Re: Make a variable single-assignment?
On 21/11/2011 20:06, Jesse Phillips wrote: What you are describing is Head Const, and is not available. http://www.d-programming-language.org/const-faq.html#head-const It will not be added as it doesn't provide any guarantees about the code that is useful to the compiler. It can't be added to the existing system without complicating the type system even more, which outweighs the benefits. snip Guarantees about the code don't need to be useful to the compiler - they can be just useful to the programmer. After all, this is the main point of DbC. And it doesn't need to be a full-fledged head const. At the simplest, a single-assignment variable could just be an rvalue - something of which the address cannot be taken and so the absence of head const becomes irrelevant. That said, it isn't much complexity to allow the address to be taken of such a thing final T data; auto ptr = data; with the following rules: - if T is a value type, immutable(something)[] or immutable(something)*, then data is an immutable(T)* - otherwise, data is a const(T)*. Once upon a time there was const/final/invariant. What exactly did final do back then? Stewart.
Re: Make a variable single-assignment?
On 12/01/2011 12:08 AM, Stewart Gordon wrote: On 21/11/2011 20:06, Jesse Phillips wrote: What you are describing is Head Const, and is not available. http://www.d-programming-language.org/const-faq.html#head-const It will not be added as it doesn't provide any guarantees about the code that is useful to the compiler. It can't be added to the existing system without complicating the type system even more, which outweighs the benefits. snip Guarantees about the code don't need to be useful to the compiler - they can be just useful to the programmer. After all, this is the main point of DbC. And it doesn't need to be a full-fledged head const. At the simplest, a single-assignment variable could just be an rvalue - something of which the address cannot be taken and so the absence of head const becomes irrelevant. That said, it isn't much complexity to allow the address to be taken of such a thing final T data; auto ptr = data; with the following rules: - if T is a value type, immutable(something)[] or immutable(something)*, then data is an immutable(T)* - otherwise, data is a const(T)*. Once upon a time there was const/final/invariant. What exactly did final do back then? Stewart. Here is some documentation from that time: http://www.digitalmars.com/d/final-const-invariant.html It did indeed mean head const, like in Java. I wonder why it was removed from the design though, it gives extra expressiveness at no cost. (maybe because then people will start pushing for readonly (tail const)? :))
Re: Make a variable single-assignment?
On 11/21/11 1:17 PM, Kapps wrote:
For one reason, public fields that lack a set without having to create a
backing field, followed by a bulky property. It does sound lazy, but
when it's something you have to repeat many times, it gets annoying.
On 21/11/2011 9:43 AM, Ary Manzana wrote:
On 11/21/11 11:04 AM, Alex Rønne Petersen wrote:
Hi,
Is there any way to make a variable single-assignment, regardless of its
type? I.e.:
void foo()
{
some magical keyword? int i = 0;
i = 2; // Error: i cannot be reassigned
}
I realize const and immutable will do this, but they are transitive and
infect the type, which I do *not* want. I simply want the variable to be
single-assignment. Is it possible?
- Alex
Why do you want that?
But that's different from what you asked. In your example i is a local
variable. You want that for public fields...
In Ruby you'd do:
attr_reader :foo
Isn't some standard mixin in D for that? (if the accesor method gets
inline then the method call shouldn't affect the performance)
Re: Make a variable single-assignment?
Don't think so. You could also wrap it in a struct with disabled opAssign, but this would also change the type.
Re: Make a variable single-assignment?
On 21-11-2011 15:48, Trass3r wrote: Don't think so. You could also wrap it in a struct with disabled opAssign, but this would also change the type. Perhaps allowing 'final' on fields and locals would be a nice way to gain this effect... - Alex
Re: Make a variable single-assignment?
On 11/21/11 11:04 AM, Alex Rønne Petersen wrote:
Hi,
Is there any way to make a variable single-assignment, regardless of its
type? I.e.:
void foo()
{
some magical keyword? int i = 0;
i = 2; // Error: i cannot be reassigned
}
I realize const and immutable will do this, but they are transitive and
infect the type, which I do *not* want. I simply want the variable to be
single-assignment. Is it possible?
- Alex
Why do you want that?
Re: Make a variable single-assignment?
For one reason, public fields that lack a set without having to create a
backing field, followed by a bulky property. It does sound lazy, but
when it's something you have to repeat many times, it gets annoying.
