Re: Reading about D: few questions
On 25.12.2011 11:28, Denis Shelomovskij wrote:
OK. As I wrote: Yes, this allocation sometimes can be optimized out but
not always.. Consider this:
---
void main()
{
int[] a = new int[5];
void f(int[] b)
{
// Here we assume that b is unchanged a.
// As these array differ we need a copy.
assert(b[0] == 0);
assert(a[0] == 1);
}
f(a[]++); // Note: compilation error now
}
---
Why not to rewrite `f(a[]++);` as `f(a); ++a[];`? Because postincrement
is expected to increment its argument when it is executed. It just
returns an unchanged copy. Analogous D code with integers illustrates this:
---
void main()
{
int a;
void f(int b)
{
assert(b == 0);
assert(a == 1);
}
f(a++);
}
---
I wasn't familiar with this postincrement behavior. I would expect both
a and b to be 0 inside f().
Anyway, what do you suggest? To leave it not compilable?
My original point was to just allow:
a[]++;
More complicated scenarios like the one above is beyond my knowledge, as
I'm only learning the language, but I think they have to be addressed as
well.
Re: Reading about D: few questions
25.12.2011 0:48, Mr. Anonymous пишет:
Actually, when I think of it:
int a_orig = a++;
int[] arr_orig = arr[]++;
Should be read as:
int a_orig = a;
++a;
int[] arr_orig = arr[];
++arr[];
(If I'm not mistaken, it was written in the TDPL book)
Which means no copy of arr is made, and both arrays (which reference to
the same block) are affected.
OK. As I wrote: Yes, this allocation sometimes can be optimized out but
not always.. Consider this:
---
void main()
{
int[] a = new int[5];
void f(int[] b)
{
// Here we assume that b is unchanged a.
// As these array differ we need a copy.
assert(b[0] == 0);
assert(a[0] == 1);
}
f(a[]++); // Note: compilation error now
}
---
Why not to rewrite `f(a[]++);` as `f(a); ++a[];`? Because postincrement
is expected to increment its argument when it is executed. It just
returns an unchanged copy. Analogous D code with integers illustrates this:
---
void main()
{
int a;
void f(int b)
{
assert(b == 0);
assert(a == 1);
}
f(a++);
}
---
Re: Reading about D: few questions
On 12/24/2011 02:02 AM, Jonathan M Davis wrote: The core problem for a number of these situations is how types are handled with regards to expressions. In an expression such as char[] arr = s ~ '.'; the type of the value being assigned is determined _before_ the assignment is done. So, even though in theory the compiler could make it work, it doesn't, because by the time it's looking at the type being assigned to, it's too late. There would need to be a fundamental change in how the language functions in order to fix issues like this. Examples of resolved issues like this: int[] foo()pure; immutable(int)[] x = foo; pure can do it when it can not because it's able to look at what the return type is and changing the result of the expression accordingly but because it has guarantees which make it so that it knows that the return value could be converted to any level of constness and still be valid. The types used in the expressions internally are generally irrelevant. So, while I completely agree that it would be an improvement if the compiler did a better job with implicit conversion when it could theoretically be done, I'm not sure how much of that we're actually going to end up seeing simply because of how the language and type system works in terms of the order of evaluation. - Jonathan M Davis I don't think this is very hard to get working.
Re: Reading about D: few questions
23.12.2011 22:51, bearophile пишет: ++a[] works, but a[]++ doesn't. Already known compiler bug. Is it a joke? Array expression in D are for performance reasons to generate x2-x100 faster code without any compiler optimisations. Link to one of these epic comments (even x100 more epic because of '%' use instead of 'x###'): https://github.com/D-Programming-Language/druntime/blob/master/src/rt/arraybyte.d#L1127 But `a[]++` should store a copy of `a`, increment elements and return stored copy. It is hidden GC allocation. We already have a silent allocation in closures, but here a _really large_ peace of data can be allocated. Yes, this allocation sometimes can be optimized out but not always. IMHO, D should not have `a[]++` operator.
Re: Reading about D: few questions
On 24.12.2011 19:01, Denis Shelomovskij wrote: 23.12.2011 22:51, bearophile пишет: ++a[] works, but a[]++ doesn't. Already known compiler bug. Is it a joke? Array expression in D are for performance reasons to generate x2-x100 faster code without any compiler optimisations. Link to one of these epic comments (even x100 more epic because of '%' use instead of 'x###'): https://github.com/D-Programming-Language/druntime/blob/master/src/rt/arraybyte.d#L1127 But `a[]++` should store a copy of `a`, increment elements and return stored copy. It is hidden GC allocation. We already have a silent allocation in closures, but here a _really large_ peace of data can be allocated. Yes, this allocation sometimes can be optimized out but not always. IMHO, D should not have `a[]++` operator. Why should it store a copy? o_O I also don't see any allocations in the code on the URL above.
Re: Reading about D: few questions
Denis Shelomovskij: IMHO, D should not have `a[]++` operator. I see, and sorry. (Those percentage comments are relative to tests done on large arrays, so they are silly.) Bye, bearophile
Re: Reading about D: few questions
2011/12/24 Mr. Anonymous mailnew4s...@gmail.com: On 24.12.2011 19:01, Denis Shelomovskij wrote: 23.12.2011 22:51, bearophile пишет: ++a[] works, but a[]++ doesn't. Already known compiler bug. Is it a joke? Array expression in D are for performance reasons to generate x2-x100 faster code without any compiler optimisations. Link to one of these epic comments (even x100 more epic because of '%' use instead of 'x###'): https://github.com/D-Programming-Language/druntime/blob/master/src/rt/arraybyte.d#L1127 But `a[]++` should store a copy of `a`, increment elements and return stored copy. It is hidden GC allocation. We already have a silent allocation in closures, but here a _really large_ peace of data can be allocated. Yes, this allocation sometimes can be optimized out but not always. IMHO, D should not have `a[]++` operator. Why should it store a copy? o_O I also don't see any allocations in the code on the URL above. int a_orig = a++; int[] arr_orig = arr[]++; If ++ is going to be applied to an array, it needs to have the same meaning as it does elsewhere. After this operation, arr_orig and arr must refer to different arrays for that to be true.
