[fpc-pascal] Default record const values

[Ryan Joseph]

Should’t this work? This would be a good way to set default record values but it doesn’t seem to be supported.


type
TMyRecord = record
public
a: integer;
b: string;
const
default: TMyRecord = (a: 100; b: 'foo');
end;

var
r: TMyRecord = TMyRecord.default;


Regards,
Ryan Joseph

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[Sven Barth via fpc-pascal]

Should’t this work? This would be a good way to set default record values
but it doesn’t seem to be supported.

TMyRecord is not yet completely parsed. There could be another field
located behind the "default" constant. Thus using the type of the record
aside from Pointers and method parameters and result is not allowed.

Regards,
Sven

[Ryan Joseph]

Not quite understanding how that’s not entirely parsed by the time you assign outside of the record. Even so I would expect it to copy what the const was able to capture, fully parsed or not.

Also, why does it work to do

begin
r := TMyRecord.default;

in the main block?

TMyRecord is not yet completely parsed. There could be another field located behind the "default" constant. Thus using the type of the record aside from Pointers and method parameters and result is not allowed.

Regards,
Ryan Joseph

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[Sven Barth via fpc-pascal]

Not quite understanding how that’s not entirely parsed by the time you
assign outside of the record. Even so I would expect it to copy what the
const was able to capture, fully parsed or not.

It's not entirely parsed by the time the constant is declared. That is
simply illegal code.

Also, why does it work to do
begin
r := TMyRecord.default;
in the main block?

It's not possible to use typed constants as initializers for variables.

Regards,
Sven

[Ryan Joseph]

This also fails.

type
TMyRecord = record
public
a: integer;
b: string;
end;
const
TMyRecord_Default: TMyRecord = (a: 100; b: 'foo');

var
r: TMyRecord = TMyRecord_Default;

TMyRecord is not yet completely parsed. There could be another field located behind the "default" constant. Thus using the type of the record aside from Pointers and method parameters and result is not allowed.

Regards,
Ryan Joseph

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[Martok]

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[Ben Grasset]

Should’t this work? This would be a good way to set default record values
but it doesn’t seem to be supported.

Personally I think the answer here is keep working on your new/alternate
implementation of default field functionality! It's a missing puzzle piece
for FPC generics in many ways.

[Ryan Joseph]

Personally I think the answer here is keep working on your new/alternate implementation of default field functionality! It's a missing puzzle piece for FPC generics in many ways.

Not sure how default fields in generics help here. I just thought it would be nice if FPC supported this so we can init records at compile time easier. I’d prefer default struct fields like C++ has but typed const defaults would be an improvement.

I looked at the sources and in ttypedconstbuilder.read_typed_const_data (I think) it simply doesn’t support default record values with typed consts, only the = (a:x;b:x) style syntax. The typed const in the class was clearly defined as a hidden static var but I couldn’t figure out how to assign the values. Maybe I’ll look into it later.

Regards,
Ryan Joseph

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[Ben Grasset]

Post by Ryan Joseph
Not sure how default fields in generics help here. I just thought it would
be nice if FPC supported this so we can init records at compile time
easier. I’d prefer default struct fields like C++ has but typed const
defaults would be an improvement.

I guess I just meant that the other work you did recently seems (to me) to
be in roughly the same "category" of functionality, and that it was also
proof that there's not a whole lot of distance to close as far as the
actual code required to implement it in FPC with regards to that kind of
thing.

[Sven Barth via fpc-pascal]

Post by Ryan Joseph
Should’t this work? This would be a good way to set default record values but it doesn’t seem to be supported.
type
TMyRecord = record
public
a: integer;
b: string;
const
default: TMyRecord = (a: 100; b: 'foo');
end;

The compiler now correctly rejects such declarations with a "Type is not
completely defined error".

Regards,
Sven
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[Ryan Joseph]

The compiler now correctly rejects such declarations with a "Type is not completely defined error".

But this syntax worked if you assigned it within blocks. Why does it need to be removed? Since I discovered it I was planning on using it instead of class functions with default values which require an implementation and are much longer to write.



