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Text File | 1998-10-28 | 56.9 KB | 1,717 lines

PPPPEEEERRRRLLLLSSSSUUUUBBBB((((1111)))) 22223333////JJJJuuuullll////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) PPPPEEEERRRRLLLLSSSSUUUUBBBB((((1111)))) NNNNAAAAMMMMEEEE perlsub - Perl subroutines SSSSYYYYNNNNOOOOPPPPSSSSIIIISSSS To declare subroutines: sub NAME; # A "forward" declaration. sub NAME(PROTO); # ditto, but with prototypes sub NAME BLOCK # A declaration and a definition. sub NAME(PROTO) BLOCK # ditto, but with prototypes To define an anonymous subroutine at runtime: $subref = sub BLOCK; # no proto $subref = sub (PROTO) BLOCK; # with proto To import subroutines: use PACKAGE qw(NAME1 NAME2 NAME3); To call subroutines: NAME(LIST); # & is optional with parentheses. NAME LIST; # Parentheses optional if predeclared/imported. &NAME; # Makes current @_ visible to called subroutine. DDDDEEEESSSSCCCCRRRRIIIIPPPPTTTTIIIIOOOONNNN Like many languages, Perl provides for user-defined subroutines. These may be located anywhere in the main program, loaded in from other files via the do, require, or use keywords, or even generated on the fly using eval or anonymous subroutines (closures). You can even call a function indirectly using a variable containing its name or a CODE reference to it. The Perl model for function call and return values is simple: all functions are passed as parameters one single flat list of scalars, and all functions likewise return to their caller one single flat list of scalars. Any arrays or hashes in these call and return lists will collapse, losing their identities--but you may always use pass-by-reference instead to avoid this. Both call and return lists may contain as many or as few scalar elements as you'd like. (Often a function without an explicit return statement is called a subroutine, but there's really no difference from the language's perspective.) Any arguments passed to the routine come in as the array @_. Thus if you called a function with two arguments, those would be stored in $_[0] and $_[1]. The array @_ is a local Page 1 (printed 10/23/98) PPPPEEEERRRRLLLLSSSSUUUUBBBB((((1111)))) 22223333////JJJJuuuullll////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) PPPPEEEERRRRLLLLSSSSUUUUBBBB((((1111)))) array, but its elements are aliases for the actual scalar parameters. In particular, if an element $_[0] is updated, the corresponding argument is updated (or an error occurs if it is not updatable). If an argument is an array or hash element which did not exist when the function was called, that element is created only when (and if) it is modified or if a reference to it is taken. (Some earlier versions of Perl created the element whether or not it was assigned to.) Note that assigning to the whole array @_ removes the aliasing, and does not update any arguments. The return value of the subroutine is the value of the last expression evaluated. Alternatively, a return statement may be used to exit the subroutine, optionally specifying the returned value, which will be evaluated in the appropriate context (list, scalar, or void) depending on the context of the subroutine call. If you specify no return value, the subroutine will return an empty list in a list context, an undefined value in a scalar context, or nothing in a void context. If you return one or more arrays and/or hashes, these will be flattened together into one large indistinguishable list. Perl does not have named formal parameters, but in practice all you do is assign to a my() list of these. Any variables you use in the function that aren't declared private are global variables. For the gory details on creating private variables, see the section on _P_r_i_v_a_t_e _V_a_r_i_a_b_l_e_s _v_i_a _m_y() and the section on _T_e_m_p_o_r_a_r_y _V_a_l_u_e_s _v_i_a _l_o_c_a_l(). To create protected environments for a set of functions in a separate package (and probably a separate file), see the section on _P_a_c_k_a_g_e_s in the _p_e_r_l_m_o_d manpage. Example: sub max { my $max = shift(@_); foreach $foo (@_) { $max = $foo if $max < $foo; } return $max; } $bestday = max($mon,$tue,$wed,$thu,$fri); Example: # get a line, combining continuation lines # that start with whitespace Page 2 (printed 10/23/98) PPPPEEEERRRRLLLLSSSSUUUUBBBB((((1111)))) 22223333////JJJJuuuullll////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) PPPPEEEERRRRLLLLSSSSUUUUBBBB((((1111)))) sub get_line { $thisline = $lookahead; # GLOBAL VARIABLES!! LINE: while (defined($lookahead = <STDIN>)) { if ($lookahead =~ /^[ \t]/) { $thisline .= $lookahead; } else { last LINE; } } $thisline; } $lookahead = <STDIN>; # get first line while ($_ = get_line()) { ... } Use array assignment to a local list to name your formal arguments: sub maybeset { my($key, $value) = @_; $Foo{$key} = $value unless $Foo{$key}; } This also has the effect of turning call-by-reference into call-by-value, because the assignment copies the values. Otherwise a function is free to do in-place modifications of @_ and change its caller's values. upcase_in($v1, $v2); # this changes $v1 and $v2 sub upcase_in { for (@_) { tr/a-z/A-Z/ } } You aren't allowed to modify constants in this way, of course. If an argument were actually literal and you tried to change it, you'd take a (presumably fatal) exception. For example, this won't work: upcase_in("frederick"); It would be much safer if the upcase_in() function were written to return a copy of its parameters instead of changing them in place: Page 3 (printed 10/23/98) PPPPEEEERRRRLLLLSSSSUUUUBBBB((((1111)))) 22223333////JJJJuuuullll////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) PPPPEEEERRRRLLLLSSSSUUUUBBBB((((1111)))) ($v3, $v4) = upcase($v1, $v2); # this doesn't sub upcase { return unless defined wantarray; # void