Re: Constraints error messages [Was: Re: Constrained Templates]
BCS: >I like the idea, but how should it interact with overloaded templates?< The syntax/semantics I have shown is just a starting point, there are probably way better ways to reach the same goal. For example a kind of run-time error/exception for templates that shows an error message and removes the template from the list of possible templates. Regarding overloaded templates, those else clauses are meant to be useful mostly when there is no template overloading, they are designed to be useful not for the usual purpose of constrains, that is to remove templates from the pool of overloads, but mostly when you have a single template that has input types and values that must satisfy some requirements. So it's a grubby idea that needs more thinking :-) Bye, bearophile
Re: Constraints error messages [Was: Re: Constrained Templates]
Hello bearophile,
A syntax like the following allows to add the user-defined error
messages too, but it's bad looking, so probably something quite better
can be invented:
template Foo(T)
if (condition1) else pragma(msg, "error message1")
if (condition2) else pragma(msg, "error message2") {
}
I like the idea, but how should it interact with overloaded templates?
template Foo(T) if(cond1!T) else pragma(msg, "a") {}
template Foo(T) if(cond2!T) else pragma(msg, "b") {}
Don't run any of the elses unless all the options fail and then run all of
them?
--
... <
Constraints error messages [Was: Re: Constrained Templates]
Yao G.:
> Something I would like to see with template constraints, is the ability to
> show text messages, like when static if is used.
This is a problem I too have.
In D1 I used to write code like:
template Foo(T) {
static assert(condition1, "error message1");
static assert(condition2, "error message2");
...
}
Now in D2 I usually write:
template Foo(T) if (condition1 && condition2) {
...
}
This D2 code is better because error compiler messages are better (at the
template instantiation point instead inside the template body), but it's worse
because I lose the error messages written by me. In simple situations the error
messages are not so important, but in some situations they are useful to
understand why the template input values are wrong.
A syntax like the following allows to add the user-defined error messages too,
but it's bad looking, so probably something quite better can be invented:
template Foo(T)
if (condition1) else pragma(msg, "error message1")
if (condition2) else pragma(msg, "error message2") {
}
Bye,
bearophile
Re: Constrained Templates
Don wrote:
This can be trivially shown to be NP-complete.
void bar(T)()
{
static if ( big_function!T) {
T t;
t.flabbergast( );
}
}
Compiler cannot determine if T needs flabbergast(), unless it evaluates
big_function. Which can be arbitrarily complicated.
But how does the compiler decide whether or not to include that code
usually? Would it not then also have to evaluate big_function?
We're taking the approach of making the default error message in such
cases as helpful as possible.
Which is likely a better choice. The idea just hit me, and I decided
to probe for reactions.
--
Simen
Re: Constrained Templates
Jason House wrote:
Simen kjaeraas Wrote:
Leandro Lucarella wrote:
> it would be nice
> to have some sort of way to tell the compiler to write the template
> constraints for us (the obvious ones at least, there might be other
> template constraints desired besides the ones the ones the compiler
can
> figure out). This way, the errors can be improved without user
> intervention.
How's about @optional? Marks an entire function as optional, i.e. will
not be included if it does not compile.
struct foo( T ) {
@optional
void bar( ) { // Will not exist if T cannot be flabbergasted.
T tmp;
t.flabbergast( );
}
}
--
Simen
Would it be a bug or a feature if bar was never included? As written,
bar should never compile...
Well spotted. And a very good reason not to have this feature (or to
write unit tests, I guess :p )
--
Simen
Re: Constrained Templates
Simen kjaeraas wrote:
Leandro Lucarella wrote:
it would be nice
to have some sort of way to tell the compiler to write the template
constraints for us (the obvious ones at least, there might be other
template constraints desired besides the ones the ones the compiler can
figure out). This way, the errors can be improved without user
intervention.
How's about @optional? Marks an entire function as optional, i.e. will
not be included if it does not compile.
struct foo( T ) {
@optional
void bar( ) { // Will not exist if T cannot be flabbergasted.
T tmp;
t.flabbergast( );
}
}
This can be trivially shown to be NP-complete.
void bar(T)()
{
static if ( big_function!T) {
T t;
t.flabbergast( );
}
}
Compiler cannot determine if T needs flabbergast(), unless it evaluates
big_function. Which can be arbitrarily complicated.
