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digitalmars.D.learn - Are named variadic arguments possible?

reply Alex <CppCoder gmail.com> writes:
Is it possible, using templates, tuples, or some other mechanism, to implement
named variadic arguments in D?

For example, I'd like to be able to do something like...
foo( 2, &bar, age : 10, status : "down");

and so forth.
Jan 04 2010
next sibling parent reply bearophile <bearophileHUGS lycos.com> writes:
Alex Wrote:


 Is it possible, using templates, tuples, or some other mechanism, to implement
named variadic arguments in D?


Do you need high performance for this code?
If not, what about using an associative array of Variant[string] as third
argument? It's not a nice solution.

Bye,
bearophile
Jan 05 2010
parent bearophile <bearophileHUGS lycos.com> writes:
 Do you need high performance for this code?
 If not, what about using an associative array of Variant[string] as third
argument? It's not a nice solution.


It's not a nice solution, but that's essentially what Python does in such
situation:
def foo(x, y, **kwds):


In D there are Typesafe Variadic Functions, you can use:
int sum(int[] ar ...) {
To define a function that takes an arbitrary number of ints or a dynamic array
of ints (and maybe fixed sized array of int too).
Another (weird! never used) alternative usage of the Typesafe Variadic
Functions is to build an object on the fly.

So that syntax may be extended to support:
void foo(Variant[string] kwds ...) {
}

I may show this silly idea in the main D newsgroup later today.

Bye,
bearophile
Jan 05 2010
prev sibling next sibling parent reply downs <default_357-line yahoo.de> writes:
Alex wrote:

 Is it possible, using templates, tuples, or some other mechanism, to implement
named variadic arguments in D?
 
 For example, I'd like to be able to do something like...
 foo( 2, &bar, age : 10, status : "down");
 
 and so forth.


Yes, with a small hack.

typedef int age_type;

age_type age(int i) { return cast(age_type) i; }

void foo(T...)(int i, Bar*, T t) {
  // Test T for age_type here.
}
Jan 05 2010
parent reply "Lars T. Kyllingstad" <public kyllingen.NOSPAMnet> writes:
downs wrote:

 Alex wrote:

 Is it possible, using templates, tuples, or some other mechanism, to implement
named variadic arguments in D?

 For example, I'd like to be able to do something like...
 foo( 2, &bar, age : 10, status : "down");

 and so forth.


 
 Yes, with a small hack.
 
 typedef int age_type;
 
 age_type age(int i) { return cast(age_type) i; }
 
 void foo(T...)(int i, Bar*, T t) {
   // Test T for age_type here.
 }



In D2 this won't be possible for much longer, since typedef is going 
away. But you can fake it:

   template Typedef(T, string name)
   {
       mixin("struct "~name~" { "~T.stringof~" x; alias x this; }");
   }

   mixin Typedef!(int, "age_type");
   age_type age(int i) { return age_type(i); }

If you're going to use it a lot, you can even automate it further:

   template NamedArg(T, string name)
   {
       mixin Typedef!(T, name~"_type");
       mixin(name~"_type "~name~"("~T.stringof~" t) {"
	~" return "~name~"_type(t); }");
   }

   mixin NamedArg!(int, "age");
   mixin NamedArg!(string, "status");

   foo(2, &bar, age(10), status("down"));

-Lars
Jan 06 2010
parent "Lars T. Kyllingstad" <public kyllingen.NOSPAMnet> writes:
Lars T. Kyllingstad wrote:

 downs wrote:

 Alex wrote:

 Is it possible, using templates, tuples, or some other mechanism, to 
 implement named variadic arguments in D?

 For example, I'd like to be able to do something like...
 foo( 2, &bar, age : 10, status : "down");

 and so forth.


 Yes, with a small hack.

 typedef int age_type;

 age_type age(int i) { return cast(age_type) i; }

 void foo(T...)(int i, Bar*, T t) {
   // Test T for age_type here.
 }


 
 
 In D2 this won't be possible for much longer, since typedef is going 
 away. But you can fake it:
 
   template Typedef(T, string name)
   {
       mixin("struct "~name~" { "~T.stringof~" x; alias x this; }");
   }
 
   mixin Typedef!(int, "age_type");
   age_type age(int i) { return age_type(i); }
 
 If you're going to use it a lot, you can even automate it further:
 
   template NamedArg(T, string name)
   {
       mixin Typedef!(T, name~"_type");
       mixin(name~"_type "~name~"("~T.stringof~" t) {"
     ~" return "~name~"_type(t); }");
   }
 
   mixin NamedArg!(int, "age");
   mixin NamedArg!(string, "status");
 
   foo(2, &bar, age(10), status("down"));


Forget that last part. I just realised you don't need the helper 
functions.. :)

   mixin Typedef!(int, "age");
   mixin Typedef!(int, "status");

   foo(2, &bar, age(10), status(down)); // should work

-Lars
Jan 06 2010
prev sibling parent reply Jacob Carlborg <doob me.com> writes:
On 1/5/10 08:50, Alex wrote:

 Is it possible, using templates, tuples, or some other mechanism, to implement
named variadic arguments in D?

 For example, I'd like to be able to do something like...
 foo( 2,&bar, age : 10, status : "down");

 and so forth.


You can try and see if you can modify this to do what you want: 
http://tango.pastebin.com/m2ae0ea3e it's named arguments but not variadic.
Jan 06 2010
parent reply Alex <CppCoder gmail.com> writes:
Sorry, I guess I wasn't clear.  What I want is to be able to do something like
the following:

void foo(...) {
  variadic [char[]] args = get_named_args(_arguments);
  foreach (k,v; args) {
    writefln("%s : %s", k, v);
  }
}

foo( name("john"), age(30), dob("6/24/1980") );

and have it print
name : john
age : 30
dob: 6/24/1980
Jan 12 2010
parent =?UTF-8?B?QWxpIMOHZWhyZWxp?= <acehreli yahoo.com> writes:
Alex wrote:

 Sorry, I guess I wasn't clear.  What I want is to be able to do something like
the following:
 
 void foo(...) {
   variadic [char[]] args = get_named_args(_arguments);
   foreach (k,v; args) {
     writefln("%s : %s", k, v);
   }
 }
 
 foo( name("john"), age(30), dob("6/24/1980") );
 
 and have it print
 name : john
 age : 30
 dob: 6/24/1980


Could opDispatch be useful in some way?

The following is nowhere clean. Additionally, VarArg.value must be some 
sort of 'any type' instead of 'string value' for int arguments like 30 
to work. Still... :)

// Worked with dmd 2.039
import std.stdio;

struct VarArg
{
     string key;
     string value;
}

struct VarArgGenerator
{
     VarArg opDispatch(string key, T)(T value)
     {
         return VarArg(key, value);
     }
}

VarArgGenerator arg;

void foo(VarArg[] args...)
{
     foreach (arg; args) {
         writefln("%s : %s", arg.key, arg.value);
     }
}

void main()
{
     foo(arg.name("john"),
         arg.age("30"),
         arg.dob("6/24/1980"));
}

Ali
Jan 12 2010