context, do nothing my @parms = @_; for (@parms) { tr/a-z/A-Z/ } return wantarray ? @parms : $parms[0]; } Notice how this (unprototyped) function doesn't care whether it was passed real scalars or arrays. Perl will see everything as one big long flat @_ parameter list. This is one of the ways where Perl's simple argument-passing style shines. The upcase() function would work perfectly well without changing the upcase() definition even if we fed it things like this: @newlist = upcase(@list1, @list2); @newlist = upcase( split /:/, $var ); Do not, however, be tempted to do this: (@a, @b) = upcase(@list1, @list2); Because like its flat incoming parameter list, the return list is also flat. So all you have managed to do here is stored everything in @a and made @b an empty list. See the section on _P_a_s_s _b_y _R_e_f_e_r_e_n_c_e for alternatives. A subroutine may be called using the "&" prefix. The "&" is optional in modern Perls, and so are the parentheses if the subroutine has been predeclared. (Note, however, that the "&" is _N_O_T optional when you're just naming the subroutine, such as when it's used as an argument to defined() or undef(). Nor is it optional when you want to do an indirect subroutine call with a subroutine name or reference using the &$subref() or &{$subref}() constructs. See the _p_e_r_l_r_e_f manpage for more on that.) Subroutines may be called recursively. If a subroutine is called using the "&" form, the argument list is optional, and if omitted, no @_ array is set up for the subroutine: the @_ array at the time of the call is visible to subroutine instead. This is an efficiency mechanism that new users may wish to avoid. &foo(1,2,3); # pass three arguments foo(1,2,3); # the same foo(); # pass a null list &foo(); # the same Page 4 (printed 10/23/98) PPPPEEEERRRRLLLLSSSSUUUUBBBB((((1111)))) 22223333////JJJJuuuullll////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) PPPPEEEERRRRLLLLSSSSUUUUBBBB((((1111)))) &foo; # foo() get current args, like foo(@_) !! foo; # like foo() IFF sub foo predeclared, else "foo" Not only does the "&" form make the argument list optional, but it also disables any prototype checking on the arguments you do provide. This is partly for historical reasons, and partly for having a convenient way to cheat if you know what you're doing. See the section on Prototypes below. Function whose names are in all upper case are reserved to the Perl core, just as are modules whose names are in all lower case. A function in all capitals is a loosely-held convention meaning it will be called indirectly by the run- time system itself. Functions that do special, pre-defined things are BEGIN, END, AUTOLOAD, and DESTROY--plus all the functions mentioned in the _p_e_r_l_t_i_e manpage. The 5.005 release adds INIT to this list. PPPPrrrriiiivvvvaaaatttteeee VVVVaaaarrrriiiiaaaabbbblllleeeessss vvvviiiiaaaa mmmmyyyy(((()))) Synopsis: my $foo; # declare $foo lexically local my (@wid, %get); # declare list of variables local my $foo = "flurp"; # declare $foo lexical, and init it my @oof = @bar; # declare @oof lexical, and init it A "my" declares the listed variables to be confined (lexically) to the enclosing block, conditional (if/unless/elsif/else), loop (for/foreach/while/until/continue), subroutine, eval, or do/require/use'd file. If more than one value is listed, the list must be placed in parentheses. All listed elements must be legal lvalues. Only alphanumeric identifiers may be lexically scoped--magical builtins like $/ must currently be localize with "local" instead. Unlike dynamic variables created by the "local" operator, lexical variables declared with "my" are totally hidden from the outside world, including any called subroutines (even if it's the same subroutine called from itself or elsewhere-- every call gets its own copy). This doesn't mean that a my() variable declared in a statically _e_n_c_l_o_s_i_n_g lexical scope would be invisible. Only the dynamic scopes are cut off. For example, the bumpx() function below has access to the lexical $x variable because both the my and the sub occurred at the same scope, presumably the file scope. my $x = 10; sub bumpx { $x++ } Page 5 (printed 10/23/98) PPPPEEEERRRRLLLLSSSSUUUUBBBB((((1111)))) 22223333////JJJJuuuullll////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) PPPPEEEERRRRLLLLSSSSUUUUBBBB((((1111)))) (An eval(), however, can see the lexical variables of the scope it is being evaluated in so long as the names aren't hidden by declarations within the eval() itself. See the _p_e_r_l_r_e_f manpage.) The parameter list to my() may be assigned to if desired, which allows you to initialize your variables. (If no initializer is given for a particular variable, it is created with the undefined value.) Commonly this is used to name the parameters to a subroutine. Examples: $arg = "fred"; # "global" variable $n = cube_root(27); print "$arg thinks the root is $n\n"; fred thinks the root is 3 sub cube_root { my $arg = shift; # name doesn't matter $arg **= 1/3; return $arg; } The "my" is simply a modifier on something you might assign to. So when you do assign to the variables in its argument list, the "my" doesn't change whether those variables are viewed as a scalar or an array. So my ($foo) = <STDIN>; # WRONG? my @FOO = <STDIN>; both supply a list context to the right-hand side, while my $foo = <STDIN>; supplies a scalar context. But the following declares only one variable: my $foo, $bar = 1; # WRONG That has the same effect as my $foo; $bar = 1; The declared variable is not introduced (is not visible) until after the current statement. Thus, my $x = $x; can be used to initialize the new $x with the value of the old $x, and the expression Page 6 (printed 10/23/98) PPPPEEEERRRRLLLLSSSSUUUUBBBB((((1111)))) 