Re: Indefinite type while writing a recursive descent parser

[Victor Porton <porton@narod.ru>]

Victor Porton wrote:

> 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

However, this seems a suboptimal solution for lists of indefinite objects: 
this way of work would lead to a definite vector of indefinite holders 
rather than to indefinite vector of indefinite objects.

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