On 21/11/2011 9:43 AM, Ary Manzana wrote:
On 11/21/11 11:04 AM, Alex Rønne Petersen wrote:
Hi,
Is there any way to make a variable single-assignment, regardless of its
type? I.e.:
void foo()
{
some magical keyword? int i = 0;
i = 2; // Error: i cannot be reassigned
}
I realize const and immutable will do this, but they are transitive and
infect the type, which I do *not* want. I simply want the variable to be
single-assignment. Is it possible?
- Alex
Why do you want that?
Re: Make a variable single-assignment?
On 11/21/2011 03:04 PM, Alex Rønne Petersen wrote:
Hi,
Is there any way to make a variable single-assignment, regardless of its
type? I.e.:
void foo()
{
some magical keyword? int i = 0;
i = 2; // Error: i cannot be reassigned
}
I realize const and immutable will do this, but they are transitive and
infect the type, which I do *not* want. I simply want the variable to be
single-assignment. Is it possible?
- Alex
How should that be possible without infecting the type?
void main(){
magical keyword int i = 0;
auto a = i;
*a = 2; // oops...
}
Re: Make a variable single-assignment?
Le 21/11/2011 15:04, Alex Rønne Petersen a écrit :
Hi,
Is there any way to make a variable single-assignment, regardless of its
type? I.e.:
void foo()
{
some magical keyword? int i = 0;
i = 2; // Error: i cannot be reassigned
}
I realize const and immutable will do this, but they are transitive and
infect the type, which I do *not* want. I simply want the variable to be
single-assignment. Is it possible?
- Alex
You can create a struct Final you could use as Final!(type) variable;
Overloading opAssign should do whatever you need.
I don't think adding to the core language what could ba achived with a
nice abstraction should be done.
Re: Make a variable single-assignment?
On 21-11-2011 17:17, Kapps wrote:
For one reason, public fields that lack a set without having to create a
backing field, followed by a bulky property. It does sound lazy, but
when it's something you have to repeat many times, it gets annoying.
On 21/11/2011 9:43 AM, Ary Manzana wrote:
On 11/21/11 11:04 AM, Alex Rønne Petersen wrote:
Hi,
Is there any way to make a variable single-assignment, regardless of its
type? I.e.:
void foo()
{
some magical keyword? int i = 0;
i = 2; // Error: i cannot be reassigned
}
I realize const and immutable will do this, but they are transitive and
infect the type, which I do *not* want. I simply want the variable to be
single-assignment. Is it possible?
- Alex
Why do you want that?
Exactly. In general, it would be useful for guaranteeing that you don't
make an accidental assignment to a local or field. Just because you
don't want it reassignable doesn't mean you don't want the *contents*
reassignable, hence why transitive immutable is not acceptable.
- Alex
Re: Make a variable single-assignment?
On 21/11/2011 14:04, Alex Rønne Petersen wrote:
Hi,
Is there any way to make a variable single-assignment, regardless of its
type? I.e.:
void foo()
{
some magical keyword? int i = 0;
i = 2; // Error: i cannot be reassigned
}
I realize const and immutable will do this, but they are transitive and
infect the type, which I do *not* want. I simply want the variable to be
single-assignment. Is it possible?
- Alex
In D1 you could use final, in D2 your choices are either const,
immutable, or as others have suggested, some sort of a wrapper. You
could also use enum if you only want to work with primitive types and
the value can be calculated at compile time.
--
Robert
http://octarineparrot.com/
Re: Make a variable single-assignment?
What you are describing is Head Const, and is not available. http://www.d-programming-language.org/const-faq.html#head-const It will not be added as it doesn't provide any guarantees about the code that is useful to the compiler. It can't be added to the existing system without complicating the type system even more, which outweighs the benefits. Tail Const, a more useful direction has been shafted for much the same reason. There is however a pull request to add support into the compiler.
Re: Make a variable single-assignment?
The only thing I can think of:
struct Once(T)
{
this(T val)
{
i = val;
}
immutable T i;
alias i this;
}
void main()
{
Once!int i = 1; // ok
i = 4; // ng
}
However it seems I've found a little hole in the system:
void foo(ref int x)
{
x = 2;
}
void main()
{
Once!int i = 1; // ok
foo(i);
assert(i == 1); // fail, changed to 2
}
Is this reported somewhere?
