List comprehensions in D?

[David Medlock]

Oskar Linde wrote:
> David Medlock wrote:
> 
>> David Medlock wrote:
>>
>>> My meager attempts to clone some list functionality in Python:
>>>
>>> r = [ y for y in array if y > 10 ];
>>
>> <snip>
>>
>>> Pretty cool, IMO.
> 
> 
> Yes, the new delegate syntax is very convenient. The above functions are 
> also in my std.array proposal (under different names). Your update 
> function with an inout argument instead of my suggested doMap with a 
> pure functional argument is interesting:
> 
> arr.update((inout int x){ x++; });
> vs
> arr.doMap((int x){ return x+1; });
> 
> A smart implementation would be able to support both versions in one 
> function. I wounder if that is appropriate. It would be helpful if there 
> were any way to tell if the arguments of a delegate type were 
> in/out/inout. I guess some .mangleof hackery could help there.

I'm not sure, but since arrays are by reference I would prefer to make 
update functions named differently (sort and reverse still bite me on 
occasion).

> 
>> in addition,
>>
>> If you consider that arrays are just machine-optimized versions of 
>> maps(or graphs) with the allowed index as integers between 0 and N,
>> then passing a predicate function on those indexes makes some sense.
>>
>> for containers: (depending on your view of the syntax)
>>
>> T[]  opIndex( bool delegate(T) dg ) { return items.where( dg ); }
>> T[]  opIndexAssign( void delegate( inout T) dg ){ items.update( dg ); }
> 
> 
> In my view of the syntax, opIndex(bool delegate) should return a view, 
> rater then a new array with copied elements, 

Do you mean a container which transforms the elements passed through 
opIndex using the delegate?  That could be an issue with the delegate 
going out of scope.

Otherwise I don't see how you could do it without making a new array.

similar to how a slice of
> an array returns a view rather than copied elements. A combined 
> select/update would also be cool:
> 
> void opIndexAssign(T delegate(T) updater, bool delegate(T) selecter)
> 
> /Oskar

Yes combined functionality is nice and getting close to the list 
comprehensions i covet from python.

I also toyed with the idea of using the (just added) opIn operator.

int[] result = container in (int a) {return a>5;};

But this seemed too 'out of the ordinary'.

PS. I will be glad to see your array lib in phobos.

-DavidM