please explain the spec: alias type specialization

[Paul Thompson]

== Quote from Russell Lewis (webmaster at villagersonline.com)'s article
> I have a set of templates which take alias parameters, and I want to
> specialize them based on the type of the aliased symbols.
> So I looked at
> http://digitalmars.com/d/2.0/template.html#TemplateAliasParameter and
> saw the following grammar (but no matching example):
> TemplateAliasParameter:
> 	alias Identifier
> 	alias Identifier TemplateAliasParameterSpecialization
> 	alias Identifier TemplateAliasParameterDefault
> 	alias Identifier TemplateAliasParameterSpecialization
> TemplateAliasParameterDefault
> TemplateAliasParameterSpecialization:
> 	 : Type
> TemplateAliasParameterDefault:
> 	 = Type
> So I wrote this code, which DMD rejects:
> 	template foo(alias A : int) {}
> How is this supposed to work?  Or, is this a DMD bug?
> Russ



I was working through the grammar and found this too.

The document at:
http://www.digitalmars.com/d/1.0/template.html#TemplateAliasParameter
http://www.digitalmars.com/d/2.0/template.html#TemplateAliasParameter

says:
Template Alias Parameters
TemplateAliasParameter:
    alias Identifier TemplateAliasParameterSpecializationopt TemplateAliasParameterDefaultopt

TemplateAliasParameterSpecialization:
     : Type          <---- ???

TemplateAliasParameterDefault:
     = Type          <---- ???




What DOES work is this:
//=======================================
int defaultSymbol;
int someParticularSymbol;


//generic version for any someThing  defaulting to defaultSymbol;
class myAliasTemplate( alias someThing = defaultSymbol )
{
    //the generic (non-specialised case)
}
//specialised version for only someParticularSymbol;
class myAliasTemplate( alias someThing:  someParticularSymbol)
{
    //the specialised case)
}
//=======================================







Another approach:
//=======================================
class myAliasTemplate( alias someThing = defaultSymbol )
{
    static if( is(typeof(someThing)==int) )
    {
        //Define contents for int       (specialised)
    }
    else
    {
        //Define contents otherwise     (generic)
    }
}
//=======================================



I'm using DMD v1.036 on linux.

Paul Thompson

(p dot thompson at acfr dot usyd dot edu dot au )