A "general" tag

[Xan]

Uf!, it's more than I can process....
It's really a **complicated** thing to do that in D.

On Monday, 16 April 2012 at 07:50:28 UTC, Denis Shelomovskij
wrote:
> 15.04.2012 0:31, Xan написал:
>> On Saturday, 14 April 2012 at 19:40:06 UTC, Aleksandar 
>> Ružičić wrote:
>>> On Saturday, 14 April 2012 at 19:17:52 UTC, Xan wrote:
>>>> Hi,
>>>>
>>>> I try to translate a script I wrote in Fantom 
>>>> [www.fantom.org]. In my
>>>> script, I have a type "Tag" defined as a triple of:
>>>> - String (the name of the tag),
>>>> - Type (the type of the tag: could be Str, Date, Int, etc.)
>>>> - Obj (the value of the tag; Fantom has Objects of Top-Class 
>>>> hierachy).
>>>>
>>>> (normally the tag has Type = Obj.Type, but you can manually 
>>>> set).
>>>>
>>>> For example,
>>>> you could have:
>>>> (name, Str#, "John")
>>>>
>>>> or
>>>>
>>>> (date, Date#, 2011-09-02)
>>>>
>>>>
>>>> (# is the Fantom way for specifying type: Str# is the 
>>>> sys::Str type)
>>>>
>>>>
>>>> Is there any way for emulating this? My main trouble is how 
>>>> to define
>>>> Type and Object in D.
>>>>
>>>> Thanks in advance,
>>>> Xan.
>>>>
>>>> PS: Please, be patient, I'm a newbee.
>>>
>>>
>>> For "Type" look at enum (http://dlang.org/enum.html) and for 
>>> "Object"
>>> look at std.variant 
>>> (http://dlang.org/phobos/std_variant.html).
>>>
>>> And since Variant can tell you what type it contains you 
>>> might no
>>> longer need that "Type" parameter.
>>
>> I think it's not what I expect. Can I have a generic object 
>> type?
>> Something like an assigment like:
>>
>> Any a
>>
>>
>> ?
>>
>> With templates?
>>
>> Please, guide me. I'm a newbee
>>
>
> What you are looking for is a boxing.
> http://en.wikipedia.org/wiki/Boxing_(computer_science)#Boxing
>
> D doesn't support auto boxing/unboxing. For a reason see, e.g. 
> this thread:
> http://forum.dlang.org/thread/ckoaum$1lbg$1@digitaldaemon.com
> http://forum.dlang.org/thread/cr7njl$18j3$1@digitaldaemon.com
>
> There was std.boxer module, but it was deprecated and removed, 
> this is the last version before removal:
> https://github.com/D-Programming-Language/phobos/blob/c20d454d63861a0c4bab647b37c01b0dd981a3f8/std/boxer.d
>
> std.variant is really what you are looking for. See example:
> ---
> import std.stdio;
> import std.variant;
> import std.string: format;
>
> struct Tag {
>     string name;
>     Variant entity;
> }
>
> class C {
>     int i;
>
>     this(int i) { this.i = i; }
>
>     string toString() { return format("C(i: %s)", i); }
> }
>
> struct SmallStruct {
>     int a;
> }
>
> struct Huge {
>     real a, b, c, d, e, f, g;
> }
>
> void writeTag(Tag tag) {
>     writeln(tag);
>     with(tag.entity)
>         if(auto intVal = peek!int()) {
>             writeln("  Contains int: ", *intVal);
>             // Arithmetic is supported too
>             writeln("  + 3: ", tag.entity + 3);
>             writeln("  * 2: ", tag.entity * 2);
>         } else if(auto hugeVal = peek!Huge())
>             // Don't use *hugeVal for now, there is a bug in 
> peek
>             writeln("  Contains Huge struct: ", 
> tag.entity.get!Huge());
>         else if(auto classInfo = cast(TypeInfo_Class)type)
>             writefln("  Contains class %s: %s", classInfo.name, 
> get!Object());
>         else if(auto structInfo = cast(TypeInfo_Struct)type)
>             writefln("  Contains struct %s: %s", 
> structInfo.name, tag.entity);
>         // else if etc.
> }
>
> void main() {
>     writeTag(Tag("tag1", Variant(12)));
>     writeTag(Tag("tag2", Variant("str")));
>     writeTag(Tag("tag3", Variant(["str1", "str2"])));
>     writeTag(Tag("tag4", Variant(new C(17))));
>     writeTag(Tag("tag4", Variant(SmallStruct(3))));
>     // Variant isn't enough to hold Huge so a copy
>     // will be accocated in GC heap.
>     // Yes, this isn't documented yet and `peek` will give you 
> garbage
>     // for this case because of a bug.
>     writeTag(Tag("tag4", Variant(Huge(1.1, 2.2, 3.3, 4.4, 5.5, 
> 6.6, 7.7))));
> }
> ---
>
> std.variant has some bugs for now but is usable:
> http://d.puremagic.com/issues/buglist.cgi?query_format=advanced&short_desc=variant&bug_status=NEW&bug_status=ASSIGNED&bug_status=REOPENED&short_desc_type=allwords
>
> Templates are usable in other cases, not this.