Re: Reading about D: few questions
On Sat, Dec 24, 2011 at 9:50 AM, Timon Gehr timon.g...@gmx.ch wrote:
On 12/24/2011 06:18 PM, Andrew Wiley wrote:
2011/12/24 Mr. Anonymousmailnew4s...@gmail.com:
On 24.12.2011 19:01, Denis Shelomovskij wrote:
23.12.2011 22:51, bearophile пишет:
++a[] works, but a[]++ doesn't.
Already known compiler bug.
Is it a joke? Array expression in D are for performance reasons to
generate x2-x100 faster code without any compiler optimisations. Link to
one of these epic comments (even x100 more epic because of '%' use
instead of 'x###'):
https://github.com/D-Programming-Language/druntime/blob/master/src/rt/arraybyte.d#L1127
But `a[]++` should store a copy of `a`, increment elements and return
stored copy. It is hidden GC allocation. We already have a silent
allocation in closures, but here a _really large_ peace of data can be
allocated. Yes, this allocation sometimes can be optimized out but not
always.
IMHO, D should not have `a[]++` operator.
Why should it store a copy? o_O
I also don't see any allocations in the code on the URL above.
int a_orig = a++;
int[] arr_orig = arr[]++;
If ++ is going to be applied to an array, it needs to have the same
meaning as it does elsewhere. After this operation, arr_orig and arr
must refer to different arrays for that to be true.
Not necessarily.
class D{
int payload;
D opUnary(string op:++)(){payload++; return this;}
}
void main() {
D d = new D;
assert(d.payload == 0);
assert(d++.payload == 1);
}
That doesn't match integer semantics:
int a = 0;
assert(a++ == 0);
assert(a == 1);
Re: Reading about D: few questions
On 12/24/2011 07:00 PM, Andrew Wiley wrote:
On Sat, Dec 24, 2011 at 9:50 AM, Timon Gehrtimon.g...@gmx.ch wrote:
On 12/24/2011 06:18 PM, Andrew Wiley wrote:
2011/12/24 Mr. Anonymousmailnew4s...@gmail.com:
On 24.12.2011 19:01, Denis Shelomovskij wrote:
23.12.2011 22:51, bearophile пишет:
++a[] works, but a[]++ doesn't.
Already known compiler bug.
Is it a joke? Array expression in D are for performance reasons to
generate x2-x100 faster code without any compiler optimisations. Link to
one of these epic comments (even x100 more epic because of '%' use
instead of 'x###'):
https://github.com/D-Programming-Language/druntime/blob/master/src/rt/arraybyte.d#L1127
But `a[]++` should store a copy of `a`, increment elements and return
stored copy. It is hidden GC allocation. We already have a silent
allocation in closures, but here a _really large_ peace of data can be
allocated. Yes, this allocation sometimes can be optimized out but not
always.
IMHO, D should not have `a[]++` operator.
Why should it store a copy? o_O
I also don't see any allocations in the code on the URL above.
int a_orig = a++;
int[] arr_orig = arr[]++;
If ++ is going to be applied to an array, it needs to have the same
meaning as it does elsewhere. After this operation, arr_orig and arr
must refer to different arrays for that to be true.
Not necessarily.
class D{
int payload;
D opUnary(string op:++)(){payload++; return this;}
}
void main() {
D d = new D;
assert(d.payload == 0);
assert(d++.payload == 1);
}
That doesn't match integer semantics:
int a = 0;
assert(a++ == 0);
assert(a == 1);
Yes, that was my point.
Re: Reading about D: few questions
On Sat, Dec 24, 2011 at 1:08 PM, Timon Gehr timon.g...@gmx.ch wrote:
On 12/24/2011 07:00 PM, Andrew Wiley wrote:
On Sat, Dec 24, 2011 at 9:50 AM, Timon Gehrtimon.g...@gmx.ch wrote:
On 12/24/2011 06:18 PM, Andrew Wiley wrote:
2011/12/24 Mr. Anonymousmailnew4s...@gmail.com:
On 24.12.2011 19:01, Denis Shelomovskij wrote:
23.12.2011 22:51, bearophile пишет:
++a[] works, but a[]++ doesn't.
Already known compiler bug.
Is it a joke? Array expression in D are for performance reasons to
generate x2-x100 faster code without any compiler optimisations. Link
to
one of these epic comments (even x100 more epic because of '%' use
instead of 'x###'):
https://github.com/D-Programming-Language/druntime/blob/master/src/rt/arraybyte.d#L1127
But `a[]++` should store a copy of `a`, increment elements and return
stored copy. It is hidden GC allocation. We already have a silent
allocation in closures, but here a _really large_ peace of data can be
allocated. Yes, this allocation sometimes can be optimized out but not
always.
IMHO, D should not have `a[]++` operator.
Why should it store a copy? o_O
I also don't see any allocations in the code on the URL above.
int a_orig = a++;
int[] arr_orig = arr[]++;
If ++ is going to be applied to an array, it needs to have the same
meaning as it does elsewhere. After this operation, arr_orig and arr
must refer to different arrays for that to be true.
