Search
Prettier iterator implementations in D?
Oct 20, 2006
Bill Baxter
Oct 20, 2006
Bill Baxter
Oct 20, 2006
Alexander Panek
Oct 20, 2006
Bill Baxter
Oct 21, 2006
Alexander Panek
Oct 20, 2006
Reiner Pope
Oct 20, 2006
Reiner Pope
Oct 20, 2006
Bill Baxter
Oct 20, 2006
David Medlock
Oct 20, 2006
Bill Baxter
Oct 20, 2006
Reiner Pope
```Forgetting about 'foreach' for the moment, I'd like to talk about the other end of things: opApply().

This code in D:
int opApply(int delegate(inout uint) dg)
{   int result = 0;

for (int i = 0; i < array.length; i++)
{
result = dg(array[i]);
if (result)
break;
}
return result;
}

seems to be rendered in Ruby-ish D as something like:

void opApply( ) |inout uint|
{
for (int i = 0; i < array.length; i++)
{
yield array[i];
}
}

Is there some way to make the real D one look as clean as the Rubified one?

Main differences:
1) need to explicitly declare a delegate,
2) need to fiddle with return values.

For 1) I'm thinking if there were only some more compact way to declare a delegate it would be golden.

How about this: with function literals, if you leave out the keyword 'delegate' or 'function' you get a delegate.  I.e.

int delegate(long c) { return 6 + b; }

is equivalent to

int (long c) { return 6 + b; }

So it seems logical that as a variable declaration:

int delegate(long c) dg;

could also be made to be equivalent to

int (long c) dg;

In that case we could say in D:

void opApply(void (inout uint) dg)
{
for (int i = 0; i < array.length; i++)
{
dg(array[i]);
}
}

(assuming that pesky 'int return' business can also be sorted out).

The void can also be omitted frequently in D.  So for a void-returning delegate that could even be shortened to:

void opApply( (inout uint) block)
{
for (int i = 0; i < array.length; i++)
{
block(array[i]);
}
}

Now *that* is something I wouldn't mind staring at at 3am after a long day coding. :-)

--bb
```
```Shorter and more beautiful is good, and has merits indeed...

But sometimes clarity comes as a price, and that's not so good.  If I were new to D, this:

void opApply( (inout uint) block)

Would look like a funky sort of cast.  That said, this:

void opApply( ) |inout uint|

Is at least clear, if not horrendously ugly to my eyes.

-[Unknown]

> Forgetting about 'foreach' for the moment, I'd like to talk about the other end of things: opApply().
>
> This code in D:
>     int opApply(int delegate(inout uint) dg)
>     {   int result = 0;
>
>     for (int i = 0; i < array.length; i++)
>     {
>         result = dg(array[i]);
>         if (result)
>         break;
>     }
>     return result;
>     }
>
> seems to be rendered in Ruby-ish D as something like:
>
>     void opApply( ) |inout uint|
>     {
>     for (int i = 0; i < array.length; i++)
>     {
>         yield array[i];
>     }
>     }
>
> Is there some way to make the real D one look as clean as the Rubified one?
>
> Main differences:
> 1) need to explicitly declare a delegate,
> 2) need to fiddle with return values.
>
> For 1) I'm thinking if there were only some more compact way to declare a delegate it would be golden.
>
> How about this: with function literals, if you leave out the keyword 'delegate' or 'function' you get a delegate.  I.e.
>
>      int delegate(long c) { return 6 + b; }
>
> is equivalent to
>
>      int (long c) { return 6 + b; }
>
> So it seems logical that as a variable declaration:
>
>      int delegate(long c) dg;
>
> could also be made to be equivalent to
>
>      int (long c) dg;
>
> In that case we could say in D:
>
>     void opApply(void (inout uint) dg)
>     {
>     for (int i = 0; i < array.length; i++)
>     {
>         dg(array[i]);
>     }
>     }
>
> (assuming that pesky 'int return' business can also be sorted out).
>
> The void can also be omitted frequently in D.  So for a void-returning delegate that could even be shortened to:
>
>     void opApply( (inout uint) block)
>     {
>     for (int i = 0; i < array.length; i++)
>     {
>         block(array[i]);
>     }
>     }
>
> Now *that* is something I wouldn't mind staring at at 3am after a long day coding. :-)
>
> --bb
```
```Unknown W. Brackets wrote:
> Shorter and more beautiful is good, and has merits indeed...
>
> But sometimes clarity comes as a price, and that's not so good.  If I were new to D, this:
>
> void opApply( (inout uint) block)
>
> Would look like a funky sort of cast.

