Error-free constant folding is a TODO

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

diff --git a/pod/perltrap.pod b/pod/perltrap.pod
index 753e721..0ad0086 100644 (file)
--- a/pod/perltrap.pod
+++ b/pod/perltrap.pod
@@ -18,6 +18,11 @@ Accustomed B<awk> 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;
@@ -143,9 +148,9 @@ gives you.
 
 =back
 
-=head2 C Traps
+=head2 C/C++ Traps
 
-Cerebral C programmers should take note of the following:
+Cerebral C and C++ programmers should take note of the following:
 
 =over 4
 
@@ -159,13 +164,14 @@ 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.
+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.  See L<perlsyn/"Loop Control">.
 
 =item *
 
-There's no switch statement.  (But it's easy to build one on the fly.)
+There's no switch statement.  (But it's easy to build one on the fly,
+see L<perlsyn/"Basic BLOCKs and Switch Statements">)
 
 =item *
 
@@ -173,7 +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 *
 
@@ -205,6 +213,11 @@ Seasoned B<sed> 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 *
@@ -254,6 +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<test> uses "=", "!=", "<" etc for string comparisons and "-eq",
+"-ne", "-lt" etc for numeric comparisons. This is the reverse of Perl, which
+uses C<eq>, C<ne>, C<lt> for string comparisons, and C<==>, C<!=> C<< < >> etc
+for numeric comparisons.
+
 =back
 
 =head2 Perl Traps
@@ -651,6 +671,16 @@ are to used around the name.
     # perl4 prints: {a}
     # perl5 prints: 2
 
+=item * Parsing
+
+When perl sees C<map {> (or C<grep {>), it has to guess whether the C<{>
+starts a BLOCK or a hash reference. If it guesses wrong, it will report
+a syntax error near the C<}> and the missing (or unexpected) comma.
+
+Use unary C<+> before C<{> on a hash reference, and unary C<+> applied
+to the first thing in a BLOCK (after C<{>), for perl to guess right all
+the time. (See L<perlfunc/map>.)
+
 =back
 
 =head2 Numerical Traps
@@ -662,18 +692,24 @@ operands, or output from same.
 
 =item * Numerical
 
-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:
 
     print 7.373504 - 0, "\n";
     printf "%20.18f\n", 7.373504 - 0;
 
     # Perl4 prints:
-    7.375039999999996141
-    7.37503999999999614
+    7.3750399999999996141
+    7.375039999999999614
 
     # Perl5 prints:
     7.373504
-    7.37503999999999614
+    7.375039999999999614
+
+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
 
@@ -688,7 +724,7 @@ If in doubt:
 
 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
+Logical tests now return a null, instead of 0
 
     $p = ($test == 1);
     print $p,"\n";
@@ -1207,6 +1243,10 @@ repeatedly, like C</x/> or C<m!x!>.
     # perl4 prints: perl4
     # perl5 prints: perl5
 
+=item * Regular Expression
+
+Unlike in Ruby, failed matches in Perl do not reset the match variables
+($1, $2, ..., C<$`>, ...).
 
 =back
 
@@ -1328,13 +1368,12 @@ within certain expressions, statements, contexts, or whatever.
 
 =item * Interpolation
 
-Double-quoted strings may no longer end with an unescaped $ or @.
+Double-quoted strings may no longer end with an unescaped $.
 
     $foo = "foo$";
-    $bar = "bar@";
-    print "foo is $foo, bar is $bar\n";
+    print "foo is $foo\n";
 
-    # perl4 prints: foo is foo$, bar is bar@
+    # perl4 prints: foo is foo$
     # perl5 errors: Final $ should be \$ or $name
 
 Note: perl5 DOES NOT error on the terminating @ in $bar
@@ -1420,13 +1459,26 @@ perl4 programs which unconsciously rely on the bugs in earlier perl versions.
 
 =item * Interpolation
 
-You also have to be careful about array references.
+You also have to be careful about array and hash brackets during
+interpolation.
+
+    print "$foo["
+
+    perl 4 prints: [
+    perl 5 prints: syntax error
 
     print "$foo{"
 
     perl 4 prints: {
     perl 5 prints: syntax error
 
+Perl 5 is expecting to find an index or key name following the respective
+brackets, as well as an ending bracket of the appropriate type.  In order
+to mimic the behavior of Perl 4, you must escape the bracket like so.
+
+    print "$foo\[";
+    print "$foo\{";
+
 =item * Interpolation
 
 Similarly, watch out for: