Re: [ANN] Data Source Name parser (ODBC etc.)

From: Nick Roberts (nick.roberts_at_acm.org)
Date: 01/15/05 

Date: Sat, 15 Jan 2005 18:42:06 +0000

Georg Bauhaus <bauhaus@futureapps.de> wrote:

> some time ago when discussion APQ or Ada and databases, Brian May
> suggested URL-like strings describing database connections. A parser
> library for such strings (data source names) is now available at
> http://home.arcor.de/bauhaus/Ada/dsn.html
> ...
> I have tried to make the interface simple, any comments as to whether it
> is usable or whether it should go down the drain will affect following
> versions.

I like the idea. I think the syntax you propose is great (it needs to be
completed). I'd like the query syntax to be defined and parsed, using the
familiar field=value form and & for conjunction.

I like the good error diagnostics, but I feel the Messages interface is too
elaborate. Surely an enumerated type is not required, but just a message
string? And surely instantiating a generic is too elaborate, as well? Surely
only one derivation needs to be returned (if there is a legal derivation)? I
think you only need a simple function, e.g.:

   package Database_IO.DSN is

      type Data_Source_Descriptor is private;

      function Parse (Name: in String) return Data_Source_Descriptor;

      function Server_Name (Descriptor: in Data_Source_Descriptor)
            return String;

      function User_Name (Descriptor: in Data_Source_Descriptor)
            return String;

      function User_Password (Descriptor: in Data_Source_Descriptor)
            return String;

      ...

      Syntax_Error: exception;

      type Parsing_Diagnosis (Max: Positive) is private;

      function Diagnose (Name: in String) return Parsing_Diagnosis;

      function Error_Count (Diagnosis: in Parsing_Diagnosis)
            return Natural;

      function Error_Message (Diagnosis: in Parsing_Diagnosis;
                              Number: in Positive)
            return String;
      ...

      Use_Error: exception renames IO_Exceptions.Use_Error;

   private
      ...
   end;

The function Parse either successfully parses the given DSN (in Name) and
returns a broken down composite value (type Data_Source_Descriptor), or it
raises the exception Syntax_Error.

The function Diagnose parses the given DSN (in Name) and returns any errors
in a composite value (type Parsing_Diagnosis).

Functions allow the interrogation of the composite values. The discriminant
Max sets a maximum on the number of different errors to be stored in
diagnosis.

The idea is that typical user code will have a nice, easy declaration like
this:

      Source: constant Data_Source_Descriptor := Parse(...);

followed by interrogation of Source to open up the appropriate database, log
on, open a schema, table, etc.

For quick and dirty applications, this enough. If the parsing fails, an
exception is raised.

For proper applications, the exception can be caught and handled, something
like this:

   exception
      when Syntax_Error =>
         declare
            Errors: constant Parsing_Diagnosis(20) := Diagnose(...);
         begin
            for i in Error_Count(Errors) loop
               Put_Line( Standard_Error, Error_Message(Errors,i) );
            end loop;
         end;
         ...

The exception Use_Error is raised if anything else goes wrong.

I'm interested in participating in the development of some kind of open
Database I/O interface package. 

-- 
Nick Roberts

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