Not necessarily.
class D{
int payload;
D opUnary(string op:++)(){payload++; return this;}
}
void main() {
D d = new D;
assert(d.payload == 0);
assert(d++.payload == 1);
}
That doesn't match integer semantics:
int a = 0;
assert(a++ == 0);
assert(a == 1);
Yes, that was my point.
Then I'm not understanding what you're trying to prove.
I'm saying that if we implement a postfix ++ operator for arrays,
keeping the language consistent would require it to make a copy if the
user stores a copy of the original array. I guess it could be argued
that since arrays have hybrid value/reference semantics, no copy
should be made and the original should change.
Actually, looking at it from that angle, a[]++ is fundamentally
ambiguous because it could have value semantics or reference
semantics, so I would argue that we shouldn't have it for that reason.
'++a' and 'a += 1' do not have such ambiguities.
Re: Reading about D: few questions
On 24.12.2011 21:22, Andrew Wiley wrote:
On Sat, Dec 24, 2011 at 1:08 PM, Timon Gehrtimon.g...@gmx.ch wrote:
On 12/24/2011 07:00 PM, Andrew Wiley wrote:
On Sat, Dec 24, 2011 at 9:50 AM, Timon Gehrtimon.g...@gmx.chwrote:
On 12/24/2011 06:18 PM, Andrew Wiley wrote:
2011/12/24 Mr. Anonymousmailnew4s...@gmail.com:
On 24.12.2011 19:01, Denis Shelomovskij wrote:
23.12.2011 22:51, bearophile пишет:
++a[] works, but a[]++ doesn't.
Already known compiler bug.
Is it a joke? Array expression in D are for performance reasons to
generate x2-x100 faster code without any compiler optimisations. Link
to
one of these epic comments (even x100 more epic because of '%' use
instead of 'x###'):
https://github.com/D-Programming-Language/druntime/blob/master/src/rt/arraybyte.d#L1127
But `a[]++` should store a copy of `a`, increment elements and return
stored copy. It is hidden GC allocation. We already have a silent
allocation in closures, but here a _really large_ peace of data can be
allocated. Yes, this allocation sometimes can be optimized out but not
always.
IMHO, D should not have `a[]++` operator.
Why should it store a copy? o_O
I also don't see any allocations in the code on the URL above.
int a_orig = a++;
int[] arr_orig = arr[]++;
If ++ is going to be applied to an array, it needs to have the same
meaning as it does elsewhere. After this operation, arr_orig and arr
must refer to different arrays for that to be true.
Not necessarily.
class D{
int payload;
D opUnary(string op:++)(){payload++; return this;}
}
void main() {
D d = new D;
assert(d.payload == 0);
assert(d++.payload == 1);
}
That doesn't match integer semantics:
int a = 0;
assert(a++ == 0);
assert(a == 1);
Yes, that was my point.
Then I'm not understanding what you're trying to prove.
I'm saying that if we implement a postfix ++ operator for arrays,
keeping the language consistent would require it to make a copy if the
user stores a copy of the original array. I guess it could be argued
that since arrays have hybrid value/reference semantics, no copy
should be made and the original should change.
Actually, looking at it from that angle, a[]++ is fundamentally
ambiguous because it could have value semantics or reference
semantics, so I would argue that we shouldn't have it for that reason.
'++a' and 'a += 1' do not have such ambiguities.
Maybe you're right, but a[]++; alone, imo, should compile.
Re: Reading about D: few questions
On 12/24/2011 08:41 PM, Mr. Anonymous wrote:
On 24.12.2011 21:22, Andrew Wiley wrote:
On Sat, Dec 24, 2011 at 1:08 PM, Timon Gehrtimon.g...@gmx.ch wrote:
On 12/24/2011 07:00 PM, Andrew Wiley wrote:
On Sat, Dec 24, 2011 at 9:50 AM, Timon Gehrtimon.g...@gmx.ch wrote:
On 12/24/2011 06:18 PM, Andrew Wiley wrote:
2011/12/24 Mr. Anonymousmailnew4s...@gmail.com:
On 24.12.2011 19:01, Denis Shelomovskij wrote:
23.12.2011 22:51, bearophile пишет:
++a[] works, but a[]++ doesn't.
Already known compiler bug.
Is it a joke? Array expression in D are for performance reasons to
generate x2-x100 faster code without any compiler optimisations.
Link
to
one of these epic comments (even x100 more epic because of '%' use
instead of 'x###'):
https://github.com/D-Programming-Language/druntime/blob/master/src/rt/arraybyte.d#L1127
But `a[]++` should store a copy of `a`, increment elements and
return
stored copy. It is hidden GC allocation. We already have a silent
allocation in closures, but here a _really large_ peace of data
can be
allocated. Yes, this allocation sometimes can be optimized out
but not
always.
IMHO, D should not have `a[]++` operator.
Why should it store a copy? o_O
I also don't see any allocations in the code on the URL above.
int a_orig = a++;
int[] arr_orig = arr[]++;
If ++ is going to be applied to an array, it needs to have the same
meaning as it does elsewhere. After this operation, arr_orig and arr
must refer to different arrays for that to be true.
Not necessarily.
class D{
int payload;
D opUnary(string op:++)(){payload++; return this;}
}
void main() {
D d = new D;
assert(d.payload == 0);
assert(d++.payload == 1);
}
That doesn't match integer semantics:
int a = 0;
assert(a++ == 0);
assert(a == 1);
Yes, that was my point.
Then I'm not understanding what you're trying to prove.
