Re: function overload on full signature?

[Sönke Ludwig]

Am 15.11.2012 22:05, schrieb Timon Gehr:
> On 11/15/2012 07:09 PM, Sönke Ludwig wrote:
>> Am 14.11.2012 20:07, schrieb Timon Gehr:
>>> On 11/14/2012 06:30 PM, Rob T wrote:
 On Wednesday, 14 November 2012 at 09:16:13 UTC, Walter Bright wrote:
>> I'm not requesting this to be a feature of D, I'm only asking why it
>> is not
>> being done.
>
> Because types are resolved bottom-up, and if the return type were part
> of the overloading, there would be no sensible rule to determine the
> types.

 But doesn't the compiler already have to perform overload-like decision
 making on return types in the "alias this" case, esp once multiple
 conversions are allowed?

 class A{
 int i;
 bool b;
 alias i this;
 alias b this;
 }

 main()
 {
 auto a = new A;
 int i = a;
 bool b = a;
 }

 --rt
>>>
>>> alias this is not the best example, but the necessary logic is basically 
>>> already in the compiler.
>>> Lambda parameter type deduction based on the expected type is a similar 
>>> task.
>>>
>>> It is not being done because it is not being done. Full type inference 
>>> would be even more fun.
>>
>> In the lambda case it's return type deduction and not overload resolution. 
>> Those are actually two
>> very different things.
>>
> 
> Yes, lambda _return_ type deduction is less related, but I have never claimed 
> otherwise.
> 
> Another case that shows how a compiler must be able to take into account the 
> left hand side of an
> assignment in order to type check the right hand side:
> 
> int foo(int);
> double foo(int);
> 
> void main(){
> double function(int) fun = &foo;
> }

The point I was trying to make is that taking the LHS into account is one thing 
(be it return type
or parameter type deduction). But selecting the right overload in presence of 
implicit type
conversions is much more than that and brings a lot of problems. In particular 
I suspect that a
solution could either be to flag every possible ambiguity as an error and thus 
possibly make
resulting APIs badly usable, or to define resolution rules to at least handle 
some ambiguities but
in turn make it difficult and non-obvious for the programmer what happens.

There can also be hidden ambiguities with number literals or custom operator 
overloads, things that
are solvable but all add up.

But then again I don't really have concrete use cases that I can think of, so 
maybe in real cases it
would not matter anyway and going with the always-error-out route would be fine.

Re: function overload on full signature?

[Rob T]

On Friday, 16 November 2012 at 01:02:59 UTC, Jonathan M Davis 
wrote:

On Thursday, November 15, 2012 17:18:43 foobar wrote:

I thought that Haskell doesn't have function overloading (which
simplifies this greatly)... Anyway, I mostly meant "standard"
imperative/OO languages. Sorry for the confusion.


It has pattern matching (which typically results in several of 
the same
function with slightly different arguments, but the overall 
types are still the
same). I'm unaware of it having full-on function overloading. 
But I don't

think that I've ever tried it.

- Jonathan M Davis


This article describes return type overloading in Haskell by way 
of a clear example. It concludes demonstrating a real-world 
implementation of the concept.


http://matthewmanela.com/blog/return-type-overloading-in-haskell/

I'm not well versed in Haskell (first look ever), so I'm not 
sure, but it does seem to look like true return type overloading.


I also read an article that described Perl's overloading on 
return type, but this is less like the overloading we've been 
talking about, and more like a function being able to inspect the 
call context to decide what type should be returned.


--rt

Re: function overload on full signature?

[Jonathan M Davis]

On Thursday, November 15, 2012 17:18:43 foobar wrote:
> I thought that Haskell doesn't have function overloading (which
> simplifies this greatly)... Anyway, I mostly meant "standard"
> imperative/OO languages. Sorry for the confusion.

It has pattern matching (which typically results in several of the same 
function with slightly different arguments, but the overall types are still the 
same). I'm unaware of it having full-on function overloading. But I don't 
think that I've ever tried it.

- Jonathan M Davis

Re: function overload on full signature?