22223333////JJJJuuuullll////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) PPPPEEEERRRRLLLLSSSSUUUUBBBB((((1111)))) my $x = 123 and $x == 123 is false unless the old $x happened to have the value 123. Lexical scopes of control structures are not bounded precisely by the braces that delimit their controlled blocks; control expressions are part of the scope, too. Thus in the loop while (defined(my $line = <>)) { $line = lc $line; } continue { print $line; } the scope of $line extends from its declaration throughout the rest of the loop construct (including the continue clause), but not beyond it. Similarly, in the conditional if ((my $answer = <STDIN>) =~ /^yes$/i) { user_agrees(); } elsif ($answer =~ /^no$/i) { user_disagrees(); } else { chomp $answer; die "'$answer' is neither 'yes' nor 'no'"; } the scope of $answer extends from its declaration throughout the rest of the conditional (including elsif and else clauses, if any), but not beyond it. (None of the foregoing applies to if/unless or while/until modifiers appended to simple statements. Such modifiers are not control structures and have no effect on scoping.) The foreach loop defaults to scoping its index variable dynamically (in the manner of local; see below). However, if the index variable is prefixed with the keyword "my", then it is lexically scoped instead. Thus in the loop for my $i (1, 2, 3) { some_function(); } the scope of $i extends to the end of the loop, but not beyond it, and so the value of $i is unavailable in some_function(). Some users may wish to encourage the use of lexically scoped variables. As an aid to catching implicit references to package variables, if you say Page 7 (printed 10/23/98) PPPPEEEERRRRLLLLSSSSUUUUBBBB((((1111)))) 22223333////JJJJuuuullll////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) PPPPEEEERRRRLLLLSSSSUUUUBBBB((((1111)))) use strict 'vars'; then any variable reference from there to the end of the enclosing block must either refer to a lexical variable, or must be fully qualified with the package name. A compilation error results otherwise. An inner block may countermand this with "no strict 'vars'". A my() has both a compile-time and a run-time effect. At compile time, the compiler takes notice of it; the principle usefulness of this is to quiet "use strict 'vars'". The actual initialization is delayed until run time, so it gets executed appropriately; every time through a loop, for example. Variables declared with "my" are not part of any package and are therefore never fully qualified with the package name. In particular, you're not allowed to try to make a package variable (or other global) lexical: my $pack::var; # ERROR! Illegal syntax my $_; # also illegal (currently) In fact, a dynamic variable (also known as package or global variables) are still accessible using the fully qualified :: notation even while a lexical of the same name is also visible: package main; local $x = 10; my $x = 20; print "$x and $::x\n"; That will print out 20 and 10. You may declare "my" variables at the outermost scope of a file to hide any such identifiers totally from the outside world. This is similar to C's static variables at the file level. To do this with a subroutine requires the use of a closure (anonymous function with lexical access). If a block (such as an eval(), function, or package) wants to create a private subroutine that cannot be called from outside that block, it can declare a lexical variable containing an anonymous sub reference: my $secret_version = '1.001-beta'; my $secret_sub = sub { print $secret_version }; &$secret_sub(); As long as the reference is never returned by any function within the module, no outside module can see the subroutine, because its name is not in any package's symbol table. Page 8 (printed 10/23/98) PPPPEEEERRRRLLLLSSSSUUUUBBBB((((1111)))) 22223333////JJJJuuuullll////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) PPPPEEEERRRRLLLLSSSSUUUUBBBB((((1111)))) Remember that it's not _R_E_A_L_L_Y called $some_pack::secret_version or anything; it's just $secret_version, unqualified and unqualifiable. This does not work with object methods, however; all object methods have to be in the symbol table of some package to be found. PPPPeeeerrrriiiisssstttteeeennnntttt PPPPrrrriiiivvvvaaaatttteeee VVVVaaaarrrriiiiaaaabbbblllleeeessss Just because a lexical variable is lexically (also called statically) scoped to its enclosing block, eval, or do FILE, this doesn't mean that within a function it works like a C static. It normally works more like a C auto, but with implicit garbage collection. Unlike local variables in C or C++, Perl's lexical variables don't necessarily get recycled just because their scope has exited. If something more permanent is still aware of the lexical, it will stick around. So long as something else references a lexical, that lexical won't be freed--which is as it should be. You wouldn't want memory being free until you were done using it, or kept around once you were done. Automatic garbage collection takes care of this for you. This means that you can pass back or save away references to lexical variables, whereas to return a pointer to a C auto is a grave error. It also gives us a way to simulate C's function statics. Here's a mechanism for giving a function private variables with both lexical scoping and a static lifetime. If you do want to create something like C's static variables, just enclose the whole function in an extra block, and put the static variable outside the function but in the block. { my $secret_val = 0; sub gimme_another { return ++$secret_val; } } # $secret_val now becomes unreachable by the outside # world, but retains its value between calls to gimme_another If this function is being sourced in from a separate file via require or use, then