Re: Collective response to := messa

[dsr@hector.UUCP (David S. Rosenblum)]

In article <124000022@inmet> stt@inmet writes:
|Here is another example of assignment leaving
|uninitialized components:
|    type Rec is record
|        F : Positive;
|        G : Positive := 5;
|    end record;
|
|    X : Rec;
|    Y : Rec;
|begin
|    Y := X;
|
|In this example predefined assignment, the compiler is NOT required to
|perform any constraint checks, and it is quite possible
|that Y.F is < 0 after the indicated assignment.
|It would be erroneous to use the value of Y.F, and it would
|be erroneous to use Y as a whole as an operand to "=" or "/=",
|but the assignment itself is not erroneous.  (See LRM 3.2.1:18 for
|discussion of erroneous use of uninitialized scalar subcomponents).

I don't have an LRM handy, but I believe that the compiler IS required
to implement the constraint checks.  But the constraint check performed by
the generated code may fail, if it is unable to distinguish an undefined
value from a correct value, or if undefined objects happen to be elaborated
in a valid state.  Thus, what is erroneous then is the reliance on a such a
quirk of the implementation.

Nevertheless, this example is hardly a justification for a further
weakening of Ada's strong typing model.  I don't feel any better knowing
that I can avoid writing an un-erroneous program in this case if I never
subsequently use the erroneously assigned object.  Yes, Ada allows unchecked
programming, Ada defines certain program constructions to be erroneous, Ada
defines representation clauses, etc.  These are red herrings in a discussion
of proposals to improve Ada's abstraction mechanisms.  Maybe I'm incredibly
naive, but I would like to see language improvements discussed based on the
assumption that we will adhere as best as possible to the spirit of Ada's
fundamental language philosophies.


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