We're taking the approach of making the default error message in such
cases as helpful as possible.
Re: Constrained Templates
:D That's weird. I just filled some typical form controls and clicked submit. No "autoban" button was clicked. Yao G. On Sun, 13 Jun 2010 20:05:04 -0500, Walter Bright wrote: Yao G. wrote: It is now displayed in the front page. http://www.reddit.com/r/programming/ On Sun, 13 Jun 2010 15:42:13 -0500, Walter Bright wrote: Yao G. wrote: Maybe I did something wrong at the moment I submitted the article. :( I sent an email to the reddit moderators asking what's up with that. Here's the email I received from the moderators: "Fixed, it was autobanned." It didn't say what rule was used to autoban it. -- Using Opera's revolutionary e-mail client: http://www.opera.com/mail/
Re: Constrained Templates
Simen kjaeraas Wrote:
> Leandro Lucarella wrote:
> > it would be nice
> > to have some sort of way to tell the compiler to write the template
> > constraints for us (the obvious ones at least, there might be other
> > template constraints desired besides the ones the ones the compiler can
> > figure out). This way, the errors can be improved without user
> > intervention.
>
> How's about @optional? Marks an entire function as optional, i.e. will
> not be included if it does not compile.
>
> struct foo( T ) {
>@optional
>void bar( ) { // Will not exist if T cannot be flabbergasted.
> T tmp;
> t.flabbergast( );
>}
> }
>
> --
> Simen
Would it be a bug or a feature if bar was never included? As written, bar
should never compile...
Re: Constrained Templates
Leandro Lucarella wrote:
it would be nice
to have some sort of way to tell the compiler to write the template
constraints for us (the obvious ones at least, there might be other
template constraints desired besides the ones the ones the compiler can
figure out). This way, the errors can be improved without user
intervention.
How's about @optional? Marks an entire function as optional, i.e. will
not be included if it does not compile.
struct foo( T ) {
@optional
void bar( ) { // Will not exist if T cannot be flabbergasted.
T tmp;
t.flabbergast( );
}
}
--
Simen
Re: Constrained Templates
Walter Bright wrote: Yao G. wrote: It is now displayed in the front page. http://www.reddit.com/r/programming/ On Sun, 13 Jun 2010 15:42:13 -0500, Walter Bright wrote: Yao G. wrote: Maybe I did something wrong at the moment I submitted the article. :( I sent an email to the reddit moderators asking what's up with that. Here's the email I received from the moderators: "Fixed, it was autobanned." It didn't say what rule was used to autoban it. This is odd. Now my post on tech talk tips has vanished from the new articles list. Andrei
Re: Constrained Templates
"Andrei Alexandrescu" wrote in message news:hv46bv$1kp...@digitalmars.com... > Walter Bright wrote: >> Nick Sabalausky wrote: >>> "and a line number pointing into the template body. The error is being >>> reported as occurring somewhere other than where the actual problem is" >>> >>> That would be a good place to point out that recent versions of DMD >>> display template instantiation traces upon template instatiation errors >>> (Does C++ do that?). Then, of course, it could go on to "but better >>> yet...". >> >> I wished to avoid quality of implementation issues, and instead focus on >> characteristics of the language. > > I think Nick does make a valid point: there is access to the error, it > just takes some more info from the compiler. > My concern was that C++ users (or other non-D-users) might get the mistaken impression that without constraints, *all* they would get is the unhelpful "Error: incompatible types for ((a) % (b)): 'int*' and 'int*'", and re-enforce their belief that templates have an innate tendency to give useless error messages, and that if you use D, constraints are the only help you get. But I may be over-thinking it and worrying about nothing.
