Re: 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.