On Tue, 13 Jul 2004, Markus Hofmann wrote:
How will P interpreter know what parameters are available? Which
parameters are optional? What are their types?
I consider this out-of-scope for the current work. The author of the
rules needs to know about the parameters and their types when writing
the rules.
For example, when I'm writing rules like
if (condition)
invoke Service_A(p1, p2, p3)
then I assume to know about Service_A and its parameters. How I learn
about this is out-of-scope (could be either human interaction or
automated).
I am not sure you can get away with this without making a major
sacrifice. I agree that how _you_ learned about the parameters is out
of scope. However, P interpreter has to check your rules and translate
them into specific OCP messages. To do that, it needs to know what
parameters Service_A has (by name or position), which of them are
optional, and what their types are.
You can say that the programmer will supply parameter names and/or use
much more dangerous positional notation. While technically possible,
it would make it impossible to check your rule set for correctness
without executing it. For example, if P interpreter does not know that
Service_A only has two parameters, then P interpreter cannot tell you
that the above code snippet with Service_A(p1, p2, p3) is buggy. Only
the service can, at runtime.
Same for parameter types. You can get away with trusting the
programmer to supply the right types, but in reality that means that
a P interpreter will not be able to detect a type mismatch.
invoke Service_A(p1, p2, p3);
or
invoke Service_A(p2, p1, p3);
will all look the same to the interpreter (but not to the service!).
We know how poor humans are at writing bug-free code. I think that
being able to validate much of P code off-line is an essential rule
language feature that we should strive to preserve. Unfortunately,
that seems to require informing the interpreter of service interface,
independent of P code itself.
Can we have a safe/validate-able service invocation without the
knowledge of service interfaces (service "feature" definition in terms
of OCP)?
Thanks,
Alex.
P.S. If P programs are "rules", what should we call P programmers?
Rulers? :-)