[Timon Gehr]

On 11/15/2012 07:09 PM, Sönke Ludwig wrote:

Am 14.11.2012 20:07, schrieb Timon Gehr:

On 11/14/2012 06:30 PM, Rob T wrote:

On Wednesday, 14 November 2012 at 09:16:13 UTC, Walter Bright wrote:

I'm not requesting this to be a feature of D, I'm only asking why it
is not
being done.


Because types are resolved bottom-up, and if the return type were part
of the overloading, there would be no sensible rule to determine the
types.


But doesn't the compiler already have to perform overload-like decision
making on return types in the "alias this" case, esp once multiple
conversions are allowed?

class A{
int i;
bool b;
alias i this;
alias b this;
}

main()
{
auto a = new A;
int i = a;
bool b = a;
}

--rt


alias this is not the best example, but the necessary logic is basically 
already in the compiler.
Lambda parameter type deduction based on the expected type is a similar task.

It is not being done because it is not being done. Full type inference would be 
even more fun.


In the lambda case it's return type deduction and not overload resolution. 
Those are actually two
very different things.



Yes, lambda _return_ type deduction is less related, but I have never 
claimed otherwise.


Another case that shows how a compiler must be able to take into account 
the left hand side of an assignment in order to type check the right 
hand side:


int foo(int);
double foo(int);

void main(){
double function(int) fun = &foo;
}

Re: function overload on full signature?

[Rob T]

On Thursday, 15 November 2012 at 17:33:24 UTC, monarch_dodra 
wrote:
I'd say because overall, you gain *very* little out of it, and 
it costs you much more complex compiler rules.




But how little, and for how much extra cost? Overloading already 
has a cost to it, and it's really difficult for me to understand 
why adding return type to the mix has to be be many times more 
costly. I will confess that I'm not a compiler designer, but I 
can still try to imagine what would be needed. Already the 
compiler MUST ensure that the return type is valid, so we're 
essentially already there from what I can see.


Most of all though, I'd say it is a bad idea in and out of 
itself: If you overload on the return type, you open the 
floodgates to call ambiguity.


Sure, but not much more so that we have already with the current 
overloading system, and the compiler can certainly prevent an 
invalid compile from happening when there is even a hint of 
ambiguity, as it does already with current overloading. Besides I 
would expect such a feature to be used by advanced programmers 
who know what they are doing because overloading in general is an 
advanced feature and it is certainly not for the easily confused.


I mean, are there even any real use-cases for overload on 
return type?


Yes, I've wanted this for a few years, and I have used a similar 
feature successfully through C++ class operator conversions.


I brought up the example of operator conversion for classes in 
C++. I know some of you have said it's not the same thing, but 
IMO it is the same thing.


int x = a;
string y = a;

Does "a" represent a class or a function? Why should it matter?

class A
{
   int i;
   string s;
   alias i this;
   alias s this; // ouch D does not allow it!
   ...
}

UFCS

int convert( A a )
{
   return a.i;
}

string convert( A a )
{
   return a.s;
}

int i = a.convert;
string s = a.convert;

A real-world use case example is to implement Variant types more 
naturally, where you could do the above and have it convert to 
int or string (and other types) on demand depending on the 
validity of data type. Obviously it will run-time error when the 
type cannot be converted, or perform whatever logic the 
programmer desires.


--rt

Re: function overload on full signature?

[Sönke Ludwig]

Am 14.11.2012 20:07, schrieb Timon Gehr:
> On 11/14/2012 06:30 PM, Rob T wrote:
>> On Wednesday, 14 November 2012 at 09:16:13 UTC, Walter Bright wrote:
 I'm not requesting this to be a feature of D, I'm only asking why it
 is not
 being done.
>>>
>>> Because types are resolved bottom-up, and if the return type were part
>>> of the overloading, there would be no sensible rule to determine the
>>> types.
>>
>> But doesn't the compiler already have to perform overload-like decision
>> making on return types in the "alias this" case, esp once multiple
>> conversions are allowed?
>>
>> class A{
>>int i;
>>bool b;
>>alias i this;
>>alias b this;
>> }
>>
>> main()
>> {
>>auto a = new A;
>>int i = a;
>>bool b = a;
>> }
>>
>> --rt
> 
> alias this is not the best example, but the necessary logic is basically 
> already in the compiler.
> Lambda parameter type deduction based on the expected type is a similar task.
> 
> It is not being done because it is not being done. Full type inference would 
> be even more fun.

In the lambda case it's return type deduction and not overload resolution. 
Those are actually two
very different things.

Re: function overload on full signature?

[bearophile]

monarch_dodra:

I mean, are there even any real use-cases for overload on 
return type?


In Haskell many functions are "overloaded" on the return type 
(like the fromString function), and it's nice. But Haskell is 
able to do it because it has a global type inferencer.


Bye,
bearophile

Re: function overload on full signature?

[monarch_dodra]

On Thursday, 15 November 2012 at 17:18:04 UTC, Rob T wrote:
I've been wondering for a couple of years about why overloading 
stops at the argument sig ...


[SNIP]

--rt


I'd say because overall, you gain *very* little out of it, and it 
costs you much more complex compiler rules.


Most of all though, I'd say it is a bad idea in and out of 
itself: If you overload on the return type, you open the 
floodgates to call ambiguity.


I mean, are there even any real use-cases for overload on return 
type?

Re: function overload on full signature?

[Rob T]

I've been wondering for a couple of years about why overloading 
stops at the argument sig in almost all languages, but so far I 
have not seen a good reason why this must be so.


From what I've read so far, the reason why full overloading is 
not being done, is because it is not being done. Other than that, 
I don't have an answer as to why it is not being done because 
clearly it can be done, and the compiler certainly has the means 
to do it already, otherwise it could not err when you assign a 
function return type to the wrong type on the LHS, ie, it most 
certainly is able to determine what the full signature is at some 
point.


So does anyone really know why it is not being done? The "it's 
too complicated" argument seems weak to me since the compiler 
already has to check for both matching argument sig as well as 
the return type, and it already does overloading on the argument 
sig. I figure "it's too complicated" only because the compiler 
was initially designed without taking full signature overloading 
into account from the very start, otherwise it would be not much 
more complicated than the regular overloading that we have now.


The argument that the compiler will get too confused seems a bit 
weak as well, since the compiler can already get confused with 
argument sig overloading, and there are certainly methods of 
working this out, otherwise function overloading would not work 
as it is now. At the end of the day, if the compiler cannot 
figure it out, then it errs, just as it does now when it cannot 
figure it out. There's already an incalculable number of ways a 
programmer can mess up a valid compile, so adding one more 
possible way to the already massive pile cannot be used as a 
reason why it should not be done.


I could argue that overloading only on function sig is too 
complicated and not worth it, but we have it, and it is useful to 
at least some or most people, so why did we stop at the argument 
sig and not go all the way? Is there a theoretical reason why we 
should stop, are their use cases that have shown that it fails to 
help the programmer or makes programming more difficult?


--rt

Re: function overload on full signature?

[foobar]

On Wednesday, 14 November 2012 at 19:12:59 UTC, Timon Gehr wrote:

On 11/14/2012 06:43 PM, foobar wrote:

On Tuesday, 13 November 2012 at 21:34:28 UTC, Rob T wrote:
I'm wondering why overloading has been implemented to only 
match on
the argument list rather than the full signature which 
includes the

return type? I know I would use it if it was available.

I'm not requesting this to be a feature of D, I'm only asking 
why it

is not being done.

--rt


This is hardly a new idea. It was implemented in a few 
languages of the
70's and it proved to be adding complexity and generally not 
worth the

trouble.


I guess they just were not doing it right then.

No language nowadays bothers with this based on those past 
lessons.


Haskell.

> fromInteger 2 :: Float
2.0


I thought that Haskell doesn't have function overloading (which 
simplifies this greatly)... Anyway, I mostly meant "standard" 
imperative/OO languages. Sorry for the confusion.

Re: function overload on full signature?

[Timon Gehr]

On 11/14/2012 06:43 PM, foobar wrote:

On Tuesday, 13 November 2012 at 21:34:28 UTC, Rob T wrote:

I'm wondering why overloading has been implemented to only match on
the argument list rather than the full signature which includes the
return type? I know I would use it if it was available.

I'm not requesting this to be a feature of D, I'm only asking why it
is not being done.

--rt


This is hardly a new idea. It was implemented in a few languages of the
70's and it proved to be adding complexity and generally not worth the
trouble.


I guess they just were not doing it right then.