Re: Constrained Templates
Walter Bright wrote: Nick Sabalausky wrote: "and a line number pointing into the template body. The error is being reported as occurring somewhere other than where the actual problem is" That would be a good place to point out that recent versions of DMD display template instantiation traces upon template instatiation errors (Does C++ do that?). Then, of course, it could go on to "but better yet...". I wished to avoid quality of implementation issues, and instead focus on characteristics of the language. I think Nick does make a valid point: there is access to the error, it just takes some more info from the compiler. Andrei
Re: Constrained Templates
Andrei Alexandrescu wrote: Walter Bright wrote: Yao G. wrote: Maybe I did something wrong at the moment I submitted the article. :( I sent an email to the reddit moderators asking what's up with that. I posted this as a test: http://www.reddit.com/r/reddit.com/comments/cemtc/tech_talk_tips/ Andrei Sorry I was on the plane and in a hurry so I posted to general reddit, not programming. I now deleted that (sorry BCS) and now reposted under programming. Vote me up please :o). http://www.reddit.com/r/programming/comments/ceo9s/tech_talk_tips/ The point of the experiment was that Walter had hypothesized it's possible his name is on a scrutiny/spam list. I removed his name from the article and the results are mixed: when I posted in the general reddit community the article didn't make it on the new articles list, however this latest post in /programming/ was listed instantly. In the meantime I saw that Walter's constrained templates article appeared but with a delay. Go figure. Andrei
Re: Constrained Templates
Nick Sabalausky wrote: "and a line number pointing into the template body. The error is being reported as occurring somewhere other than where the actual problem is" That would be a good place to point out that recent versions of DMD display template instantiation traces upon template instatiation errors (Does C++ do that?). Then, of course, it could go on to "but better yet...". I wished to avoid quality of implementation issues, and instead focus on characteristics of the language.
Re: Constrained Templates
Yao G. wrote: It is now displayed in the front page. http://www.reddit.com/r/programming/ On Sun, 13 Jun 2010 15:42:13 -0500, Walter Bright wrote: Yao G. wrote: Maybe I did something wrong at the moment I submitted the article. :( I sent an email to the reddit moderators asking what's up with that. Here's the email I received from the moderators: "Fixed, it was autobanned." It didn't say what rule was used to autoban it.
Re: Constrained Templates
Hello Nick, "Walter Bright" wrote in message news:hv39tt$87...@digitalmars.com... http://www.drdobbs.com/blog/archives/2010/06/constrained_tem.html Anyone want to do the honors and post to reddit, ycombinator, etc. ? "and a line number pointing into the template body. The error is being reported as occurring somewhere other than where the actual problem is" That would be a good place to point out that recent versions of DMD display template instantiation traces upon template instatiation errors (Does C++ do that?). I think GCC does. But IIRC it ends up being an almost unreadable one. --- Not sent from an iPhone. -- ... <
Re: Constrained Templates
Hello Ali,
The method that I learned from Phobos is not the best, but at least
the function interface reads better:
T gcd(T)(T a, T b)
if (supports_modulus!T)
{
T result;
// ...
return result;
}
That makes it clear that T must support the modulus operation.
Here is how supports_modulus may be defined:
template supports_modulus(T)
{
const bool supports_modulus = is (typeof(
{
T a;
T b;
T c = a % b;
}()));
}
this shorter vertion also works:
template supports_modulus(T)
{
const bool supports_modulus = is (typeof(T.init%T.init) == T);
}
--
... <
Re: Constrained Templates
Leandro Lucarella writes: > When you get used to this idiom, it might not look to bad, but I'm > seeing people new to D wonder "why in the hell was that syntax used?" > while they try to decode what is(typeof()) means. The syntax doesn't bother me as much as the suggestion to repeat details of the template's implementation. C++'s concepts were going in this direction too, adding code that suffers the same problem as documentation: It can fall out of step with "the real code", and keeping it aligned requires repeating details that should only be stated once -- in "the real code". Here we're debating how to state that some type can participate in a modulus operation, but that we even need to apply this operation to the function's arguments is an internal detail. It could change after the first pass at implementing the function, adopting a different algorithm, and we could wind up continuing to impose requirements on the types that are no longer relevant to the revised implementation. It looks like yet another maintenance burden. -- Steven E. Harris
Re: Constrained Templates
"Walter Bright" wrote in message news:hv39tt$87...@digitalmars.com... > http://www.drdobbs.com/blog/archives/2010/06/constrained_tem.html > > Anyone want to do the honors and post to reddit, ycombinator, etc. ? "and a line number pointing into the template body. The error is being reported as occurring somewhere other than where the actual problem is" That would be a good place to point out that recent versions of DMD display template instantiation traces upon template instatiation errors (Does C++ do that?). Then, of course, it could go on to "but better yet...". --- Not sent from an iPhone.
