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digitalmars.D.learn - What do you use opDispatch for?

reply Philippe Sigaud <philippe.sigaud gmail.com> writes:
OK, I know opDispatch just appeared in DMD, but I remember a *huge* thread on
it, where people were jumping up and down waiting for it.

Me, I have no wonderful idea, though I feel some potential in it.
The only interesting use I found for now is making a class/struct extensible:

mixin template Extensible()
{
    auto opDispatch(string s, T...)(T ts)
    {
        mixin("return " ~ s ~ "(this, ts);");
    }
}

So given:

class C { 
    mixin Extensible;
}

and 

class D : C {} // (D inherits from C's extensibility).

If can 'add' methods to C and D by defining external functions: 

int foo(C c, int i) { return i;}
C c = new C;
c.foo(3); // becomes foo(c, 3).

It allows me to imitate what the compiler does for arrays:

"abc".toupper.reverse; // -> "CBA"

This functions can be templated, giving a new functionality to all extensible
classes:

string typeof(C)(C c) { return C.stringof;}

C build(C, T...)(C c, T t) if (is(C == class)) { return new C(t);}
C build(C, T...)(C c, T t) if (is(C == struct)) { return C(t);}

bool isSubtypeOf(C, D)(C c, D d) if (is(C : D)) { return true;}
bool isSubtypeOf(C, D)(C c, D d) if (!is(C : D)) { return false;}

etc.

So, is this a good idea or not? I use something like .build to map factory
functions on struct ranges, building another range of structs with different
values, but I do not _need_ mixin Extensible for this...

As for you, what are your experiences / projects with opDispatch?

Philippe
Mar 21 2010
next sibling parent BCS <none anon.com> writes:
Hello Philippe,


 OK, I know opDispatch just appeared in DMD, but I remember a *huge*
 thread on it, where people were jumping up and down waiting for it.
 
 Me, I have no wonderful idea, though I feel some potential in it. The
 only interesting use I found for now is making a class/struct
 extensible:
 




 
 As for you, what are your experiences / projects with opDispatch?
 



My unitted type uses it for it's value<->unit properties to get a single 
point of definition for each unit:
http://www.dsource.org/projects/scrapple/browser/trunk/units/si2.d

I've also been thinking of a way to build a compile time LINQ like program. 
If the comparison and boolean operators are overloadable, you could build 
prepared SQL queries from expressions at compile time.

using(myDatabase.tables.baz.bar) 
   foreach(Row!(int) row; select!("baz.foo").whare(for.a != someInt && baz.c 
== bar.c))
      do(row.foo);





 Philippe
 



... <IXOYE><
Mar 21 2010
prev sibling next sibling parent reply Philippe Sigaud <philippe.sigaud gmail.com> writes:
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On Sun, Mar 21, 2010 at 23:02, BCS <none anon.com> wrote:



 My unitted type uses it for it's value<->unit properties to get a single
 point of definition for each unit:
 http://www.dsource.org/projects/scrapple/browser/trunk/units/si2.d



and generate what you need. Good idea.
Why did you make OfType opDispatch a static function?




 I've also been thinking of a way to build a compile time LINQ like program.
 If the comparison and boolean operators are overloadable, you could build
 prepared SQL queries from expressions at compile time.

 using(myDatabase.tables.baz.bar)   foreach(Row!(int) row;
 select!("baz.foo").whare(for.a != someInt && baz.c == bar.c))
     do(row.foo);



If you have a limited number of methods like .whare, why use opDispach?
(Sorry if my question is naive).