No language nowadays bothers with this based on those past lessons.


Haskell.

> fromInteger 2 :: Float
2.0

Re: function overload on full signature?

[Timon Gehr]

On 11/14/2012 06:30 PM, Rob T wrote:

On Wednesday, 14 November 2012 at 09:16:13 UTC, Walter Bright wrote:

I'm not requesting this to be a feature of D, I'm only asking why it
is not
being done.


Because types are resolved bottom-up, and if the return type were part
of the overloading, there would be no sensible rule to determine the
types.


But doesn't the compiler already have to perform overload-like decision
making on return types in the "alias this" case, esp once multiple
conversions are allowed?

class A{
   int i;
   bool b;
   alias i this;
   alias b this;
}

main()
{
   auto a = new A;
   int i = a;
   bool b = a;
}

--rt


alias this is not the best example, but the necessary logic is basically 
already in the compiler. Lambda parameter type deduction based on the 
expected type is a similar task.


It is not being done because it is not being done. Full type inference 
would be even more fun.

Re: function overload on full signature?

[monarch_dodra]

On Wednesday, 14 November 2012 at 06:52:57 UTC, Rob T wrote:
In C++ there are conversion operators, which are not exactly 
the same as function overloading, but the correct function is 
selected based on the type on the left hand side.


Example

class A
{
   operator bool(){ return _b; }
   operator int(){ return _i; }
   int _i; bool _b;
}

A a;
bool b = a; // executes bool()
int i = a; // executes int()

Is there anything like C++ conversion operators in D? I have 
used conversion ops in C++ and may want to use a similar 
feature in D if available.



--rt


Not really, that has *nothing* to do with "the correct function 
is selected based on the type on the left hand side", but just 
C++ trying to match the signature of the constructor it is trying 
to call. As a matter of fact, as a rule of thumb, there is no 
"left hand side" for the compiler. Just functions and arguments.


It is absolutely no different from:
//
void foob(bool);
void fooi(int);
A a;
foob(a); //Executes bool()
fooi(a); //Executes int()
//

Re: function overload on full signature?

[monarch_dodra]

On Wednesday, 14 November 2012 at 06:52:57 UTC, Rob T wrote:
In C++ there are conversion operators, which are not exactly 
the same as function overloading, but the correct function is 
selected based on the type on the left hand side.


Example

class A
{
   operator bool(){ return _b; }
   operator int(){ return _i; }
   int _i; bool _b;
}

A a;
bool b = a; // executes bool()
int i = a; // executes int()

Is there anything like C++ conversion operators in D? I have 
used conversion ops in C++ and may want to use a similar 
feature in D if available.



--rt


Not really, that has *nothing* to do with "the correct function 
is selected based on the type on the left hand side", but just 
C++ trying to match the signature of the constructor it is trying 
to call. As a matter of fact, as a rule of thumb, there is no 
"left hand side" for the compiler. Just functions and arguments.


It is absolutely no different from:
//
void foob(bool);
void fooi(int);
A a;
foob(a); //Executes bool()
fooi(a); //Executes int()
//

Re: function overload on full signature?

[foobar]

On Tuesday, 13 November 2012 at 21:34:28 UTC, Rob T wrote:
I'm wondering why overloading has been implemented to only 
match on the argument list rather than the full signature which 
includes the return type? I know I would use it if it was 
available.


I'm not requesting this to be a feature of D, I'm only asking 
why it is not being done.


--rt


This is hardly a new idea. It was implemented in a few languages 
of the 70's and it proved to be adding complexity and generally 
not worth the trouble. No language nowadays bothers with this 
based on those past lessons.

Re: function overload on full signature?

[Rob T]

On Wednesday, 14 November 2012 at 09:16:13 UTC, Walter Bright 
wrote:
I'm not requesting this to be a feature of D, I'm only asking 
why it is not

being done.


Because types are resolved bottom-up, and if the return type 
were part of the overloading, there would be no sensible rule 
to determine the types.


But doesn't the compiler already have to perform overload-like 
decision making on return types in the "alias this" case, esp 
once multiple conversions are allowed?


class A{
  int i;
  bool b;
  alias i this;
  alias b this;
}

main()
{
  auto a = new A;
  int i = a;
  bool b = a;
}

--rt

Re: function overload on full signature?

[Walter Bright]

On 11/13/2012 1:34 PM, Rob T wrote:

I'm wondering why overloading has been implemented to only match on the argument
list rather than the full signature which includes the return type? I know I
would use it if it was available.

I'm not requesting this to be a feature of D, I'm only asking why it is not
being done.


Because types are resolved bottom-up, and if the return type were part of the 
overloading, there would be no sensible rule to determine the types.

Re: function overload on full signature?

[Jonathan M Davis]

On Wednesday, November 14, 2012 13:05:41 Gor Gyolchanyan wrote:
> Actually the (x) => y style delegates compute their return type exactly by
> looking at the left-hand side. This exact thing is already being done. If
> the ambiguity cannot be resolved, the return type is explicitly set OR the
> result is casted to a type.
> Having normal functions behave this way doesn't add anything new. This
> already exists.

I think that you're wrong about that. (x) => y lambdas are no different from 
any other delegate literals save for  their syntax. The type of x is dependent 
on what's being passed to it, and the type of y depends on x and what the 
function does. It shouldn't have any need whatsoever to look at the expression 
that the lambda is being used in, just what's passed to it.

- Jonathan M Davis

Re: function overload on full signature?

[Gor Gyolchanyan]

Actually the (x) => y style delegates compute their return type exactly by
looking at the left-hand side. This exact thing is already being done. If
the ambiguity cannot be resolved, the return type is explicitly set OR the
result is casted to a type.
Having normal functions behave this way doesn't add anything new. This
already exists.


On Wed, Nov 14, 2012 at 12:30 PM, Rob T  wrote:

> On Wednesday, 14 November 2012 at 08:25:30 UTC, Rob T wrote:
>
>> Was the single conversion limitation specified by design, or do we have
>> room to expand it to allow for multiple conversions?
>>
>>
> http://d.puremagic.com/issues/**show_bug.cgi?id=6083
>
> I guess it will be expanded to allow multiple conversions.
>
> --rt
>



-- 
Bye,
Gor Gyolchanyan.

Re: function overload on full signature?

[Rob T]

On Wednesday, 14 November 2012 at 08:25:30 UTC, Rob T wrote:
Was the single conversion limitation specified by design, or do 
we have room to expand it to allow for multiple conversions?




http://d.puremagic.com/issues/show_bug.cgi?id=6083

I guess it will be expanded to allow multiple conversions.

--rt

Re: function overload on full signature?

[Rob T]

On Wednesday, 14 November 2012 at 07:26:44 UTC, Tove wrote:
it would be a very useful feature to allow overload on void and 
1 other type... as sometimes the return is very expensive to 
calculate... I have seen this trick used by compiler build-in 
functions.


struct A
{
  int i;
  string s;

  alias i this;
  alias s this;
}

but... 2 alias this are not currently allowed.


Was the single conversion limitation specified by design, or do 
we have room to expand it to allow for multiple conversions?


In C++ I used multiple conversions for a variant class and it 
worked like a charm. The alternative was to manually specify a 
named function based on the type to return, which is not as fun 
to work with.


--rt

Re: function overload on full signature?

[Jacob Carlborg]

On 2012-11-14 07:52, Rob T wrote:


Is there anything like C++ conversion operators in D? I have used
conversion ops in C++ and may want to use a similar feature in D if
available.


There's "alias this", but you can currently only do one conversion for a 
given type.


--
/Jacob Carlborg

Re: function overload on full signature?

[Tove]

On Wednesday, 14 November 2012 at 06:52:57 UTC, Rob T wrote:
On Wednesday, 14 November 2012 at 02:01:56 UTC, Jonathan M 
Davis wrote:
Is there anything like C++ conversion operators in D? I have 
used conversion ops in C++ and may want to use a similar 
feature in D if available.

--rt


it would be a very useful feature to allow overload on void and 1 
other type... as sometimes the return is very expensive to 
calculate... I have seen this trick used by compiler build-in 
functions.


struct A
{
  int i;
  string s;

  alias i this;
  alias s this;
}

but... 2 alias this are not currently allowed.

Re: function overload on full signature?

