How'd I store possible types for each item of a list of Variants ?

oleobal olivier.contrib at
Fri Sep 20 08:58:09 UTC 2019


I'm writing a small parser for some command line app. A given 
command-line parameter can be of one of multiple types, so I 
thought I'd store this type information somewhere and make use of 
it in a function such as this:

   Algebraic!T parse(T...)(string s)
     foreach(t; T)
         return Algebraic!T(to!t(s));
       catch (Exception e)
       { }

My issue is storing the that type information in a way that makes 
sense, the compiler can understand, and requires as little change 
as possible when a new one is added. Ideally, I'd have some kind 
of associative array that would direct command-line flags to the 

   Algebraic!(ulong, string) number = "any"; // default value
   Algebraic!T[string] params = ["-n": number, "--number": number];

   // parse the value
   params[args[i]] = 

But obviously, this is not legal code, as I can't define a 
pointer to "Algebraic!T".

No problem, I thought. Simply define a Parameter class to wrap 
around the variant, and then a templated "TypedParameter" class 
that inherits it. This way, I can define an array of Parameter 
values. Here are some definitions:

   class Parameter
     Variant value;
   class TypedParameter(T...): Parameter
     Algebraic!(T) value;

Let's try it:

   Parameter number = new TypedParameter!(ulong, string)();
   number.value = "any";
   Parameter[string] params = ["-n": number];

This compiles fine. However, parsing..

   params[args[i]].value = 

   > Error: array index [0] is outside array bounds [0 .. 0]

My understanding is D has no dynamic dispatch, so .AllowedTypes 
results in an empty list, because the base "Parameter" class uses 
a generic Variant.

I'm kind of out of ideas at this point. Would anyone have an 
elegant solution to this problem?

Thanks !

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