NAME

perltrap - Perl traps for the unwary

DESCRIPTION

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

Awk Traps

Accustomed awk users should take special note of the following:

• The English module, loaded via

      use English;

  allows you to refer to special variables (like $RS) as though they were in awk; see perlvar for details.

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

• Curly brackets are required on ifs and whiles.

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

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

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

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

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

• 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.

• 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 perlvar.

• $<digit> does not refer to fields--it refers to substrings matched by the last match pattern.

• The print() statement does not add field and record separators unless you set $, and $.. You can set $OFS and $ORS if you're using the English module.

• You must open your files before you print to them.

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

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

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

• The concatenation operator is ".", not the null string. (Using the null string would render /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.)

• The next, exit, and continue keywords work differently.

• 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    $;

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

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

C Traps

Cerebral C programmers should take note of the following:

• Curly brackets are required on if's and while's.

• You must use elsif rather than else if.

• The break and continue keywords from C become in Perl last and next, respectively. Unlike in C, these do NOT work within a do { } while construct.

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

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

• 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.

• Comments begin with "#", not "/*".

• You can't take the address of anything, although a similar operator in Perl 5 is the backslash, which creates a reference.

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

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

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

Sed Traps

Seasoned sed programmers should take note of the following:

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

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

• The range operator is ..., rather than comma.

Shell Traps

Sharp shell programmers should take note of the following:

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

• The backtick operator does no translation of the return value, unlike csh.

• Shells (especially 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.

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

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

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

Perl Traps

Practicing Perl Programmers should take note of the following:

• Remember that many operations behave differently in a list context than they do in a scalar one. See perldata for details.

• 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.

• 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 only be list operators, never unary ones.) See perlop.

• 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.

• 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!

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

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

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

• Use my() for local variables whenever you can get away with it (but see 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.

• If you localize an exported variable in a module, its exported value will not change. The local name becomes an alias to a new value but the external name is still an alias for the original.

Perl4 Traps

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

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

• 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 -w switch to find such places.

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

• 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";

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

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

• 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;

• 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.

• 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.

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

• You can't do a goto into a block that is optimized away. Darn.

• 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.

• 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.

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

• reverse is no longer allowed as the name of a sort subroutine.

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

• Double-quoted strings may no longer end with an unescaped $ or @.

• The archaic while/if BLOCK BLOCK syntax is no longer supported.

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

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

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

• Setting $#array lower now discards array elements.

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

• The construct "this is $$x" used to interpolate the pid at that point, but now tries to dereference $x. $$ by itself still works fine, however.

• The meaning of foreach has changed slightly when it is iterating over a list which is not an array. This used to assign the list to a 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. To retain Perl 4 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 /x/, @list) { ... }

  to

      foreach $var (my @tmp = grep /x/, @list) { ... }

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

• Some error messages will be different.

• Some bugs may have been inadvertently removed.