[Rob T]

On Wednesday, 14 November 2012 at 02:01:56 UTC, Jonathan M Davis 
wrote:
It would also screw up covariant return types if overloading 
too the return
type into account. I believe that the _big_ reason though is 
simply because
the type of the expression is determined by the return type, 
not what it's
assigned to. For instance, with how the language works, the 
type of the right-
hand side of an assignment must always be determined 
independently of the type
on the left, so the type of the expression on the right-hand 
side cannot
depend on the type of the left. I suspect that it would 
complicate things
considerably to try and make the return type have anything to 
do with function
overloading. I'm sure that you could find it being discussions 
on it somewhere

online for other C-based laguages though.

Regardless, I've never even heard of a language which tried to 
include the
return type in overload resolution. It sounds like a huge 
complication to me.


- Jonathan M Davis


I can envision simple ways for a programmer to supply hints to 
the compiler for easy resolution based on return type.


For example, when calling an overloaded function without 
assignment to anything:


int f() { ... }
void f() { ... }

f(); // should choose void return
(void)f(); // calls void return
(int)f(); // calls int return

In C++ there are conversion operators, which are not exactly the 
same as function overloading, but the correct function is 
selected based on the type on the left hand side.


Example

class A
{
   operator bool(){ return _b; }
   operator int(){ return _i; }
   int _i; bool _b;
}

A a;
bool b = a; // executes bool()
int i = a; // executes int()

Is there anything like C++ conversion operators in D? I have used 
conversion ops in C++ and may want to use a similar feature in D 
if available.



--rt

Re: function overload on full signature?

[Jonathan M Davis]

On Tuesday, November 13, 2012 22:37:37 Peter Alexander wrote:
> On Tuesday, 13 November 2012 at 21:34:28 UTC, Rob T wrote:
> > I'm wondering why overloading has been implemented to only
> > match on the argument list rather than the full signature which
> > includes the return type? I know I would use it if it was
> > available.
> 
> If it worked on return type, how would it decide the overload?
> 
> int foo() { ... }
> string foo() { ... }
> 
> void bar(int x) { ... }
> void bar(string y) { ... }
> 
> auto z = foo(); // what foo?
> bar(foo()); // what foo?

It would also screw up covariant return types if overloading too the return 
type into account. I believe that the _big_ reason though is simply because 
the type of the expression is determined by the return type, not what it's 
assigned to. For instance, with how the language works, the type of the right-
hand side of an assignment must always be determined independently of the type 
on the left, so the type of the expression on the right-hand side cannot 
depend on the type of the left. I suspect that it would complicate things 
considerably to try and make the return type have anything to do with function 
overloading. I'm sure that you could find it being discussions on it somewhere 
online for other C-based laguages though.

Regardless, I've never even heard of a language which tried to include the 
return type in overload resolution. It sounds like a huge complication to me.

- Jonathan M Davis

Re: function overload on full signature?

[Rob T]

On Tuesday, 13 November 2012 at 21:37:38 UTC, Peter Alexander 
wrote:

On Tuesday, 13 November 2012 at 21:34:28 UTC, Rob T wrote:
I'm wondering why overloading has been implemented to only 
match on the argument list rather than the full signature 
which includes the return type? I know I would use it if it 
was available.


If it worked on return type, how would it decide the overload?

int foo() { ... }
string foo() { ... }

void bar(int x) { ... }
void bar(string y) { ... }

auto z = foo(); // what foo?
bar(foo()); // what foo?


Don't use auto in such cases. If the compiler cannot resolve 
ambiguity, it will issue an error telling you that you must 
resolve the situation with explicit typing. I don't see a 
problem, because this is similar to the situation we already have 
when you err and create two functions with the same argument sig, 
the compiler cannot tell which function to use and issues an 
error with appropriate message.


--rt

Re: function overload on full signature?

[Peter Alexander]

On Tuesday, 13 November 2012 at 21:34:28 UTC, Rob T wrote:
I'm wondering why overloading has been implemented to only 
match on the argument list rather than the full signature which 
includes the return type? I know I would use it if it was 
available.


If it worked on return type, how would it decide the overload?

int foo() { ... }
string foo() { ... }

void bar(int x) { ... }
void bar(string y) { ... }

auto z = foo(); // what foo?
bar(foo()); // what foo?