I'm saying that if we implement a postfix ++ operator for arrays,
keeping the language consistent would require it to make a copy if the
user stores a copy of the original array. I guess it could be argued
that since arrays have hybrid value/reference semantics, no copy
should be made and the original should change.
Actually, looking at it from that angle, a[]++ is fundamentally
ambiguous because it could have value semantics or reference
semantics, so I would argue that we shouldn't have it for that reason.
'++a' and 'a += 1' do not have such ambiguities.
Maybe you're right, but a[]++; alone, imo, should compile.
+1.
Re: Reading about D: few questions
On 12/24/2011 08:22 PM, Andrew Wiley wrote:
On Sat, Dec 24, 2011 at 1:08 PM, Timon Gehrtimon.g...@gmx.ch wrote:
On 12/24/2011 07:00 PM, Andrew Wiley wrote:
On Sat, Dec 24, 2011 at 9:50 AM, Timon Gehrtimon.g...@gmx.chwrote:
On 12/24/2011 06:18 PM, Andrew Wiley wrote:
2011/12/24 Mr. Anonymousmailnew4s...@gmail.com:
On 24.12.2011 19:01, Denis Shelomovskij wrote:
23.12.2011 22:51, bearophile пишет:
++a[] works, but a[]++ doesn't.
Already known compiler bug.
Is it a joke? Array expression in D are for performance reasons to
generate x2-x100 faster code without any compiler optimisations. Link
to
one of these epic comments (even x100 more epic because of '%' use
instead of 'x###'):
https://github.com/D-Programming-Language/druntime/blob/master/src/rt/arraybyte.d#L1127
But `a[]++` should store a copy of `a`, increment elements and return
stored copy. It is hidden GC allocation. We already have a silent
allocation in closures, but here a _really large_ peace of data can be
allocated. Yes, this allocation sometimes can be optimized out but not
always.
IMHO, D should not have `a[]++` operator.
Why should it store a copy? o_O
I also don't see any allocations in the code on the URL above.
int a_orig = a++;
int[] arr_orig = arr[]++;
If ++ is going to be applied to an array, it needs to have the same
meaning as it does elsewhere. After this operation, arr_orig and arr
must refer to different arrays for that to be true.
Not necessarily.
class D{
int payload;
D opUnary(string op:++)(){payload++; return this;}
}
void main() {
D d = new D;
assert(d.payload == 0);
assert(d++.payload == 1);
}
That doesn't match integer semantics:
int a = 0;
assert(a++ == 0);
assert(a == 1);
Yes, that was my point.
Then I'm not understanding what you're trying to prove.
I'm saying that if we implement a postfix ++ operator for arrays,
keeping the language consistent would require it to make a copy if the
user stores a copy of the original array.
And I said: not necessarily
That is because reference types have had semantics that go well with not
making a copy all along, so there is no danger of making things more
inconsistent.
I guess it could be argued
that since arrays have hybrid value/reference semantics, no copy
should be made and the original should change.
Actually, looking at it from that angle, a[]++ is fundamentally
ambiguous because it could have value semantics or reference
semantics, so I would argue that we shouldn't have it for that reason.
'++a' and 'a += 1' do not have such ambiguities.
I don't think a[]++ should necessarily be there either.
Re: Reading about D: few questions
On Sat, Dec 24, 2011 at 1:45 PM, Timon Gehr timon.g...@gmx.ch wrote:
On 12/24/2011 08:41 PM, Mr. Anonymous wrote:
On 24.12.2011 21:22, Andrew Wiley wrote:
On Sat, Dec 24, 2011 at 1:08 PM, Timon Gehrtimon.g...@gmx.ch wrote:
On 12/24/2011 07:00 PM, Andrew Wiley wrote:
On Sat, Dec 24, 2011 at 9:50 AM, Timon Gehrtimon.g...@gmx.ch wrote:
On 12/24/2011 06:18 PM, Andrew Wiley wrote:
2011/12/24 Mr. Anonymousmailnew4s...@gmail.com:
On 24.12.2011 19:01, Denis Shelomovskij wrote:
23.12.2011 22:51, bearophile пишет:
++a[] works, but a[]++ doesn't.
Already known compiler bug.
Is it a joke? Array expression in D are for performance reasons to
generate x2-x100 faster code without any compiler optimisations.
Link
to
one of these epic comments (even x100 more epic because of '%' use
instead of 'x###'):
https://github.com/D-Programming-Language/druntime/blob/master/src/rt/arraybyte.d#L1127
But `a[]++` should store a copy of `a`, increment elements and
return
stored copy. It is hidden GC allocation. We already have a silent
allocation in closures, but here a _really large_ peace of data
can be
allocated. Yes, this allocation sometimes can be optimized out
but not
always.
IMHO, D should not have `a[]++` operator.
Why should it store a copy? o_O
I also don't see any allocations in the code on the URL above.
int a_orig = a++;
int[] arr_orig = arr[]++;
If ++ is going to be applied to an array, it needs to have the same
meaning as it does elsewhere. After this operation, arr_orig and arr
must refer to different arrays for that to be true.
Not necessarily.
class D{
int payload;
D opUnary(string op:++)(){payload++; return this;}
}
void main() {
D d = new D;
assert(d.payload == 0);
assert(d++.payload == 1);
}
That doesn't match integer semantics:
int a = 0;
assert(a++ == 0);
assert(a == 1);
Yes, that was my point.
Then I'm not understanding what you're trying to prove.
I'm saying that if we implement a postfix ++ operator for arrays,
keeping the language consistent would require it to make a copy if the
user stores a copy of the original array. I guess it could be argued
that since arrays have hybrid value/reference semantics, no copy
should be made and the original should change.