Regards,
Ryan Joseph

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[Ryan Joseph]

Post by Ryan Joseph
But this syntax worked if you assigned it within blocks. Why does it need to be removed? Since I discovered it I was planning on using it instead of class functions with default values which require an implementation and are much longer to write.

Here’s an example of what I was doing before. A constant is so much better and doesn’t require the implementation and we still get the same . syntax, i.e., TPoint.Up.

Instead of removing it maybe give the error unless it’s the last field of the record and in which case can assumed to be fully defined.

type
TPoint = record
x: TFloat;
y: TFloat;
class function Up (_x: TFloat = 0; _y: TFloat = -1): TPoint; static; inline;

Regards,
Ryan Joseph

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[Sven Barth via fpc-pascal]

Post by Ryan Joseph

Post by Ryan Joseph
But this syntax worked if you assigned it within blocks. Why does it

need to be removed? Since I discovered it I was planning on using it
instead of class functions with default values which require an
implementation and are much longer to write.
Here’s an example of what I was doing before. A constant is so much better
and doesn’t require the implementation and we still get the same . syntax,
i.e., TPoint.Up.
Instead of removing it maybe give the error unless it’s the last field of
the record and in which case can assumed to be fully defined.

No, that is too random for a language.
This change is not up for discussion as it could lead to incorrect code.

Regards,
Sven

[John Doe]

On Mon, Nov 12, 2018 at 1:51 AM Sven Barth via fpc-pascal <

Post by Sven Barth via fpc-pascal
The compiler now correctly rejects such declarations with a "Type is
not completely defined error".

Making this not work is a ridiculous removal of a feature for no logical
reason that serves no benefit to anyone whatsoever. It just makes records
less useful, and that's it.

I've already encountered several large libraries that are pretty massively
broken by this pointless change. You're very visibly just making up
arbitrary "rules" out of nowhere that certainly aren't actually defined
anywhere and implementing (or de-implementing) whatever strikes your fancy
on a given day.

Also:

On Sat, Nov 10, 2018 at 4:56 AM Sven Barth via fpc-pascal <

Post by Sven Barth via fpc-pascal
It's not possible to use typed constants as initializers for variables.

What are you talking about? Of course that's possible. People do it
literally all the time.

[Sven Barth via fpc-pascal]

Post by John Doe
On Mon, Nov 12, 2018 at 1:51 AM Sven Barth via fpc-pascal
The compiler now correctly rejects such declarations with a "Type
is not completely defined error".
Making this not work is a ridiculous removal of a feature for no
logical reason that serves no benefit to anyone whatsoever. It just
makes records less useful, and that's it.

Just to avoid any confusion: I am talking about a typed constant
declared inside a record's declaration of the same type as the record.
Constants outside a record, even inside other records work as before.
Also there can't be that many libraries affected, because this only ever
worked in trunk. In 3.0.x the compiler simply crashed when encountering
such a constant.

Post by John Doe
I've already encountered several large libraries that are pretty
massively broken by this pointless change. You're very visibly just
making up arbitrary "rules" out of nowhere that certainly aren't
actually defined anywhere and implementing (or de-implementing)
whatever strikes your fancy on a given day.
On Sat, Nov 10, 2018 at 4:56 AM Sven Barth via fpc-pascal
It's not possible to use typed constants as initializers for variables.
What are you talking about? Of course that's possible. People do it
literally all the time.

=== code begin ===

type
  TTest = record
  public
    a: LongInt;
    b: LongInt;
  public //const
    //Default: array of TTest;// = (a: 42; b: 21);
  end;

const
  Test: TTest = (a: 42; b: 21);

var
  TestVar: TTest = Test; // <<<< this fails, because typed constants
can't be used as initializers (would also be the case if Test and
TestVar had any other type; only untyped constants are supported here)

=== code end ===

Regards,
Sven

[Benjamin Rosseaux]

program Test123;
{$ifdef fpc}
{$mode delphi}
{$endif}

type
TTest = record
public
a: LongInt;
b: LongInt;
end;

TTestHelper = record helper for TTest
public
const Default: TTest = (a: 1; b: 2);
end;

var
Test: TTest;
begin
Test := TTest.Default;
end.