this is probably just fine. If it's all in the main program, you'll need to arrange for the my() to be executed early, either by putting the whole block above your main program, or more likely, placing merely a BEGIN sub around it to make sure it gets executed before your program starts to run: Page 9 (printed 10/23/98) PPPPEEEERRRRLLLLSSSSUUUUBBBB((((1111)))) 22223333////JJJJuuuullll////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) PPPPEEEERRRRLLLLSSSSUUUUBBBB((((1111)))) sub BEGIN { my $secret_val = 0; sub gimme_another { return ++$secret_val; } } See the section on _P_a_c_k_a_g_e _C_o_n_s_t_r_u_c_t_o_r_s _a_n_d _D_e_s_t_r_u_c_t_o_r_s in the _p_e_r_l_m_o_d manpage about the BEGIN function. If declared at the outermost scope, the file scope, then lexicals work someone like C's file statics. They are available to all functions in that same file declared below them, but are inaccessible from outside of the file. This is sometimes used in modules to create private variables for the whole module. TTTTeeeemmmmppppoooorrrraaaarrrryyyy VVVVaaaalllluuuueeeessss vvvviiiiaaaa _l_o_c_a_l() NNNNOOOOTTTTEEEE: In general, you should be using "my" instead of "local", because it's faster and safer. Exceptions to this include the global punctuation variables, filehandles and formats, and direct manipulation of the Perl symbol table itself. Format variables often use "local" though, as do other variables whose current value must be visible to called subroutines. Synopsis: local $foo; # declare $foo dynamically local local (@wid, %get); # declare list of variables local local $foo = "flurp"; # declare $foo dynamic, and init it local @oof = @bar; # declare @oof dynamic, and init it local *FH; # localize $FH, @FH, %FH, &FH ... local *merlyn = *randal; # now $merlyn is really $randal, plus # @merlyn is really @randal, etc local *merlyn = 'randal'; # SAME THING: promote 'randal' to *randal local *merlyn = \$randal; # just alias $merlyn, not @merlyn etc A local() modifies its listed variables to be "local" to the enclosing block, eval, or do FILE--and to _a_n_y _s_u_b_r_o_u_t_i_n_e _c_a_l_l_e_d _f_r_o_m _w_i_t_h_i_n _t_h_a_t _b_l_o_c_k. A local() just gives temporary values to global (meaning package) variables. It does nnnnooootttt create a local variable. This is known as dynamic scoping. Lexical scoping is done with "my", which works more like C's auto declarations. If more than one variable is given to local(), they must be placed in parentheses. All listed elements must be legal lvalues. This operator works by saving the current values of those variables in its argument list on a hidden stack Page 10 (printed 10/23/98) PPPPEEEERRRRLLLLSSSSUUUUBBBB((((1111)))) 22223333////JJJJuuuullll////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) PPPPEEEERRRRLLLLSSSSUUUUBBBB((((1111)))) and restoring them upon exiting the block, subroutine, or eval. This means that called subroutines can also reference the local variable, but not the global one. The argument list may be assigned to if desired, which allows you to initialize your local variables. (If no initializer is given for a particular variable, it is created with an undefined value.) Commonly this is used to name the parameters to a subroutine. Examples: for $i ( 0 .. 9 ) { $digits{$i} = $i; } # assume this function uses global %digits hash parse_num(); # now temporarily add to %digits hash if ($base12) { # (NOTE: not claiming this is efficient!) local %digits = (%digits, 't' => 10, 'e' => 11); parse_num(); # parse_num gets this new %digits! } # old %digits restored here Because local() is a run-time command, it gets executed every time through a loop. In releases of Perl previous to 5.0, this used more stack storage each time until the loop was exited. Perl now reclaims the space each time through, but it's still more efficient to declare your variables outside the loop. A local is simply a modifier on an lvalue expression. When you assign to a localized variable, the local doesn't change whether its list is viewed as a scalar or an array. So local($foo) = <STDIN>; local @FOO = <STDIN>; both supply a list context to the right-hand side, while local $foo = <STDIN>; supplies a scalar context. A note about local() and composite types is in order. Something like local(%foo) works by temporarily placing a brand new hash in the symbol table. The old hash is left alone, but is hidden "behind" the new one. This means the old variable is completely invisible via the symbol table (i.e. the hash entry in the *foo typeglob) for the duration of the dynamic scope within which the local() was seen. This has the effect of allowing one to Page 11 (printed 10/23/98) PPPPEEEERRRRLLLLSSSSUUUUBBBB((((1111)))) 22223333////JJJJuuuullll////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) PPPPEEEERRRRLLLLSSSSUUUUBBBB((((1111)))) temporarily occlude any magic on composite types. For instance, this will briefly alter a tied hash to some other implementation: tie %ahash, 'APackage'; [...] { local %ahash; tie %ahash, 'BPackage'; [..called code will see %ahash tied to 'BPackage'..] { local %ahash; [..%ahash is a normal (untied) hash here..] } } [..%ahash back to its initial tied self again..] As another example, a custom implementation of %ENV might look like this: { local %ENV; tie %ENV, 'MyOwnEnv'; [..do your own fancy %ENV manipulation here..] } [..normal %ENV behavior here..] It's also worth taking a moment to explain what happens when you localize a member of a composite type (i.e. an array or hash element). In this case, the element is localized _b_y _n_a_m_e. This means that when the scope of the local() ends, the saved value will be restored to the hash element whose key was named in the local(), or the array element whose index was named in the local(). If that element was deleted while the local() was in effect (e.g. by a delete() from a hash or a shift() of an array), it will spring back into existence, possibly extending an array and filling in the skipped elements with undef. For instance, if you say Page 12 (printed 10/23/98) PPPPEEEERRRRLLLLSSSSUUUUBBBB((((1111)))) 22223333////JJJJuuuullll////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) PPPPEEEERRRRLLLLSSSSUUUUBBBB((((1111)))) %hash = ( 'This' => 'is', 'a' => 'test' ); @ary = ( 0..5 ); { local($ary[5]) = 6; local($hash{'a'}) = 'drill'; while (my $e = pop(@ary)) { print "$e . . .