Well if you're new to D then there's a whole *lot* of things that are going to look funky.  Like  foo!(int)() -- looks like some kind of negation of a cast of nothing.  Funky!

> That said, this:
>
> void opApply( ) |inout uint|
>
> Is at least clear, if not horrendously ugly to my eyes.

I agree that it's not so ugly, but I don't think it's any clearer unless you've been reading Ruby code.  And it doesn't have any analogues anywhere else in D.  At least leaving out the keyword 'delegate' and the keyword 'void' is already sanctioned practice elsewhere in D.  Being able to leave it out on variable declarations actually makes things *more* consistent.

--bb
```
```I know Ruby, and it's loops are very cool, indeed. But this is D, and as a full blown *system and application programming language*, you can't compare it to Ruby. I don't think syntactic sugar should be added too hasty. Apart from that I kinda like the delegate / function ptr syntax as it is.

Alex

On Fri, 2006-10-20 at 12:41 +0900, Bill Baxter wrote:
> Forgetting about 'foreach' for the moment, I'd like to talk about the other end of things: opApply().
>
> This code in D:
>      int opApply(int delegate(inout uint) dg)
>      {   int result = 0;
>
> 	for (int i = 0; i < array.length; i++)
> 	{
> 	    result = dg(array[i]);
> 	    if (result)
> 		break;
> 	}
> 	return result;
>      }
>
> seems to be rendered in Ruby-ish D as something like:
>
>      void opApply( ) |inout uint|
>      {
> 	for (int i = 0; i < array.length; i++)
> 	{
> 	    yield array[i];
> 	}
>      }
>
> Is there some way to make the real D one look as clean as the Rubified one?
>
> Main differences:
> 1) need to explicitly declare a delegate,
> 2) need to fiddle with return values.
>
> For 1) I'm thinking if there were only some more compact way to declare a delegate it would be golden.
>
> How about this: with function literals, if you leave out the keyword 'delegate' or 'function' you get a delegate.  I.e.
>
>       int delegate(long c) { return 6 + b; }
>
> is equivalent to
>
>       int (long c) { return 6 + b; }
>
> So it seems logical that as a variable declaration:
>
>       int delegate(long c) dg;
>
> could also be made to be equivalent to
>
>       int (long c) dg;
>
> In that case we could say in D:
>
>      void opApply(void (inout uint) dg)
>      {
> 	for (int i = 0; i < array.length; i++)
> 	{
> 	    dg(array[i]);
> 	}
>      }
>
> (assuming that pesky 'int return' business can also be sorted out).
>
> The void can also be omitted frequently in D.  So for a void-returning delegate that could even be shortened to:
>
>      void opApply( (inout uint) block)
>      {
> 	for (int i = 0; i < array.length; i++)
> 	{
> 	    block(array[i]);
> 	}
>      }
>
> Now *that* is something I wouldn't mind staring at at 3am after a long day coding. :-)
>
> --bb

```
```Alexander Panek wrote:
> I know Ruby, and it's loops are very cool, indeed. But this is D, and as
> a full blown *system and application programming language*, you can't
> compare it to Ruby.

What is D if not "the speed of C++ with the ease of Ruby"?

> I don't think syntactic sugar should be added too
> hasty.

I agree.  That's why plenty of discussion is needed.  So start poking holes!

> Apart from that I kinda like the delegate / function ptr syntax
> as it is.

And you could still use it as is if you like, because the suggestion is to make 'delegate' optional, not mandate it's removal.

About whether sugar is warranted here:  it seems Walter sees this delegate mechanism as becoming *the* primary technique for iteration in D.  That being the case, it should be as easy to read and write as possible.   Being easier to use than C++ iterators is the primary reason he gives for liking it, in fact.  So let's figure out how to make absolutely as simple as possible.

Right now, say you want to write a generic array iterator factory function 'traverser', which you do because it allows you to do this:

foreach(int i; iarray.traverser()) {
...
}
to iterate in some custom way over the elements of any array.

Here is the signature for that now (returns a delegate that takes a delegate parameter):

int delegate(int delegate(inout typeof(ArrayT[0])))
traverser(ArrayT)(inout ArrayT array)
{
}
...

The int delegate(int delegate ...) business is just too verbose to grok easily.   If I change the ints to voids and drop the 'delegate':

(( inout typeof(ArrayT[0]) )) reversed(ArrayT)(inout ArrayT array)

I think that's easier to look at if for no other reason than being shorter.  But I'll admit it probably is more mind boggling at first. But it's not a stretch to say most everybody could get used to reading

(int) somefunc() {...}

as a function that returns a void-returning, int-taking delegate.  Just think of the (int) as a lone argument list ["takes an int"] and it's pretty clear.