Should’t this work? This would be a good way to set default record values
but it doesn’t seem to be supported.
type
TMyRecord = record
public
a: integer;
b: string;
const
default: TMyRecord = (a: 100; b: 'foo');
end;
var
r: TMyRecord = TMyRecord.default;
Regards,
Ryan Joseph
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[Sven Barth via fpc-pascal]

Post by Benjamin Rosseaux
program Test123;
{$ifdef fpc}
  {$mode delphi}
{$endif}
type
  TTest = record
  public
    a: LongInt;
    b: LongInt;
  end;
  TTestHelper = record helper for TTest
  public
    const Default: TTest = (a: 1; b: 2);
  end;
var
  Test: TTest;
begin
  Test := TTest.Default;
end.

That is indeed a good idea and with the extension to allow multiple
helpers there wouldn't even be a negative impact... 🀔

Regards,
Sven

[Benjamin Rosseaux]

I'm using this solution myself for the vector and matrix data types at

https://github.com/BeRo1985/pasvulkan/blob/master/src/PasVulkan.Math.pas
https://github.com/BeRo1985/pasvulkan/blob/master/src/PasVulkan.Math.TpvVector2.Swizzle.Definitions.inc
https://github.com/BeRo1985/pasvulkan/blob/master/src/PasVulkan.Math.TpvVector2.Swizzle.Implementations.inc
https://github.com/BeRo1985/pasvulkan/blob/master/src/PasVulkan.Math.TpvVector2Helper.Swizzle.Definitions.inc
https://github.com/BeRo1985/pasvulkan/blob/master/src/PasVulkan.Math.TpvVector2Helper.Swizzle.Implementations.inc
https://github.com/BeRo1985/pasvulkan/blob/master/src/PasVulkan.Math.TpvVector3.Swizzle.Definitions.inc
https://github.com/BeRo1985/pasvulkan/blob/master/src/PasVulkan.Math.TpvVector3.Swizzle.Implementations.inc
https://github.com/BeRo1985/pasvulkan/blob/master/src/PasVulkan.Math.TpvVector3Helper.Swizzle.Definitions.inc
https://github.com/BeRo1985/pasvulkan/blob/master/src/PasVulkan.Math.TpvVector3Helper.Swizzle.Implementations.inc
https://github.com/BeRo1985/pasvulkan/blob/master/src/PasVulkan.Math.TpvVector4.Swizzle.Definitions.inc
https://github.com/BeRo1985/pasvulkan/blob/master/src/PasVulkan.Math.TpvVector4.Swizzle.Implementations.inc
https://github.com/BeRo1985/pasvulkan/blob/master/src/PasVulkan.Math.TpvVector4Helper.Swizzle.Definitions.inc
https://github.com/BeRo1985/pasvulkan/blob/master/src/PasVulkan.Math.TpvVector4Helper.Swizzle.Implementations.inc

without any bigger issues, except that the Delphi IDE has some runtime
CodeInsight record lookup issues from time to time.


On Wed, Nov 28, 2018 at 8:47 PM Sven Barth via fpc-pascal <

Post by Benjamin Rosseaux
program Test123;
{$ifdef fpc}
{$mode delphi}
{$endif}
type
TTest = record
public
a: LongInt;
b: LongInt;
end;
TTestHelper = record helper for TTest
public
const Default: TTest = (a: 1; b: 2);
end;
var
Test: TTest;
begin
Test := TTest.Default;
end.
That is indeed a good idea and with the extension to allow multiple
helpers there wouldn't even be a negative impact... 🀔
Regards,
Sven
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[Ryan Joseph]

Post by Benjamin Rosseaux
TTestHelper = record helper for TTest
public
const Default: TTest = (a: 1; b: 2);
end;

I didn’t even know you could add consts to record helpers. That’s good to know, thanks.

Multi scoped helpers will make this a safe solution which you don’t have to worry about being overwritten in other units. Hopefully I can submit a patch for the trunk soon.

Regards,
Ryan Joseph

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