Actually, looking at it from that angle, a[]++ is fundamentally
ambiguous because it could have value semantics or reference
semantics, so I would argue that we shouldn't have it for that reason.
'++a' and 'a += 1' do not have such ambiguities.
Maybe you're right, but a[]++; alone, imo, should compile.
+1.
You could special case this, but I'd be happy with an error that told
you to use one of the working alternatives.
Re: Reading about D: few questions
On 24.12.2011 19:18, Andrew Wiley wrote: 2011/12/24 Mr. Anonymousmailnew4s...@gmail.com: On 24.12.2011 19:01, Denis Shelomovskij wrote: 23.12.2011 22:51, bearophile пишет: ++a[] works, but a[]++ doesn't. Already known compiler bug. Is it a joke? Array expression in D are for performance reasons to generate x2-x100 faster code without any compiler optimisations. Link to one of these epic comments (even x100 more epic because of '%' use instead of 'x###'): https://github.com/D-Programming-Language/druntime/blob/master/src/rt/arraybyte.d#L1127 But `a[]++` should store a copy of `a`, increment elements and return stored copy. It is hidden GC allocation. We already have a silent allocation in closures, but here a _really large_ peace of data can be allocated. Yes, this allocation sometimes can be optimized out but not always. IMHO, D should not have `a[]++` operator. Why should it store a copy? o_O I also don't see any allocations in the code on the URL above. int a_orig = a++; int[] arr_orig = arr[]++; If ++ is going to be applied to an array, it needs to have the same meaning as it does elsewhere. After this operation, arr_orig and arr must refer to different arrays for that to be true. Actually, when I think of it: int a_orig = a++; int[] arr_orig = arr[]++; Should be read as: int a_orig = a; ++a; int[] arr_orig = arr[]; ++arr[]; (If I'm not mistaken, it was written in the TDPL book) Which means no copy of arr is made, and both arrays (which reference to the same block) are affected.
Re: Reading about D: few questions
Am 23.12.2011 16:25, schrieb Mr. Anonymous:
Hi guys!
I'm mostly familiar with C (and a bit of PHP). I've stumbled upon the D
language, and I must say I really like it.
Now I'm reading the The D Programming Language book, and I have a
couple of questions:
[]
3. const and immutable.
Is there any use for const when defining variables?
As I see it, there is no use for using e.g. const int x;, as it can't be
modified anyway;
So with immutable, const is only good for reference variables that are
initialized to refer to another variable (like a function const ref
parameter).
Am I right?
Right. There's no point in a const int but of course there is a big
difference between const(int)* and immutable(int)*.
4. if (lhs != rhs)?
std.algorithm has this in it's swap function.
Is it different than if (lhs !is rhs)?
Just wondering.
They're not the same at all. is checks if the two operands have binary
equality.
To understand, you have to keep in mind that lhs and rhs are references
and could refer to one and the same variable as in:
int a = 0; swap(a, a);
Now, if you want to know if two refs refer to the same variable, you
use lhs == rhs. If want to know if two class instances are the same,
you use is. If you want to know if two things (instances or anything
else) are equal, you use ==.
lhs == rhs (makes only sense with refs)
rhs is lhs (always true, if the above is true)
rhs == lhs (always true, if the above is true)
import std.stdio;
void f(ref int[] a, ref int[] b) {
writefln(%s %s %s, a == b, a is b, a == b);
}
void main() {
auto u = [1, 2, 3];
auto u2 = u;
auto v = [1, 2, 3];
auto w = [4, 5, 6];
f(u, u); // true true true
f(u, u2);// false true true
f(u, v); // false false true
f(u, w); // false false false
}
[...]
Mafi
Re: Reading about D: few questions
On 23.12.2011 19:47, Ali Çehreli wrote:
On 12/23/2011 07:25 AM, Mr. Anonymous wrote:
I have a couple of questions:
I prefer separate threads for each. :)
Should I resend the questions as separate messages?
1. Uninitialized Arrays and GC.
http://dlang.org/memory.html#uninitializedarrays
It's said here ^ that:
The uninitialized data that is on the stack will get scanned by the
garbage collector looking for any references to allocated memory.
With the given example of: byte[1024] buffer = void;
So does the GC really scan this byte array? Or (sounds more logical to
me) does it scan only reference types?
I am not an expert on garbage collectors but I've never heard about
differentiating the bits of data. The GC would have to need to keep meta
data about every part of the allocated space as such and it would not be
practical.
Maybe not every part, but only reference parts.
If the latter is true, I think the example should use some kind of a
pointer array. Also, in this case, I can't see why Uninitialized data
can be a source of bugs and trouble, even when used correctly.?
I don't think that the last part is any different than the initialize
all of your variables advice. The uninitialized data has come from
memory that has been used earlier in the program and may have valid data
(and references) to existing or already-destroyed data. Hard to debug.
2. Setting Dynamic Array Length.
http://dlang.org/arrays.html#resize
A more practical approach would be to minimize the number of resizes
The solution works but is not as clean as just using array ~= c;
Is there any way (language, runtime, or phobos) to declare an array that
would reallocate memory by chunks, which are multiple of x?
Array expansion is already more efficient than they look at first. This
article is a good read:
http://www.dsource.org/projects/dcollections/wiki/ArrayArticle
Thanks, I'll take a look.
3. const and immutable.
Is there any use for const when defining variables?
As I see it, there is no use for using e.g. const int x;, as it can't be
modified anyway;
So with immutable, const is only good for reference variables that are
initialized to refer to another variable (like a function const ref
parameter).
Am I right?
Right. I have two observations myself:
- To be more useful, function parameters should not insist on immutable
data, yet we type string all over the place.
