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Re^6: Creating dispatch table with variable in function name

by Mr. Muskrat (Canon)
on Nov 22, 2017 at 18:55 UTC ( #1204056=note: print w/replies, xml ) Need Help??


in reply to Re^5: Creating dispatch table with variable in function name
in thread Creating dispatch table with variable in function name

What worries me is that, while strict is perfectly happy with it, the code below has a run-time error that may not be triggered for hours/days/weeks/months/...

Okay... So does the equivalent static dispatch table.

use strict; use warnings; ;; sub hiya { print 'hi from ', (caller(1))[3]; } ;; sub _x_foo { hiya; }; sub _x_bar { hiya; }; ;; my %disp = ( foo => \&_x_foo, bar => \&_x_bar, zot => \&_x_zot, ); dd \%disp; ;; $disp{foo}->(); $disp{zot}->(); $disp{bar}->();

You have to write code for each piece in order for the whole thing to work reliably, but you may not discover that it doesn't until a very inopportune moment. If everything could be thoroughly tested prior to deployment, the queasy feeling in my stomach might be eased a bit, but even so...

This is true for all Perl code though. There's a gajillion ways to write a script that "compiles" with strictures enabled but fails at runtime. This is why we write tests (unit, functional, etc).

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Re^7: Creating dispatch table with variable in function name
by AnomalousMonk (Bishop) on Nov 23, 2017 at 03:52 UTC
    So does the equivalent static dispatch table.

    Hmm... I hadn't tested that. Ok, how about:

    c:\@Work\Perl\monks>perl -MData::Dump -le "use warnings; use strict; ;; sub _x_foo { hiya(@_); } sub _x_bar { hiya(@_); } ;; my %disp = map { my $agent = qq{_x_$_}; $_ => __PACKAGE__->UNIVERSAL::can($agent) || die qq{dispatch via unknown agent '$agent'}; } qw(foo bar BAM) ; ::dd \%disp; ;; sub hiya { print 'hi from ', (caller(1))[3], qq{ (@_)}; } ;; $disp{foo}(); $disp{POW}(qw(x y z)); $disp{bar}(99, 88); " dispatch via unknown agent '_x_BAM' at -e line 1.
    This throws both compile- and run-time exceptions, although I don't like the obscure wording of the run-time error message.

    How about putting the whole thing into a lexically scoped package? This allows full control of the composition of the run-time error message, and makes the dispatch hash entirely private. It's also all ready to be moved off to its own module. I can't say I'm fond the  Dispatch::me('foo') invocation syntax, but putting this in its own module would give you some more syntax choices.

    c:\@Work\Perl\monks>perl -MData::Dump -le "use warnings; use strict; ;; { package Dispatch; ;; sub _x_foo { hiya(@_); } sub _x_bar { hiya(@_); } ;; my %disp = map { my $agent = qq{_x_$_}; $_ => __PACKAGE__->UNIVERSAL::can($agent) || die qq{dispatch via unknown agent '$agent'}; } qw(foo bar BAM) ; ::dd \%disp; ;; sub me { my $tag = shift; ;; die qq{unknown dispatcher '$tag' used} unless exists $disp{$tag}; ;; return $disp{$tag}(@_); } ;; sub hiya { print 'hi from ', (caller(1))[3], qq{ (@_)}; } } ;; Dispatch::me('foo'); Dispatch::me('POW', qw(x y z)); Dispatch::me('bar', 99, 88); " dispatch via unknown agent '_x_BAM' at -e line 1.


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