[Aldor-l] exports and constants

[Bill Page]

On July 24, 2006 5:10 PM Gaby wrote:

> | > ...
> | > Bill Page wrote:
> | ... 
> | Could you define for me what you mean by "dynamic type"?
> | I think the usual definition involves type checking during
> | run time.
> 
> Not really.  If you take the C language for example, there is the
> notion of "static type" and "dynamic type" (sometimes called
> "effective type"), yet C does not require type checking during
> runtime.  
> 
> When you have the notion of variable declaration and the possibility
> of "cast", i.e. in the sense of change of *type of expression* used
> to access data, you naturally get into distinguishing "static type"
> from "dynamic type".  You don't necessarily need to have runtime
> type checking involved.

Oh, the water is so muddy! Let's reserve talking about Aldor's
'pretend' for another time, but recall that 'rep' and 'pre' are
intended as a "type-safe" implementation of this idea.

> 
> "static type" concerns with the properties of the program that
> can be derived only from "static" analysis without taking into
> account any runtime semantics.
> 
> "dynamic type" concernes the properties of the program that can
> be derived by taking into account runtime semantics.

Yes, this definition seems fine to me.

Do you agree that Aldor uses only "static type"?

> 
> | Since Aldor is statically typed how can we define this in a
> | useful way in Aldor?
> | 
> | Should we say something like this?
> | 
> |  The "dynamic type" of Y is
> | 
> |    Join( x for x in Category where Y has x)
> 
> that is close -- and certainly matches practice in object oriented
> languages and non-object orinted ones like C.
> 
> | But then dynamic type would be just type satisfaction as I
> | said earlier, no?
> 
> no, "type satisfaction" is a generic term that applies to both
> the static semantics and the dynamic semantics.

I meant "type satisfaction" as it is used in the Aldor Users Guide
in the definition of the "has expression".

> ... 
> I suspect the trouble here is that in some context "has" is
> evaluated statically, and in other context it is evalued
> dynamically.  So you fail to appreciate the distinction.

The Aldor Users Guide says that "has" expressions are evaluated
at run time.

Regards,
Bill Page.