Re: Constrained Templates
It is now displayed in the front page. http://www.reddit.com/r/programming/ On Sun, 13 Jun 2010 15:42:13 -0500, Walter Bright wrote: Yao G. wrote: Maybe I did something wrong at the moment I submitted the article. :( I sent an email to the reddit moderators asking what's up with that. -- Using Opera's revolutionary e-mail client: http://www.opera.com/mail/
Re: Constrained Templates
Walter Bright wrote: Yao G. wrote: Maybe I did something wrong at the moment I submitted the article. :( I sent an email to the reddit moderators asking what's up with that. I posted this as a test: http://www.reddit.com/r/reddit.com/comments/cemtc/tech_talk_tips/ Andrei
Re: Constrained Templates
Leandro Lucarella wrote:
> Walter Bright, el 13 de junio a las 12:01 me escribiste:
>> http://www.drdobbs.com/blog/archives/2010/06/constrained_tem.html
>>
>> Anyone want to do the honors and post to reddit, ycombinator, etc. ?
>
> Nice article, but when I read:
>
>T gcd(T)(T a, T b)
>if (is(typeof(a % b)))
>{
>...
>}
>
> Under the presence of such beauty as template constraint syntax is,
> "is(typeof(a % b))" makes my eyes hurt, and my brain wonder.
The method that I learned from Phobos is not the best, but at least the
function interface reads better:
T gcd(T)(T a, T b)
if (supports_modulus!T)
{
T result;
// ...
return result;
}
That makes it clear that T must support the modulus operation.
Here is how supports_modulus may be defined:
template supports_modulus(T)
{
const bool supports_modulus = is (typeof(
{
T a;
T b;
T c = a % b;
}()));
}
Here are what is at play to get that "named constraint":
- The curly braces after the typeof define a closure (delegate?) literal
- The body includes how the type should be used to satisfy the
'supports_modulus' constraint
- The closure is "executed" with the empty parethesis at the end of it
(actually, not executed at all; because the expression is a parameter to
typeof, which never executes its argument)
- [This is my assumption] typeof produces an invalid type for invalid
expressions
- One of the uses of the 'is' expression produces a bool result if the
type that it receives is not valid
- When a template contains just one definition and that definition
matches the name of the template, the template instantiation is the same
as the definition that it contains. i.e. instead of writing
(supports_modulus!T).supports_modulus
to mean the bool value, one merely writes
supports_modulus!T
A lot of D features! :) But once that code structure is accepted as an
idiom, it works and gives names to constraints.
Ali
Re: Constrained Templates
Yao G. wrote: Maybe I did something wrong at the moment I submitted the article. :( I sent an email to the reddit moderators asking what's up with that.
Re: Constrained Templates
Maybe I did something wrong at the moment I submitted the article. :( Somebody else should try to submit it too. On Sun, 13 Jun 2010 15:15:25 -0500, Walter Bright wrote: Yao G. wrote: http://www.reddit.com/r/programming/comments/cel4o/constrained_templates_in_the_d_programming/ Thanks! Anyone know why this doesn't appear on the "new" page? http://www.reddit.com/r/programming/new/ -- Using Opera's revolutionary e-mail client: http://www.opera.com/mail/
Re: Constrained Templates
Yao G. wrote: http://www.reddit.com/r/programming/comments/cel4o/constrained_templates_in_the_d_programming/ Thanks! Anyone know why this doesn't appear on the "new" page? http://www.reddit.com/r/programming/new/
Re: Constrained Templates
Andrei used that idiom quite extensive in Phobos, unfortunately, I dare to
say. It makes the code a lot harder to read. But the alternative, using
__traits(compiles, ... ) is even worse IMO, because even as is a little
more obvious, is quite verbose.
Something I would like to see with template constraints, is the ability to
show text messages, like when static if is used.
Yao G.
