Re: [Aldor-l] [Axiom-developer] "has" and "with" (was curious algebra failure)

[Bill Page]

On 8/13/07, Gabriel Dos Reis wrote:
> On Mon, 13 Aug 2007, Bill Page wrote:
>
> [...]
>
> | > > Similarly, an expression of type PositiveInteger is coercible to
> | > > Integer bcause PositiveInteger is a subdomain of Integer.  Similarly,
> | > > a domain expression of type C1 is coercible to C2 if C2 appears
> | > > in the list of named categories extended by C1.
> | > >
> | > > The exact rules are implemented by the function coerce() defined in
> | > > compiler.boot.  They are split into three categories:
> | > >
> | > >   (1) easy coercion -- ceorceEasy
> | > >   (2) subset coercion -- coerceSubset
> | > >   (3) hard coercion -- coerceHard
> |
> | I think coercions from subdomain to domain and from subcategory to
> | category (presumably (2) above) are of a different kind than coercions
> | provided by the programmer. These are guaranteed to be correct by the
> | nature of what we mean by these subtypes. That is why we need the
> | concept of subdomain to be be "builtin" to the language instead of
> | pasted on externally via an exported coerce operation.
>
> So, what are discussing now?  To change the existing semantics?
>

No. subdomain is already part of Spad. Spad implements things this way
but Aldor does not. If I am discussing changing anything it would be
Aldor not Spad.

>
> If we believe that we should treat values and types uniformely, I'm not
> sure we should start with the principle that coercions from subdomain
> to domain and from subcategory to category are different.  If we want
> uniformity, we should have a single rule.
>

I agree, however these coercions do arise from fundamentally different
properties of domains and categories.

> As for coercions, I'm not so sure I agree that they violate the
> principle of least surprise.
>

There are many easy examples of this when you use the Axiom interpreter.

Regards,
Bill Page.