Re: Indefinite type while writing a recursive descent parser

[Victor Porton <porton@narod.ru>]

Victor Porton wrote:

> I am writing a recursive descent parser. (It parses RDF relations rather
> than a text file, but that's not very important for my question, except
> for the fact that I cannot use an existing text parser package.)
> 
> While parsing I need to extract data (numbers, booleans, arrays, etc.)
> 
> My first idea was:
> 
> generic
>    type Data_Type(<>) is private;
> package Base_Predicate is
>    type Base_Predicate_Parser is tagged
>       record
>          Predicate: URI_Type;
>          Data: access Data_Type;
>       end record;
>    not overriding
>    procedure Parse (Parser: Base_Predicate_Parser; Node: Node_Type) is
>    null;
> end Base_Predicate;
> 
> I wanted my node parsers to assign to Data.all if Data/=null.
> 
> But then I realized that this won't work for an indefinite Data_Type.
> 
> So I have an Ada (2012) specific issue on possible ways to assign data
> while parsing. Please give advice.
> 
> I could use Indefinite_Holders everywhere but this would be a step into
> the direction of being like a dynamic language instead of strong typing
> system which Ada offers. I don't like to make a step in this direction.

It seems for me the right idea to:

- restrict Data_Type only to definite types as a record member where we are 
to store the data is anyway not supporting indefinite types

- whenever we need an indefinite type, replace it with an indefinite holder 
defined specifically for this type

-- 
Victor Porton - http://portonvictor.org