  Philippe

--00032555990aa1abeb04826a5ba1
Content-Type: text/html; charset=ISO-8859-1
Content-Transfer-Encoding: quoted-printable

<div class=3D"gmail_quote">On Sun, Mar 21, 2010 at 23:02, BCS <span dir=3D"=
ltr">&lt;<a href=3D"mailto:none anon.com">none anon.com</a>&gt;</span> wrot=
e:<br><div>=A0</div><blockquote class=3D"gmail_quote" style=3D"margin: 0pt =
0pt 0pt 0.8ex; border-left: 1px solid rgb(204, 204, 204); padding-left: 1ex=
;">
My unitted type uses it for it&#39;s value&lt;-&gt;unit properties to get a=
 single point of definition for each unit:<br>
<a href=3D"http://www.dsource.org/projects/scrapple/browser/trunk/units/si2=
.d" target=3D"_blank">http://www.dsource.org/projects/scrapple/browser/trun=
k/units/si2.d</a><br>
<br></blockquote><div><br>Ah, I see, that&#39;s nice. You use the received =
string to feed another template and generate what you need. Good idea.<br>W=
hy did you make OfType opDispatch a static function?<br><br>=A0</div><block=
quote class=3D"gmail_quote" style=3D"margin: 0pt 0pt 0pt 0.8ex; border-left=
: 1px solid rgb(204, 204, 204); padding-left: 1ex;">

I&#39;ve also been thinking of a way to build a compile time LINQ like prog=
ram. If the comparison and boolean operators are overloadable, you could bu=
ild prepared SQL queries from expressions at compile time.<br>
<br>
using(myDatabase.tables.baz.bar) =A0 foreach(Row!(int) row; select!(&quot;b=
az.foo&quot;).whare(for.a !=3D someInt &amp;&amp; baz.c =3D=3D bar.c))<br>
 =A0 =A0 do(row.foo);<br></blockquote><div><br>If you have a limited number=
 of methods like .whare, why use opDispach? (Sorry if my question is naive)=
.<br><br><br>=A0 Philippe <br></div></div><br>

--00032555990aa1abeb04826a5ba1--
Mar 22 2010
parent BCS <none anon.com> writes:
Hello Philippe,


 On Sun, Mar 21, 2010 at 23:02, BCS <none anon.com> wrote:
 

 My unitted type uses it for it's value<->unit properties to get a
 single point of definition for each unit:
 http://www.dsource.org/projects/scrapple/browser/trunk/units/si2.d
 



 template
 and generate what you need. Good idea.
 Why did you make OfType opDispatch a static function?



So that "OfType.someunit(val)" becomes a free function.


 I've also been thinking of a way to build a compile time LINQ like
 program. If the comparison and boolean operators are overloadable,
 you could build prepared SQL queries from expressions at compile
 time.
 
 using(myDatabase.tables.baz.bar)   foreach(Row!(int) row;
 select!("baz.foo").whare(for.a != someInt && baz.c == bar.c))
 do(row.foo);
 



 opDispach? (Sorry if my question is naive).



The opDispatch gets used for the .a and .c bits. The above expression would 
get translated so that the !=/== expressions just collect values at runtime 
and pass them off as parameters of a prepared SQL statement whose string 
is built at compile time. For instance the above might result in the following 
string literal being used:

"select baz.foo from baz join bar where a = % and baz.c == bar.c" 

-- 
... <IXOYE><
Mar 22 2010
prev sibling parent Mike Parker <aldacron gmail.com> writes:
Philippe Sigaud wrote:

 
 
 As for you, what are your experiences / projects with opDispatch?
 
 Philippe



I was toying around with the idea of using it for a quick & dirty logger 
and came up with this:

**************************
import std.stdio;
import std.string;

struct Log
{
	File file;
		
	this(string filename)
	{
		file.open(filename, "w");
	}
	
	void opDispatch(string s, string f = __FILE__, uint line = __LINE__, 
S...)(S args)
	{		
		file.writeln("[", f, "   ", line, "][", toupper(s), "]", args);		
	}
}
**************************

And then use it like this:

log.error("This is an error.");
log.info("Wouldn't you like to know something?");
debug log.trace("Hey, this is happening now.");
version(AudioStats) log.audio("Some stats: ", foo, bar);


To me, this is much cleaner than the alternative, which would be 
something like:

log.write("ERROR", "This is an error");
Mar 22 2010