by pdcawley (Hermit)
 on Apr 19, 2002 at 06:58 UTC ( #160449=note: print w/replies, xml ) Need Help??

Strong typing can be useful at runtime too. For instance, here's a semi strong typing thing implemented with objects.
```package Quantity;

...
;

sub new {
my \$proto = shift;
my \$quantity = shift;
my \$unit = shift;

bless {quantity => \$quantity, unit => \$unit},
ref(\$proto)||\$proto;
}

my \$self = shift;
my \$target = shift;

croak "Oi! You can't add \$\$self{unit}s to \$\$target{unit}s!"
unless \$\$self{unit} eq \$\$target{unit};
\$self->new(\$self->{quantity + \$target->{quantity},
\$self->{unit});
}
Ooh looky, useful strong typing:
```  \$a = Quantity->new(10, 'apple');
\$b = Quantity->new(20, 'orange');

...

\$c = \$a + \$b; # dies "Oi! you can't add apples to oranges!"
Note that the above example can be (and has been) useful extended to do dimensional analysis (is this a valid calculation), derived units (metres / seconds -> m/sec). Further possible extensions include upcasting (10 oranges + 20 apples = 30 fruit) and deferred calculations (10 EUR + 10 USD = new Sum(10 EUR, 10 USD), we'll worry about completing the calculation when we know what units we want the answer in, however 10 EUR * 10 USD = die "Meaningless calculation, don't be so bloody silly!")

Strong typing does have it's uses. And not just in the 'making the compiler go quickly' sense. Replacing your dimensionless numbers with strongly typed quantities can be a really powerful tool for helping to track down calculation errors (no more pesky mile/kilometre confusion whilst doing calculations in orbital mechanics mister rocket scientist...). And that's even before you take into account the possible benefits of deferred calculation that can be easily hung on the side of such a system.

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