *

B<taintperl> is no longer a separate executable.  There is now a B<-T>
switch to turn on tainting when it isn't turned on automatically.

=item *

Double-quoted strings may no longer end with an unescaped C<$> or C<@>.

=item *

The archaic C<while/if> BLOCK BLOCK syntax is no longer supported.


=item *

Negative array subscripts now count from the end of the array.

=item *

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

=item *

The C<**> operator now binds more tightly than unary minus.  
It was documented to work this way before, but didn't.

=item *

Setting C<$#array> lower now discards array elements.

=item *

delete() is not guaranteed to return the old value for tie()d arrays,
since this capability may be onerous for some modules to implement.

=item *

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.

=item *

Some error messages will be different.

=item *

Some bugs may have been inadvertently removed.

=back