[fpc-devel] Re: [fpc-l] type discussion

[Michael Van Canneyt]

On Thu, 2 Jun 2005, Sebastian Kaliszewski wrote:

> Michael Van Canneyt wrote:
>> This is only 'logical' if the hypothesis
>> 
>> "productivity is inversely related to the syntax verbosity"
>> 
>> is correct.
>
> And it's not.
>
> What is correct is "productivity is directly related to the number of 
> separate language constructs developer has to put in program to acomplish the 
> task"

Inversely related, I assume you want to say :-)

> So, for example, need to put separate finally block to free memory means 
> additional few constructs. Or lack of standard libary supplied container 
> apropriate for a task requires programmer to develop his/her own or to adapt 
> something less usable.
>
> Geeintg rid of end in begin / end wont help much (as they can;t be separated, 
> they count as single construct).
>
>
> So here is some little idea which seems to me Pascalish enough to be 
> considered:
>
> how about new keyword: local
> Class variable declared local will be automatically freed upon every exit 
> from the scope (i.e. something along the lines of implicit try/finally for 
> some builtin types).

While I can see the use, there are all kinds of problems associated with it.

So instead of

Var
    S : TStringList;

ManagedVar  // or whatever. All that goes here is freed again by the compiler.
    S : TStringList;

begin
   S:=TStringList.Create;
   // code here
end;   // Compiler frees S here.

But the compiler needs to check many things:

1. Initialize the S with Nil.
2. Check that S is assigned only once during the lifetime of the
    procedure.
    This is actually a bigger restriction than you might think,
    unless you want to introduce reference counting.
3. Put a try/finally block and generate a call to S.Destroy at the end.
    It must also catch any errors that may occur when s.destroy is
    called. For classes, this danger is very real; For ansistrings it is
    not (well, very small)
4. It cannot assign S to anything, since that could mean that the
    lifetime of S could be prolonged. The alternative is again again ref. counting.

This is not so easy, and reference counting is always a mess...


Michael.