\n"; last unless $e > 3; } if (@ary) { $hash{'only a'} = 'test'; delete $hash{'a'}; } } print join(' ', map { "$_ $hash{$_}" } sort keys %hash),".\n"; print "The array has ",scalar(@ary)," elements: ", join(', ', map { defined $_ ? $_ : 'undef' } @ary),"\n"; Perl will print 6 . . . 4 . . . 3 . . . This is a test only a test. The array has 6 elements: 0, 1, 2, undef, undef, 5 PPPPaaaassssssssiiiinnnngggg SSSSyyyymmmmbbbboooollll TTTTaaaabbbblllleeee EEEEnnnnttttrrrriiiieeeessss ((((ttttyyyyppppeeeegggglllloooobbbbssss)))) [Note: The mechanism described in this section was originally the only way to simulate pass-by-reference in older versions of Perl. While it still works fine in modern versions, the new reference mechanism is generally easier to work with. See below.] Sometimes you don't want to pass the value of an array to a subroutine but rather the name of it, so that the subroutine can modify the global copy of it rather than working with a local copy. In perl you can refer to all objects of a particular name by prefixing the name with a star: *foo. This is often known as a "typeglob", because the star on the front can be thought of as a wildcard match for all the funny prefix characters on variables and subroutines and such. When evaluated, the typeglob produces a scalar value that represents all the objects of that name, including any filehandle, format, or subroutine. When assigned to, it causes the name mentioned to refer to whatever "*" value was assigned to it. Example: Page 13 (printed 10/23/98) PPPPEEEERRRRLLLLSSSSUUUUBBBB((((1111)))) 22223333////JJJJuuuullll////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) PPPPEEEERRRRLLLLSSSSUUUUBBBB((((1111)))) sub doubleary { local(*someary) = @_; foreach $elem (@someary) { $elem *= 2; } } doubleary(*foo); doubleary(*bar); Note that scalars are already passed by reference, so you can modify scalar arguments without using this mechanism by referring explicitly to $_[0] etc. You can modify all the elements of an array by passing all the elements as scalars, but you have to use the * mechanism (or the equivalent reference mechanism) to push, pop, or change the size of an array. It will certainly be faster to pass the typeglob (or reference). Even if you don't want to modify an array, this mechanism is useful for passing multiple arrays in a single LIST, because normally the LIST mechanism will merge all the array values so that you can't extract out the individual arrays. For more on typeglobs, see the section on _T_y_p_e_g_l_o_b_s _a_n_d _F_i_l_e_h_a_n_d_l_e_s in the _p_e_r_l_d_a_t_a manpage. WWWWhhhheeeennnn ttttoooo SSSSttttiiiillllllll UUUUsssseeee _l_o_c_a_l() Despite the existence of my(), there are still three places where the local() operator still shines. In fact, in these three places, you _m_u_s_t use local instead of my. especially $_. 1. You need to give a global variable a temporary value, The global variables, like @ARGV or the punctuation variables, must be localized with local(). This block reads in /_e_t_c/_m_o_t_d, and splits it up into chunks separated by lines of equal signs, which are placed in @Fields. { local @ARGV = ("/etc/motd"); local $/ = undef; local $_ = <>; @Fields = split /^\s*=+\s*$/; } It particular, it's important to localize $_ in any routine that assigns to it. Look out for implicit assignments in while conditionals. function. 2. You need to create a local file or directory handle or a local Page 14 (printed 10/23/98) PPPPEEEERRRRLLLLSSSSUUUUBBBB((((1111)))) 22223333////JJJJuuuullll////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) PPPPEEEERRRRLLLLSSSSUUUUBBBB((((1111)))) A function that needs a filehandle of its own must use local() uses local() on complete typeglob. This can be used to create new symbol table entries: sub ioqueue { local (*READER, *WRITER); # not my! pipe (READER, WRITER); or die "pipe: $!"; return (*READER, *WRITER); } ($head, $tail) = ioqueue(); See the Symbol module for a way to create anonymous symbol table entries. Because assignment of a reference to a typeglob creates an alias, this can be used to create what is effectively a local function, or at least, a local alias. { local *grow = \&shrink; # only until this block exists grow(); # really calls shrink() move(); # if move() grow()s, it shrink()s too } grow(); # get the real grow() again See the section on _F_u_n_c_t_i_o_n _T_e_m_p_l_a_t_e_s in the _p_e_r_l_r_e_f manpage for more about manipulating functions by name in this way. hash. 3. You want to temporarily change just one element of an array or You can localize just one element of an aggregate. Usually this is done on dynamics: { local $SIG{INT} = 'IGNORE'; funct(); # uninterruptible } # interruptibility automatically restored here But it also works on lexically declared aggregates. Prior to 5.005, this operation could on occasion misbehave. PPPPaaaassssssss bbbbyyyy RRRReeeeffffeeeerrrreeeennnncccceeee If you want to pass more than one array or hash into a function--or return them from it--and have them maintain their integrity, then you're going to have to use an explicit pass-by-reference. Before you do that, you need to understand references as detailed in the _p_e_r_l_r_e_f manpage. Page 15 (printed 