Re: object states
On Sunday, 7 October 2012 at 20:46:09 UTC, Henning Pohl wrote:
On Sunday, 7 October 2012 at 20:18:15 UTC, Ali Çehreli wrote:
Sounds good.
Thanks.
You haven't mentioned the invariant keyword, so perhaps you
are not aware of that feature?
http://dlang.org/class.html#Invariant
http://dlang.org/dbc.html
Although the docs seem to favor classes, invariant is
available for structs as well.
Ali
How would you resolve the issue I wrote in my first post using
invariants? You cannot add/remove checks from invariants while
using the object. That's what I made different. But there are
still some limits given by the language. Look at point 7 for
instance.
The problem about contract programming in general is you cannot
use it in public library code. Distinction between logic and
user input is a very important thing, we really need to improve
this. You may have noticed that I did not make use of either
assert or enforce. I didn't want to specify this.
You can create a wrapper struct that includes an invariant:
import std.stdio;
struct Image
{
int width;
void doSomething()
{
}
void modifiesWidth()
{
--width;
}
}
void func(Image img)
{
struct ImgWrapper
{
Image* img;
this(ref Image img)
{
this.img = img;
}
invariant()
{
assert(img.width == 512);
}
void opDispatch(string s)()
{
mixin(img.~s~(););
}
}
auto wrapper = ImgWrapper(img);
wrapper.doSomething();
wrapper.modifiesWidth(); // assertion failure
}
void main()
{
Image img = { 512 };
func(img);
}
Re: object states
On Monday, 8 October 2012 at 08:53:39 UTC, simendsjo wrote:
You can create a wrapper struct that includes an invariant:
import std.stdio;
struct Image
{
int width;
void doSomething()
{
}
void modifiesWidth()
{
--width;
}
}
void func(Image img)
{
struct ImgWrapper
{
Image* img;
this(ref Image img)
{
this.img = img;
}
invariant()
{
assert(img.width == 512);
}
void opDispatch(string s)()
{
mixin(img.~s~(););
}
}
auto wrapper = ImgWrapper(img);
wrapper.doSomething();
wrapper.modifiesWidth(); // assertion failure
}
void main()
{
Image img = { 512 };
func(img);
}
There is a difference between invariants and what I call states.
States clear things up when you actually need to do a runtime
check and when not. Invariants are always checked, because you
don't care about performance in debug mode. A Numberpositive is
positive already and will stay positive while you can access it.
De facto you cannot change a Numberpositive to a negative
number.
Re: object states
On 10/07/2012 11:44 AM, Henning Pohl wrote:
Imagine you want an image to keep the width of 512:
void func(Image img) {
assert(img.width == 512);
img.doSomething();
assert(img.width == 512);
while (img.somethingElse()) {
assert(img.width == 512)
someFunc(img);
assert(img.width == 512);
}
}
In theory, every call to a function can change the width of the image.
What do you think about this:
https://dl.dropbox.com/u/34762907/temp/prop1.html
I do know it can be implemented in D4 only.
Sounds good. You haven't mentioned the invariant keyword, so perhaps you
are not aware of that feature?
http://dlang.org/class.html#Invariant
http://dlang.org/dbc.html
Although the docs seem to favor classes, invariant is available for
structs as well.
Ali
--
D Programming Language Tutorial: http://ddili.org/ders/d.en/index.html
Re: object states
On Sunday, 7 October 2012 at 20:18:15 UTC, Ali Çehreli wrote: Sounds good. Thanks. You haven't mentioned the invariant keyword, so perhaps you are not aware of that feature? http://dlang.org/class.html#Invariant http://dlang.org/dbc.html Although the docs seem to favor classes, invariant is available for structs as well. Ali How would you resolve the issue I wrote in my first post using invariants? You cannot add/remove checks from invariants while using the object. That's what I made different. But there are still some limits given by the language. Look at point 7 for instance. The problem about contract programming in general is you cannot use it in public library code. Distinction between logic and user input is a very important thing, we really need to improve this. You may have noticed that I did not make use of either assert or enforce. I didn't want to specify this.
