> "Bill Page" <[EMAIL PROTECTED]> writes: > ... > > > > I think mixed union constructions should be consider syntax errors. > Stephen Wilson wrote: > I personally agree as well. Unfortunately, the current Spad parser > is, well, difficult to change. I can check the uniformity of Union > branches at a post-parsing stage to at least get the assertion into > the system. Perhaps in time my new parser could be used as a full > replacement for the current one.
'> >
On 7/9/07, Waldek Hebisch wrote:
A little nitpick: if mixed union constructions are considered errors for me it is clearly a semantic error. I think it is better to catch such errors on a post-parsing stage. Why this may matter: syntax errors frequently leave parser in a confused state, making hard to detect other errors. Also for users is seem easier to have simple syntax + set of semantic constraints than to encode constraints in syntax.
On reflection, I am inclined to agree that it should not be a syntax error but perhaps for different reasons. Aldor defines a uniform concept not in Spad called a "keyword argument". Among other things keyword arguments can be used when calling a function. E.g. the definition: f:(x:Integer)->Integer allows f to be called like this: f(x==10) Syntactically a construct like "x:Integer" can be used anywhere the name "Integer" can be used. It associates a "keyword" or tag with a type but otherwise remains the specification of a type. So really in Aldor when writes Union(x:Integer, y:String) one is using the same conventions. So as Waldek implied, the fact that Aldor currently does not allow Union(Integer, String) is not really a syntactic issue but rather a semantic one. Thinking out load: Perhaps this use of ':' should be given explict operator semantics so that it could be considered as just a pretty form of type constructor, e.g. Keyword(x:Symbol,T:Type): ... with KeywordCategory ... == T add ... (not withstanding that it's usage here is recursive). Then as a type it would be easy to insist that a parameter to a constructor be of this type by requiring KeywordCategory. Does this make sense? Regards, Bill Page. _______________________________________________ Axiom-developer mailing list Axiom-developer@nongnu.org http://lists.nongnu.org/mailman/listinfo/axiom-developer
