Re: Ada Interfaces and the Liskov Substitution Principle

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On Wed, 23 May 2007 21:47:32 +0200, Stefan Lucks wrote:

> to me, it seems as if Ada 2005 is bluntly violating the Liskov 
> Substitution Prinicple.

Huh, as well as Ada 83 did. There is no need in much code, just write

   subtype Non_LSP_Subtype is String (1..80);

or

   subtype Positive is Integer range 1..Integer'Last;

or, for that matter  "constant", "in", "out", "not null", etc. All these
are examples of non-LSP subtypes.

> My understanding of the Liskov substitution principle, see
>    http://en.wikipedia.org/wiki/Liskov_substitution_principle
> is that as Partens.Parent implicitely (by not being limited) provides 
> certain primitve operations, such as ":=" and "=", and Stepchild.Object 
> takes away these primitive operations,

It is illegal, but if it were legal, then yes, disallowing operations
breaks LSP. However, see above, mere passing a variable as "in" does it as
well in the sense that "in T" is not an LSP-subtype of T.

> What do you guys think about this?

LPS is totally irrelevant as long as substitutability violation can be
detected at compile time. This is why "constant" does not worry anybody. A
method disallowing is perfectly OK, if you cannot call it.

LSP violation becomes a problem when substitutability is indeterminable
until run-time. In may cases we still choose to live with that. Constrained
Ada subtypes is an example of. Another is multi-methods Foo (X, Y : T),
when called on different children of T. In such cases Ada adds
Constraint_Error to the interface of each subprogram and things become
"substitutable" again.

LSP violation is catastrophic when undetected. I think that renaming array
objects with changing bounds falls under this category. (Apart from
"unchecked" stuff)

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de