- To be more useful, functions should not insist on the mutability of
the data that they return.
The following function makes a new string:
char[] endWithDot(const(char)[] s)
{
return s ~ '.';
}
char[] s;
s ~= hello;
auto a = endWithDot(s);
It is good that the parameter is const(char) so that I could pass the
mutable s to it.
But the orthogonal problem of the type of the return is troubling. The
result is clearly mutable yet it can't be returned as such:
Error: cannot implicitly convert expression (s ~ '.') of type
const(char)[] to char[]
We've talked about this before. There is nothing in the language that
makes me say the returned object is unique; you can cast it to mutable
or immutable freely.
I saw that std.string functions use assumeUnique from std.exception.
As for your example, it probably should be:
char[] endWithDot(const(char)[] s)
{
return s.dup ~ '.';
}
5. Align attribute.
http://dlang.org/attribute.html#align
struct S {
align(4) byte a; // placed at offset 0
align(4) byte b; // placed at offset 1
}
Explain this please.
I don't know more than what the documentation says but I remember
reading bugs about align().
6. Array slices manipulation.
a[] += 1; works but a[]++ doesn't.
Not so important but just wondering: why, and is it intended?
Again, I remember discussion and limitations about this feature.
Fixed-length arrays have better support and the regular increment works:
double[3] a = [ 10, 20, 30 ];
++a[];
++a[] works, but a[]++ doesn't.
7. Anonymous structs.
In C you can write:
struct { int a; } s = {10};
printf(%d\n, s.a);
In D you must declare the struct first:
struct S { int a; };
S s = {10};
writeln(s.a);
Why doesn't D allow anonymous structs?
It may be related to palsing. D does not require the semicolon at the
end of the struct definition, so it wouldn't know what 's' is:
struct { int a; } // definition (of unmentionable type :) )
s = {10}; // unknown s
There could be special casing but I don't think that it would be worth it.
Sounds reasonable.
Ali
Re: Reading about D: few questions
Mr. Anonymous:
With the given example of: byte[1024] buffer = void;
So does the GC really scan this byte array?
The current D GC is not precise, so I think the current DMD+GC scan this array.
Future better compilers/runtimes probably will be able to avoid it (with a
shadow stack the gives precise typing information at runtime, used by a precise
GC).
The solution works but is not as clean as just using array ~= c;
Is there any way (language, runtime, or phobos) to declare an array that
would reallocate memory by chunks, which are multiple of x?
Appending to built-in D arrays is several times slower than doing the same
thing to a C++ vector, but in many situations the performance is enough. When
it's not enough there is the capacity function in the object module. Or for
even better performance the appender in std.array, that gives performance just
a little worse than the C++ vector push back.
Is there any use for const when defining variables?
As I see it, there is no use for using e.g. const int x;, as it can't be
modified anyway;
const int x = 5 + foo(y) * bax(z);
It's better to use immutable or const everywhere this is possible and doesn't
give you too many problems. In my D2 code about 70-90% of variables are now
const or better immutable. This avoids some bugs and will help future compilers
optimize code better.
5. Align attribute.
http://dlang.org/attribute.html#align
struct S {
align(4) byte a; // placed at offset 0
align(4) byte b; // placed at offset 1
}
Explain this please.
I don't know. Keep in mind that DMD has many bugs, almost 50-100 gets removed
every month.
6. Array slices manipulation.
a[] += 1; works but a[]++ doesn't.
Not so important but just wondering: why, and is it intended?
It's a compiler bug. I think it's already in Bugzilla (but take a look in
Bugzilla if you want to be sure).
7. Anonymous structs.
In C you can write:
struct { int a; } s = {10};
printf(%d\n, s.a);
In D you must declare the struct first:
struct S { int a; };
S s = {10};
writeln(s.a);
Why doesn't D allow anonymous structs?
D doesn't allow top-level anonymous structs.
++a[] works, but a[]++ doesn't.
Already known compiler bug.
Ali:
There is nothing in the language that makes me say the returned object is
unique; you can cast it to mutable or immutable freely.
The return value of strongly pure functions is implicitly castable to immutable.
And sometimes inout helps.
Bye,
bearophile
Re: Reading about D: few questions
5. Align attribute.
http://dlang.org/attribute.html#align
struct S {
align(4) byte a; // placed at offset 0
align(4) byte b; // placed at offset 1
}
Explain this please.
align is a huge mess imo.
It matches the corresponding C compiler behavior
So what's the point of align in the first place, if the compiler does what
it wants anyway, see above?
The only thing that really works is
align(1) struct S {...}
for packed structs.
Re: Reading about D: few questions
On 23.12.2011 19:47, Ali Çehreli wrote: On 12/23/2011 07:25 AM, Mr. Anonymous wrote: 2. Setting Dynamic Array Length. http://dlang.org/arrays.html#resize A more practical approach would be to minimize the number of resizes The solution works but is not as clean as just using array ~= c; Is there any way (language, runtime, or phobos) to declare an array that would reallocate memory by chunks, which are multiple of x? Array expansion is already more efficient than they look at first. This article is a good read: http://www.dsource.org/projects/dcollections/wiki/ArrayArticle std.array.Appender is what I was talking about :)
Re: Reading about D: few questions
On 12/23/2011 11:51 AM, bearophile wrote:
Ali:
There is nothing in the language that makes me say the returned
object is unique; you can cast it to mutable or immutable freely.
The return value of strongly pure functions is implicitly castable to
immutable.
