On Nov 3, 2006, at 1:11 PM, Marcus Boerger wrote:
Liskov applies to static methods as soon as calls via objects are
common
which is true for PHP. Actually in PHP static methods are inherited as
any
other method (also true for a lot of other languages). Now given Liskov
rules you can as well add default parameter values as add new
parameters
with default values and even change type hints (when respecting the
rules
correctly). With C++ the first language has proven that changing
default
parameter values is a bad idea. There however mainly because they are
bound
at compile time based on the compile time types, which results in
default
values that are mostly not what you would expect. In PHP it might work
as
expected but then all programmers that come from langiages like C++ get
confused. Also it would disallow a few optimizer things later (going
the C++
way of compile time function invocatoin binding). The same holds for
new
additional parameters. In most languages that is no option because it
would
effectively be a different function. In PHP it would eventually work
but add
more confusion that it would help. The last point, changing type hints
in
derived class' methods, was discussed at PDM and declined. The main
reason
for that decision were that all languages we knew of do not support it
and
that most people even do not understand the rules which are quite the
opposite of what most people think.
Hi Marcus,
Enlightening explanation in a long and confusing thread.
To restate the last point for clarification, if the PHP's rules were
following Liskov's rules, this
php -d"error_reporting=8191" -r 'class Foo {} class FooBar extends Foo
{} class T {function f(FooBar $x){}} class S extends T {function f(Foo
$x){}}'
and possibly
php -d"error_reporting=8191" -r 'class T {function f(StdClass $x){}}
class S extends T {function f($x){}}'
would not be E_STRICT violations as they currently are. In both
examples the subclass weakens the parent classes' preconditions (type
hints). So PHP's E_STRICT rules are (intentionally) 'stricter' than
Liskov's rules.
Best Regards,
Jeff
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