10/23/98) PPPPEEEERRRRLLLLSSSSUUUUBBBB((((1111)))) 22223333////JJJJuuuullll////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) PPPPEEEERRRRLLLLSSSSUUUUBBBB((((1111)))) This section may not make much sense to you otherwise. Here are a few simple examples. First, let's pass in several arrays to a function and have it pop all of then, return a new list of all their former last elements: @tailings = popmany ( \@a, \@b, \@c, \@d ); sub popmany { my $aref; my @retlist = (); foreach $aref ( @_ ) { push @retlist, pop @$aref; } return @retlist; } Here's how you might write a function that returns a list of keys occurring in all the hashes passed to it: @common = inter( \%foo, \%bar, \%joe ); sub inter { my ($k, $href, %seen); # locals foreach $href (@_) { while ( $k = each %$href ) { $seen{$k}++; } } return grep { $seen{$_} == @_ } keys %seen; } So far, we're using just the normal list return mechanism. What happens if you want to pass or return a hash? Well, if you're using only one of them, or you don't mind them concatenating, then the normal calling convention is ok, although a little expensive. Where people get into trouble is here: (@a, @b) = func(@c, @d); or (%a, %b) = func(%c, %d); That syntax simply won't work. It sets just @a or %a and clears the @b or %b. Plus the function didn't get passed into two separate arrays or hashes: it got one long list in @_, as always. If you can arrange for everyone to deal with this through references, it's cleaner code, although not so nice to look at. Here's a function that takes two array references as arguments, returning the two array elements in order of how Page 16 (printed 10/23/98) PPPPEEEERRRRLLLLSSSSUUUUBBBB((((1111)))) 22223333////JJJJuuuullll////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) PPPPEEEERRRRLLLLSSSSUUUUBBBB((((1111)))) many elements they have in them: ($aref, $bref) = func(\@c, \@d); print "@$aref has more than @$bref\n"; sub func { my ($cref, $dref) = @_; if (@$cref > @$dref) { return ($cref, $dref); } else { return ($dref, $cref); } } It turns out that you can actually do this also: (*a, *b) = func(\@c, \@d); print "@a has more than @b\n"; sub func { local (*c, *d) = @_; if (@c > @d) { return (\@c, \@d); } else { return (\@d, \@c); } } Here we're using the typeglobs to do symbol table aliasing. It's a tad subtle, though, and also won't work if you're using my() variables, because only globals (well, and local()s) are in the symbol table. If you're passing around filehandles, you could usually just use the bare typeglob, like *STDOUT, but typeglobs references would be better because they'll still work properly under use strict 'refs'. For example: splutter(\*STDOUT); sub splutter { my $fh = shift; print $fh "her um well a hmmm\n"; } $rec = get_rec(\*STDIN); sub get_rec { my $fh = shift; return scalar <$fh>; } Another way to do this is using *HANDLE{IO}, see the _p_e_r_l_r_e_f manpage for usage and caveats. If you're planning on generating new filehandles, you could Page 17 (printed 10/23/98) PPPPEEEERRRRLLLLSSSSUUUUBBBB((((1111)))) 22223333////JJJJuuuullll////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) PPPPEEEERRRRLLLLSSSSUUUUBBBB((((1111)))) do this: sub openit { my $name = shift; local *FH; return open (FH, $path) ? *FH : undef; } Although that will actually produce a small memory leak. See the bottom of the open() entry in the _p_e_r_l_f_u_n_c manpage for a somewhat cleaner way using the IO::Handle package. PPPPrrrroooottttoooottttyyyyppppeeeessss As of the 5.002 release of perl, if you declare sub mypush (\@@) then mypush() takes arguments exactly like push() does. The declaration of the function to be called must be visible at compile time. The prototype affects only the interpretation of new-style calls to the function, where new-style is defined as not using the & character. In other words, if you call it like a builtin function, then it behaves like a builtin function. If you call it like an old-fashioned subroutine, then it behaves like an old-fashioned subroutine. It naturally falls out from this rule that prototypes have no influence on subroutine references like \&foo or on indirect subroutine calls like &{$subref}. Method calls are not influenced by prototypes either, because the function to be called is indeterminate at compile time, because it depends on inheritance. Because the intent is primarily to let you define subroutines that work like builtin commands, here are the prototypes for some other functions that parse almost exactly like the corresponding builtins. Declared as Called as Page 18 (printed 10/23/98) PPPPEEEERRRRLLLLSSSSUUUUBBBB((((1111)))) 22223333////JJJJuuuullll////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) PPPPEEEERRRRLLLLSSSSUUUUBBBB((((1111)))) sub mylink ($$) mylink $old, $new sub myvec ($$$) myvec $var, $offset, 1 sub myindex ($$;$) myindex &getstring, "substr" sub mysyswrite ($$$;$) mysyswrite $buf, 0, length($buf) - $off, $off sub myreverse (@) myreverse $a, $b, $c sub myjoin ($@) myjoin ":", $a, $b, $c sub mypop (\@) mypop @array sub mysplice (\@$$@) mysplice @array, @array, 0, @pushme sub mykeys (\%) mykeys %{$hashref} sub myopen (*;$) myopen HANDLE, $name sub mypipe (**) mypipe READHANDLE, WRITEHANDLE sub mygrep (&@) mygrep { /foo/ } $a, $b, $c sub myrand ($) myrand 42 sub mytime () mytime Any backslashed prototype character represents an actual argument that absolutely must start with that character. The value passed to the subroutine (as part of @_) will be a reference to the actual argument given in the subroutine call, obtained by applying \ to that argument. Unbackslashed prototype characters have special meanings. Any unbackslashed @ or % eats all the rest of the arguments, and forces list context. An argument represented by $ forces scalar context. An & requires