Is that working yet? The commented-out lines below don't compile with 2.057:
void main()
{
char[] s = hello.dup;
char[]am = endWithDot(s);
const(char)[] ac = endWithDot(s);
const(char[]) acc = endWithDot(s);
// immutable(char)[] ai = endWithDot(s);
// immutable(char[]) aii = endWithDot(s);
}
pure char[] endWithDot(const(char)[] s)
{
char[] result = s.dup;
result ~= '.';
return result;
}
Also note that I could not use the better line below in endWithDot():
return s ~ '.';
as the type of the result is const(char)[]. I insist that it too should
be castable to any mutable or immutable type.
And sometimes inout helps.
Yes but it is only when the types of the parameters and the result
should be related.
Bye,
bearophile
Ali
Re: Reading about D: few questions
On Friday, December 23, 2011 09:47:35 Ali Çehreli wrote:
- To be more useful, function parameters should not insist on immutable
data, yet we type string all over the place.
That depends. If they're going to have to idup the data anyway, then it's
better to require that the argument be immutable so that that cost is clear.
The worst is taking const and then iduping, because then you're forced to idup
strings which didn't need to be iduped.
And in general, operating on strings is more efficient than mutable character
arrays, because you can slice them with impunity, whereas you often have to
dup or idup mutable arrays in order to avoid altering the original data. The
area where the immutability becomes problematic is when you actually want to
directly mutate a string - but that's generally a rather iffy thing to do with
UTF-8 anyway, since you have to deal with the varying length of the various
code points within the string.
That being said, an increasing number of functions in Phobos are templated on
string type so that you can use whatever string type that you want with them.
And there is a push (at least with toString) to add the ability to put the
result of a string function into an existing string of some variety (be it
using a delegate or an output range). So, you'll be forced to use string less,
but the reality of the matter is that in the general case you should probably
be using string anyway (there are, of course, always exceptions).
- To be more useful, functions should not insist on the mutability of
the data that they return.
The following function makes a new string:
char[] endWithDot(const(char)[] s)
{
return s ~ '.';
}
char[] s;
s ~= hello;
auto a = endWithDot(s);
It is good that the parameter is const(char) so that I could pass the
mutable s to it.
But the orthogonal problem of the type of the return is troubling. The
result is clearly mutable yet it can't be returned as such:
Error: cannot implicitly convert expression (s ~ '.') of type
const(char)[] to char[]
We've talked about this before. There is nothing in the language that
makes me say the returned object is unique; you can cast it to mutable
or immutable freely.
In general, D doesn't have features where the programmer says that something
is okay. It's too interested in making guarantees for that. Either it can
guarantee something, or you force it with a cast. I can't think of even one
feature where you say that _you_ guarantee that something is okay. Casting is
your only option.
That being said, the language is improving in what it can guarantee and in
what it can do thanks to those guarantees. For instance, if you have a pure
function and the compiler can guarantee that the return value doesn't
reference anything in the argumetns passed in, then the return value is
implicitly convertible to whatever const-ness you want.
If you want to be making such guarantees yourself, then what you typically
have to do is templatize the function and take advantage of static if and D's
compile-time reflection capabilities. Phobos does this quite a bit to improve
performance and avoid having to duplicate data.
Your particular example is quite easily fixed though. The issue is that the
string which was passed in is typed as const(char)[], and the expression s ~
'.' naturally results in the same type. But it's quite clear that the
resulting string could be of any constness, since it's a new string. So, just
tell it what constness to have by casting it.
- Jonathan M Davis
Re: Reading about D: few questions
On Friday, December 23, 2011 20:19:28 Mr. Anonymous wrote:
I saw that std.string functions use assumeUnique from std.exception.
As for your example, it probably should be:
char[] endWithDot(const(char)[] s)
{
return s.dup ~ '.';
}
No, that allocates _two_ strings - one from dup and one as the result of the
concatenation. It should either be
auto retval = s.dup;
retval ~= '.';
return retval;
or
return cast(char[])(s ~ '.');
The problem is that because s is const(char)[], the result of the
concatenation is that type. But it's guaranteed to be a new string, so the
cast is fine. It's arguably better to use the first version though, since it
doesn't require a cast.
- Jonathan M Davis
Re: Reading about D: few questions
On Friday, December 23, 2011 14:51:06 bearophile wrote:
And sometimes inout helps.
Yeah, good point. I keep forgetting about inout, since it didn't work properly
before. So, the best way to implement Ali's function would be
inout(char)[] endWithDot(inout(char)[] s)
{
return s ~ '.';
}
- Jonathan M Davis
Re: Reading about D: few questions
On Friday, December 23, 2011 14:51:21 Ali Çehreli wrote:
On 12/23/2011 11:51 AM, bearophile wrote:
Ali:
There is nothing in the language that makes me say the returned
object is unique; you can cast it to mutable or immutable freely.
The return value of strongly pure functions is implicitly castable to
immutable.
Is that working yet? The commented-out lines below don't compile with 2.057:
void main()
{
char[] s = hello.dup;
char[]am = endWithDot(s);
const(char)[] ac = endWithDot(s);
const(char[]) acc = endWithDot(s);
// immutable(char)[] ai = endWithDot(s);
// immutable(char[]) aii = endWithDot(s);
}
pure char[] endWithDot(const(char)[] s)
{
char[] result = s.dup;
result ~= '.';
return result;
}
Well, that's not strongly pure - only weakly pure - so if the optimization is
only for strongly pure functions, then that won't work. I know that it _could_
be done with weakly pure functions as well (such as your example here), but
I'm not exactly sure what it does right now. The feature is new, so it doesn't
yet work in all of the cases that it should, and it's not entirely clear
exactly far it will go. IIRC, Daniel Murphy and Steven were discussing it a
while back, and it clearly didn't do as much as it could, and it wasn't
entirely clear that it ever would beacuse of the increased complications
involved. However, it wil almost certainly work in more cases in the future
than it does now as the feature is improved.