On Sun, 13 Jun 2010 14:36:14 -0500, Leandro Lucarella
wrote:
Walter Bright, el 13 de junio a las 12:01 me escribiste:
http://www.drdobbs.com/blog/archives/2010/06/constrained_tem.html
Anyone want to do the honors and post to reddit, ycombinator, etc. ?
Nice article, but when I read:
T gcd(T)(T a, T b)
if (is(typeof(a % b)))
{
...
}
Under the presence of such beauty as template constraint syntax is,
"is(typeof(a % b))" makes my eyes hurt, and my brain wonder. When you
get used to this idiom, it might not look to bad, but I'm seeing people
new to D wonder "why in the hell was that syntax used?" while they try
to decode what is(typeof()) means.
I think is really a shame, all the beauty and clarity gained by the
simple "if" of template constraints, is lost with the cryptic
is(typeof()) (well, maybe not all, but a significant part).
I would love to see this fixed/simplified. For example, by applying the
changes in suggested in bug 3702 [1]. This looks much better, and is
pretty clear to anyone (even people that doesn't know D):
T gcd(T)(T a, T b)
if (meta.compiles(a % b))
{
...
}
I guess at this point this is not going to happen for D2, a real shame
:S
The, the article made me think that, even when template constraints are
useful not only for improving error reporting, they are often used only
for that. Since the compiler knows all the operations a template
parameter will need to satisfy from the function body, it would be nice
to have some sort of way to tell the compiler to write the template
constraints for us (the obvious ones at least, there might be other
template constraints desired besides the ones the ones the compiler can
figure out). This way, the errors can be improved without user
intervention.
[1] http://d.puremagic.com/issues/show_bug.cgi?id=3702
--
Using Opera's revolutionary e-mail client: http://www.opera.com/mail/
Re: Constrained Templates
Walter Bright, el 13 de junio a las 12:01 me escribiste:
> http://www.drdobbs.com/blog/archives/2010/06/constrained_tem.html
>
> Anyone want to do the honors and post to reddit, ycombinator, etc. ?
Nice article, but when I read:
T gcd(T)(T a, T b)
if (is(typeof(a % b)))
{
...
}
Under the presence of such beauty as template constraint syntax is,
"is(typeof(a % b))" makes my eyes hurt, and my brain wonder. When you
get used to this idiom, it might not look to bad, but I'm seeing people
new to D wonder "why in the hell was that syntax used?" while they try
to decode what is(typeof()) means.
I think is really a shame, all the beauty and clarity gained by the
simple "if" of template constraints, is lost with the cryptic
is(typeof()) (well, maybe not all, but a significant part).
I would love to see this fixed/simplified. For example, by applying the
changes in suggested in bug 3702 [1]. This looks much better, and is
pretty clear to anyone (even people that doesn't know D):
T gcd(T)(T a, T b)
if (meta.compiles(a % b))
{
...
}
I guess at this point this is not going to happen for D2, a real shame
:S
The, the article made me think that, even when template constraints are
useful not only for improving error reporting, they are often used only
for that. Since the compiler knows all the operations a template
parameter will need to satisfy from the function body, it would be nice
to have some sort of way to tell the compiler to write the template
constraints for us (the obvious ones at least, there might be other
template constraints desired besides the ones the ones the compiler can
figure out). This way, the errors can be improved without user
intervention.
[1] http://d.puremagic.com/issues/show_bug.cgi?id=3702
--
Leandro Lucarella (AKA luca) http://llucax.com.ar/
--
GPG Key: 5F5A8D05 (F8CD F9A7 BF00 5431 4145 104C 949E BFB6 5F5A 8D05)
--
Salvajes, de traje, me quieren enseñar
Salvajes, de traje, me quieren educar
Re: Constrained Templates
http://www.reddit.com/r/programming/comments/cel4o/constrained_templates_in_the_d_programming/ On Sun, 13 Jun 2010 14:01:16 -0500, Walter Bright wrote: http://www.drdobbs.com/blog/archives/2010/06/constrained_tem.html Anyone want to do the honors and post to reddit, ycombinator, etc. ? -- Using Opera's revolutionary e-mail client: http://www.opera.com/mail/
Constrained Templates
http://www.drdobbs.com/blog/archives/2010/06/constrained_tem.html Anyone want to do the honors and post to reddit, ycombinator, etc. ?