an anonymous subroutine, which, if passed as the first argument, does not require the "sub" keyword or a subsequent comma. A * does whatever it has to do to turn the argument into a reference to a symbol table entry. A semicolon separates mandatory arguments from optional arguments. (It is redundant before @ or %.) Note how the last three examples above are treated specially by the parser. mygrep() is parsed as a true list operator, myrand() is parsed as a true unary operator with unary precedence the same as rand(), and mytime() is truly without arguments, just like time(). That is, if you say mytime +2; you'll get mytime() + 2, not mytime(2), which is how it would be parsed without the prototype. The interesting thing about & is that you can generate new syntax with it: Page 19 (printed 10/23/98) PPPPEEEERRRRLLLLSSSSUUUUBBBB((((1111)))) 22223333////JJJJuuuullll////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) PPPPEEEERRRRLLLLSSSSUUUUBBBB((((1111)))) sub try (&@) { my($try,$catch) = @_; eval { &$try }; if ($@) { local $_ = $@; &$catch; } } sub catch (&) { $_[0] } try { die "phooey"; } catch { /phooey/ and print "unphooey\n"; }; That prints "unphooey". (Yes, there are still unresolved issues having to do with the visibility of @_. I'm ignoring that question for the moment. (But note that if we make @_ lexically scoped, those anonymous subroutines can act like closures... (Gee, is this sounding a little Lispish? (Never mind.)))) And here's a reimplementation of grep: sub mygrep (&@) { my $code = shift; my @result; foreach $_ (@_) { push(@result, $_) if &$code; } @result; } Some folks would prefer full alphanumeric prototypes. Alphanumerics have been intentionally left out of prototypes for the express purpose of someday in the future adding named, formal parameters. The current mechanism's main goal is to let module writers provide better diagnostics for module users. Larry feels the notation quite understandable to Perl programmers, and that it will not intrude greatly upon the meat of the module, nor make it harder to read. The line noise is visually encapsulated into a small pill that's easy to swallow. It's probably best to prototype new functions, not retrofit prototyping into older ones. That's because you must be especially careful about silent impositions of differing list versus scalar contexts. For example, if you decide that a function should take just one parameter, like this: Page 20 (printed 10/23/98) PPPPEEEERRRRLLLLSSSSUUUUBBBB((((1111)))) 22223333////JJJJuuuullll////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) PPPPEEEERRRRLLLLSSSSUUUUBBBB((((1111)))) sub func ($) { my $n = shift; print "you gave me $n\n"; } and someone has been calling it with an array or expression returning a list: func(@foo); func( split /:/ ); Then you've just supplied an automatic scalar() in front of their argument, which can be more than a bit surprising. The old @foo which used to hold one thing doesn't get passed in. Instead, the func() now gets passed in 1, that is, the number of elements in @foo. And the split() gets called in a scalar context and starts scribbling on your @_ parameter list. This is all very powerful, of course, and should be used only in moderation to make the world a better place. CCCCoooonnnnssssttttaaaannnntttt FFFFuuuunnnnccccttttiiiioooonnnnssss Functions with a prototype of () are potential candidates for inlining. If the result after optimization and constant folding is either a constant or a lexically-scoped scalar which has no other references, then it will be used in place of function calls made without & or do. Calls made using & or do are never inlined. (See _c_o_n_s_t_a_n_t._p_m for an easy way to declare most constants.) The following functions would all be inlined: sub pi () { 3.14159 } # Not exact, but close. sub PI () { 4 * atan2 1, 1 } # As good as it gets, # and it's inlined, too! sub ST_DEV () { 0 } sub ST_INO () { 1 } sub FLAG_FOO () { 1 << 8 } sub FLAG_BAR () { 1 << 9 } sub FLAG_MASK () { FLAG_FOO | FLAG_BAR } Page 21 (printed 10/23/98) PPPPEEEERRRRLLLLSSSSUUUUBBBB((((1111)))) 22223333////JJJJuuuullll////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) PPPPEEEERRRRLLLLSSSSUUUUBBBB((((1111)))) sub OPT_BAZ () { not (0x1B58 & FLAG_MASK) } sub BAZ_VAL () { if (OPT_BAZ) { return 23; } else { return 42; } } sub N () { int(BAZ_VAL) / 3 } BEGIN { my $prod = 1; for (1..N) { $prod *= $_ } sub N_FACTORIAL () { $prod } } If you redefine a subroutine that was eligible for inlining, you'll get a mandatory warning. (You can use this warning to tell whether or not a particular subroutine is considered constant.) The warning is considered severe enough not to be optional because previously compiled invocations of the function will still be using the old value of the function. If you need to be able to redefine the subroutine you need to ensure that it isn't inlined, either by dropping the () prototype (which changes the calling semantics, so beware) or by thwarting the inlining mechanism in some other way, such as sub not_inlined () { 23 if $]; } OOOOvvvveeeerrrrrrrriiiiddddiiiinnnngggg BBBBuuuuiiiillllttttiiiinnnn FFFFuuuunnnnccccttttiiiioooonnnnssss Many builtin functions may be overridden, though this should be tried only occasionally and for good reason. Typically this might be done by a package attempting to emulate missing builtin functionality on a non-Unix system. Overriding may be done only by importing the name from a module--ordinary predeclaration isn't good enough. However, the subs pragma (compiler directive) lets you, in effect, predeclare subs via the import syntax, and these names may then override the builtin ones: use subs 'chdir', 'chroot', 'chmod', 'chown'; chdir $somewhere; sub chdir { ... } To unambiguously refer to the builtin form, one may