And in that light it's not hard to see the extra sets of parens (( )) mean 'a void returning delegate that takes a void returning delegate'. At some point (( type )) just becomes second nature as the basic return signature for an iterator factory.  It's easy to see once you know to look for it, because its so short and (( )) stands out if spaced properly.

Another alternative is standard aliases for those complicated types:

template Types(ArgT) {
static if( is (ArgT[0]) ) {
alias typeof(ArgT[0]) elem_t;
}
else static if( is(ArgT.elem_t) ) {
alias ArgT.elem_t elem_t;
}
else {
alias ArgT elem_t;
}
alias int delegate(int delegate(inout elem_t)) iter_t;
// this works for anything that is either array-like or has an
// elem_t alias.
}

Then you can use those aliases like:

Types!(ArrayT).iter_t  reversed(ArrayT)(inout ArrayT array)
{ ... }

that still looks kinda klunky :-/, but maybe better.  At least it's short enough to fit on one line.  But it just replaces having to know what (( )) means with having to know what Types!().iter_t means.  So I'm not sure it's really better.

Well at least it is something that works now!

--bb

```
```The proposal looks good (as long as it creates no syntactical ambiguities) but if we had variadic template parameters (see http://www.generic-programming.org/~dgregor/cpp/variadic-templates.html for a good version for C++) then you could do it much more easily with templates:

template IterFunc(type ...)
{
alias int delegate(type ...) IterFunc;
}

and then you could just declare your iterators:

int opApply(IterFunc!(int) dg)

template IndexedIterFunc(type ...)
{
alias int delegate(size_t index, type ...) IndexedIterFunc;
}

or if the return status was instead the first function parameter (to allow for return values, not void):

template NewIterFunc(retVal, type ...)
{
alias retVal delegate(out int status, type ...) NewIterFunc;
}

Unfortunately, this wouldn't handle 'inout' parameters, since you can't instantiate a template with an inout parameter...

Cheers,

Reiner
```
```Alexander Panek wrote:
> I know Ruby, and it's loops are very cool, indeed. But this is D, and as
> a full blown *system and application programming language*, you can't
> compare it to Ruby.
The fundamental difference between D and Ruby is static typing. This leads on to D being more suitable for compilation than Ruby, and thus its speed, type-safety and meta-programming, and D not as concisely handling some aspects of reflection. Other than that, they are the leading languages in their fields (interpreted and compiled languages), and can learn a lot from each other.

Cheers,

Reiner
```
```Bill Baxter wrote:
> Forgetting about 'foreach' for the moment, I'd like to talk about the
> other end of things: opApply().
> <snip>

You could always use this handy template which automatically exposes any array or assoc-array in a class you want, with a single mixin.

class Foo
{
Bar[]  bar;

mixin applyThis!(Bar,array);
}

Bar b = new Bar();
foreach( Bar b; foo ) {...}

-DavidM

```
```David Medlock wrote:
> Bill Baxter wrote:
>
>> Forgetting about 'foreach' for the moment, I'd like to talk about the other end of things: opApply().
>> <snip>
>
>
> You could always use this handy template which automatically exposes any array or assoc-array in a class you want, with a single mixin.
> [...]

Wow.  That is very cool.  D is spiffy. :-)
But I still think the core syntax should be clean.
Makes me wonder, though, if C++ had mixins like that, maybe proper iterators wouldn't be so hard to do correctly there either?  Just a matter of adding in a line:
iteratorThis!(...)

--bb
```
```Reiner Pope wrote:
> The proposal looks good (as long as it creates no syntactical ambiguities) but if we had variadic template parameters (see http://www.generic-programming.org/~dgregor/cpp/variadic-templates.html for a good version for C++) then you could do it much more easily with templates:
>
> template IterFunc(type ...)
> {
>     alias int delegate(type ...) IterFunc;
> }

Ooh, I missed this "Implicit Template Properties" thing.  That's an improvement over what I had even without the variadic bit.  Just means I need an IterFunc1, IterFunc2,... etc.

>
> and then you could just declare your iterators:
>
> int opApply(IterFunc!(int) dg)
> [...]
> Unfortunately, this wouldn't handle 'inout' parameters, since you can't instantiate a template with an inout parameter...

Ah, that's a pity.  Is that a spec limitation or just something still on the "todo" list?

--bb
```
« First   ‹ Prev
1 2