- Jonathan M Davis
Re: Reading about D: few questions
Jonathan M Davis: The feature is new, so it doesn't yet work in all of the cases that it should, and it's not entirely clear exactly far it will go. IIRC, Daniel Murphy and Steven were discussing it a while back, I have very recently opened another thread about it, but unfortunately it didn't attract a lot of attention: http://www.digitalmars.com/webnews/newsgroups.php?art_group=digitalmars.Darticle_id=153041 and it wasn't entirely clear that it ever would beacuse of the increased complications involved. Yeah. Ali: return s ~ '.'; as the type of the result is const(char)[]. I insist that it too should be castable to any mutable or immutable type. Add a request in Bugzilla if it's not already present. I don't know how much complex it is to implement in the compiler. Bye, bearophile
Re: Reading about D: few questions
On 12/23/2011 03:16 PM, Jonathan M Davis wrote:
On Friday, December 23, 2011 09:47:35 Ali Çehreli wrote:
- To be more useful, function parameters should not insist on immutable
data, yet we type string all over the place.
That depends. If they're going to have to idup the data anyway, then it's
better to require that the argument be immutable so that that cost is
clear.
The worst is taking const and then iduping, because then you're
forced to idup
strings which didn't need to be iduped.
That would be leaking an implementation detail to the user. Besides, it
doesn't solve the problem if the user is in the middle:
void user(const(char)[] p)
{
writeln(endWithDot(p));
}
The user itself would be forced to take immutable, but this time the
reason is different: not because he is passing a copy optimization of
its own, but because he is passing endWithDot()'s copy optimization to
its caller.
immutable would have to be leaked all the way up just because a low
level function decided to make a copy!
Perhaps the guideline should be: Everybody should take by immutable
references so that this leaking of immutable through all layers should
not be a problem in case a low-level function decided to make a copy.
And in general, operating on strings is more efficient than mutable
character
arrays, because you can slice them with impunity, whereas you often
have to
dup or idup mutable arrays in order to avoid altering the original data.
Agreed. But immutable on the parameter list is an insistence: The
function insists that the data be immutable. Why? Because it is going to
store it for later use? Perhaps share it between threads? It is
understandable when there is such a legitimate reason. Then the caller
would see the reason too: oh, takes immutable; that means my data may
be used later as is.
That being said, an increasing number of functions in Phobos are
templated on
string type so that you can use whatever string type that you want
with them.
And there is a push (at least with toString) to add the ability to
put the
result of a string function into an existing string of some variety
(be it
using a delegate or an output range). So, you'll be forced to use
string less,
Good. Ranges are more and more becoming thinking in D. Perhaps we
should be talking about a range that appends a dot at the end of the
existing elements.
but the reality of the matter is that in the general case you should
probably
be using string anyway (there are, of course, always exceptions).
I am looking for simple guidelines when designing functions. It is
simple in C++: take data by reference to const if you are not going to
modify it. (It is questionable whether small structs should be passed by
value instead, but that's beside the point.)
In C++, passing by reference to const works because the function accepts
any type of mutability and a copy is avoided because it's a reference.
In D, immutable is not more const than const (which was my initial
assumption); it is an additional requirement: give me data that should
never change. My point is that this requirement makes sense only in rare
cases. Why would a function like endWithDot() insist on how mutable the
user's data is?
- To be more useful, functions should not insist on the mutability of
the data that they return.
The following function makes a new string:
char[] endWithDot(const(char)[] s)
{
return s ~ '.';
}
char[] s;
s ~= hello;
auto a = endWithDot(s);
It is good that the parameter is const(char) so that I could pass the
mutable s to it.
But the orthogonal problem of the type of the return is troubling. The
result is clearly mutable yet it can't be returned as such:
Error: cannot implicitly convert expression (s ~ '.') of type
const(char)[] to char[]
We've talked about this before. There is nothing in the language that
makes me say the returned object is unique; you can cast it to mutable
or immutable freely.
In general, D doesn't have features where the programmer says that
something
is okay. It's too interested in making guarantees for that. Either it can
guarantee something, or you force it with a cast. I can't think of
even one
feature where you say that _you_ guarantee that something is okay.
Casting is
your only option. [...]
I know. I used the wrong words. Yes, the compiler should see what I see:
the returned object is unique and can be elevated to any mutability level.
Your particular example is quite easily fixed though. The issue is
that the
string which was passed in is typed as const(char)[], and the
expression s ~
'.' naturally results in the same type. But it's quite clear that the
resulting string could be of any constness, since it's a new string.
So, just
tell it what constness to have by casting it.
That's the other side of the problem: Why would the function dictate how
the caller should treat this piece of data?
Re: Reading about D: few questions
The core problem for a number of these situations is how types are handled with regards to expressions. In an expression such as char[] arr = s ~ '.'; the type of the value being assigned is determined _before_ the assignment is done. So, even though in theory the compiler could make it work, it doesn't, because by the time it's looking at the type being assigned to, it's too late. There would need to be a fundamental change in how the language functions in order to fix issues like this. pure can do it when it can not because it's able to look at what the return type is and changing the result of the expression accordingly but because it has guarantees which make it so that it knows that the return value could be converted to any level of constness and still be valid. The types used in the expressions internally are generally irrelevant. So, while I completely agree that it would be an improvement if the compiler did a better job with implicit conversion when it could theoretically be done, I'm not sure how much of that we're actually going to end up seeing simply because of how the language and type system works in terms of the order of evaluation. - Jonathan M Davis