precede Page 22 (printed 10/23/98) PPPPEEEERRRRLLLLSSSSUUUUBBBB((((1111)))) 22223333////JJJJuuuullll////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) PPPPEEEERRRRLLLLSSSSUUUUBBBB((((1111)))) the builtin name with the special package qualifier CORE::. For example, saying CORE::open() will always refer to the builtin open(), even if the current package has imported some other subroutine called &open() from elsewhere. Library modules should not in general export builtin names like "open" or "chdir" as part of their default @EXPORT list, because these may sneak into someone else's namespace and change the semantics unexpectedly. Instead, if the module adds the name to the @EXPORT_OK list, then it's possible for a user to import the name explicitly, but not implicitly. That is, they could say use Module 'open'; and it would import the open override, but if they said use Module; they would get the default imports without the overrides. The foregoing mechanism for overriding builtins is restricted, quite deliberately, to the package that requests the import. There is a second method that is sometimes applicable when you wish to override a builtin everywhere, without regard to namespace boundaries. This is achieved by importing a sub into the special namespace CORE::GLOBAL::. Here is an example that quite brazenly replaces the glob operator with something that understands regular expressions. package REGlob; require Exporter; @ISA = 'Exporter'; @EXPORT_OK = 'glob'; sub import { my $pkg = shift; return unless @_; my $sym = shift; my $where = ($sym =~ s/^GLOBAL_// ? 'CORE::GLOBAL' : caller(0)); $pkg->export($where, $sym, @_); } sub glob { my $pat = shift; my @got; local(*D); if (opendir D, '.') { @got = grep /$pat/, readdir D; closedir D; } @got; } 1; Page 23 (printed 10/23/98) PPPPEEEERRRRLLLLSSSSUUUUBBBB((((1111)))) 22223333////JJJJuuuullll////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) PPPPEEEERRRRLLLLSSSSUUUUBBBB((((1111)))) And here's how it could be (ab)used: #use REGlob 'GLOBAL_glob'; # override glob() in ALL namespaces package Foo; use REGlob 'glob'; # override glob() in Foo:: only print for <^[a-z_]+\.pm\$>; # show all pragmatic modules Note that the initial comment shows a contrived, even dangerous example. By overriding glob globally, you would be forcing the new (and subversive) behavior for the glob operator for eeeevvvveeeerrrryyyy namespace, without the complete cognizance or cooperation of the modules that own those namespaces. Naturally, this should be done with extreme caution--if it must be done at all. The REGlob example above does not implement all the support needed to cleanly override perl's glob operator. The builtin glob has different behaviors depending on whether it appears in a scalar or list context, but our REGlob doesn't. Indeed, many perl builtins have such context sensitive behaviors, and these must be adequately supported by a properly written override. For a fully functional example of overriding glob, study the implementation of File::DosGlob in the standard library. AAAAuuuuttttoooollllooooaaaaddddiiiinnnngggg If you call a subroutine that is undefined, you would ordinarily get an immediate fatal error complaining that the subroutine doesn't exist. (Likewise for subroutines being used as methods, when the method doesn't exist in any base class of the class package.) If, however, there is an AUTOLOAD subroutine defined in the package or packages that were searched for the original subroutine, then that AUTOLOAD subroutine is called with the arguments that would have been passed to the original subroutine. The fully qualified name of the original subroutine magically appears in the $AUTOLOAD variable in the same package as the AUTOLOAD routine. The name is not passed as an ordinary argument because, er, well, just because, that's why... Most AUTOLOAD routines will load in a definition for the subroutine in question using eval, and then execute that subroutine using a special form of "goto" that erases the stack frame of the AUTOLOAD routine without a trace. (See the standard AutoLoader module, for example.) But an AUTOLOAD routine can also just emulate the routine and never define it. For example, let's pretend that a function that wasn't defined should just call system() with those arguments. All you'd do is this: Page 24 (printed 10/23/98) PPPPEEEERRRRLLLLSSSSUUUUBBBB((((1111)))) 22223333////JJJJuuuullll////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) PPPPEEEERRRRLLLLSSSSUUUUBBBB((((1111)))) sub AUTOLOAD { my $program = $AUTOLOAD; $program =~ s/.*:://; system($program, @_); } date(); who('am', 'i'); ls('-l'); In fact, if you predeclare the functions you want to call that way, you don't even need the parentheses: use subs qw(date who ls); date; who "am", "i"; ls -l; A more complete example of this is the standard Shell module, which can treat undefined subroutine calls as calls to Unix programs. Mechanisms are available for modules writers to help split the modules up into autoloadable files. See the standard AutoLoader module described in the _A_u_t_o_L_o_a_d_e_r manpage and in the _A_u_t_o_S_p_l_i_t manpage, the standard SelfLoader modules in the _S_e_l_f_L_o_a_d_e_r manpage, and the document on adding C functions to perl code in the _p_e_r_l_x_s manpage. SSSSEEEEEEEE AAAALLLLSSSSOOOO See the _p_e_r_l_r_e_f manpage for more about references and closures. See the _p_e_r_l_x_s manpage if you'd like to learn about calling C subroutines from perl. See the _p_e_r_l_m_o_d manpage to learn about bundling up your functions in separate files. Page 25 (printed 10/23/98) PPPPEEEERRRRLLLLSSSSUUUUBBBB((((1111)))) 22223333////JJJJuuuullll////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) PPPPEEEERRRRLLLLSSSSUUUUBBBB((((1111)))) Page 26 (printed 10/23/98)