[ruby-talk:03574] Re: Static Typing( Was: Waffling between Python and Ruby)

Albert Wagner <[e-mail]> writes:

> Exactly what does DBC mean in the context of this thread?

I'm not sure if this is a straightforward question or a profound one
;-)

Assuming it's straightforward:

DBC is design by Contract, Meyer's concept tht you can embed within
code the specification of a class by giving an overall invariant, and
pre- and post- conditions for each of the methods. The invariant
defines the class's overall state, the pre-conditions constrain the
context in which methods may be called, and the postconditions verify
that the methods do what they are supposed to. DbC is primarily a
design technique, but in some languages the contracts are checked as
the programs run, so you can enjoy warm fuzzy feelings that your code
is behaving. DbC is more than just assertion checking, as it imposes
constraints on the behavior of classes derived from classes with
contracts. 

I believe that the discussion hinges around whether static typing is a
requirement/benefit for specifying a pre-condition, or whether static
typing is orthogonal to DbC. My view is that in statically typed
languages, the types are a part of the precondition simply because the
types are a part of everything. The preconditions are orthogonal to
the type system, as the types would still be there even if the
preconditions were taken away. Types neither strengthen nor weaken the 
precondition per se.

In an untyped language, the preconditions can choose to interrogate
the input parameters to determine if they supply the required
interface/protocol, but in most cases that seems somewhat over the
top, as the method will find out soon enough anyway. So again, DbC is
orthogonal to the type system.

But, these are just late-night ramblings, and I'm sure the Eiffel
folks will patiently explain the error of my ways... ;-)

DbC is discussed in Object-Oriented Software Construction II. There's
also a section on it in The Pragmatic Programmer (heh, I'm allowed the 
occasional plug ;-)


Regards


Dave