[fpc-devel] XML node dump feature

[J. Gareth Moreton]

No-one said this would be an easy feature to implement.  This might be a 
step-by-step process, starting with simple function calls and building 
up to complex constant assignment. Nevertheless, at least initially, I 
would prefer to avoid the creation of a pure function that deals with a 
pointer in any way, including objects, since their values or successful 
creation (e.g. out of memory errors or the calling of "Fail") cannot be 
determined at design time.

Still, looking at your example, there's nothing to say that those record 
methods cannot be made pure, since the records already exist by the time 
they're called.  If anything, that looks like a relatively good use case 
to go on.

My initial complex use cases (after simple tests) will be the following:

*function *Factorial(N: QWord): QWord; *pure*; *inline*;
*var*
   X: Integer;
*begin*
   Result := 1;
*if *N < 2 *then*
     Exit;

*for *X := 2 *to *N *do*
     Result := Result * N;
*end*;

*function *RecursiveFactorial(N: QWord): QWord; *pure*; /{ Cannot be 
inlined on account of the recursive call, but can be made pure }/
*begin*
*if *N < 2 *then*
*begin*
     Result := 1;
     Exit;
*end*;

   Result := Result * RecursiveFactorial(N - 1);
*end*;

*function *RoughExp(X: Double): Double; *pure*;
*var*
   Count: Integer;
   WorkingValue: Double;
*begin*
/{ Calculate RoughExp(X) as e^X = 1 + X + X^2/2! + X^3/3! + ... + X^8/8! }/
   Result := Exponent;

*for *Count := 2 *to *8 *do*
*begin*
     X := X * Exponent; { Contains "X^Count" }
     Result := Result + (X / RecursiveFactorial(Count));
*end*;
end;

*function *Bad(X: QWord): QWord; *pure*;
*begin*
   Result := 0;
*while *True *do*
/{ Run forever }/;
*end*;

*const*
   Factorial6 = Factorial(6);/{ Should convert the function call into 
the immediate value 720 }/
   Factorial21 = Factorial(21); /{ Should return an error due to 
arithmetic overflow }/
   BigNumber = BadCount(0); /{ Should return an error upon realising 
it's in an infinite loop }/

   RFactorial20: Double = 1 / Factorial(20); /{ Typed constant }/
   SqrtE = RoughExp(0.5); /{ This also tests compiler efficiency by 
remembering past results of RecursiveFactorial with identical inputs }/
   ESquared = RoughExp(2); /{ ... as does this one if a non-recursive 
factorial is used, due to the previous call to RoughExp(0.5) }/

I may ask you guys for more complex examples down the line.  This is 
something I really want to get working!

Gareth aka. Kit

P.S. Just to clarify... we happy with the directive being '*pure*'?


On 25/06/2019 06:20, Sven Barth via fpc-devel wrote:
> Am 25.06.2019 um 06:20 schrieb Marģers . via fpc-devel:
>>
>> ----- Reply to message -----
>> Subject: Re: [fpc-devel] XML node dump feature
>> Date: otrd., 25 jūn., 03:16
>> From:  Ben Grasset <operator97 at gmail.com>
>> To:  FPC developers' list
>> <fpc-devel at lists.freepascal.org>
>>
>>> const
>>>    A: TVec3F = (X: 1.2; Y: 2.4; Z: 3.8);
>>>    B: TVec3F = (X: 2.1; Y: 4.2; Z: 8.3);
>>>    // You can't do the next part currently, obviously
>>>    C: TVec3F = A + B;
>>>    D: TVec3F = A - B;
>>>    E: TVec3F = A * B;
>>>    F: TVec3F = A / B;
>>>
>> Sorry to say but, this should not work even with
>> *pure* function.  Typed constants are not truly
>> constants.
> Correct. Though considering that we're talking about *compile time* 
> the compiler *could* treat consts (in {$J-} mode) as constants if pure 
> functions are involved... Though it would need a bit more work as 
> currently the compiler is "simply" piping the constant values of typed 
> constants into the output assembly.
>
> Regards,
> Sven
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> fpc-devel maillist  -  fpc-devel at lists.freepascal.org
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>
>


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