www.digitalmars.com         C & C++   DMDScript  

digitalmars.D - suggestion: default values

reply bobef <adas adasd.asdasd> writes:
It would be nice if there was some character like $ or something, if used as
function argument, the default value for this argument to be used. What I mean
is this:

void myfun(int a,char[] b,bool d=false,int e=55){}

void main()
{
    myfun(5,"5",$,$); // which should act like myfun(5,"5",false,55);
}
Apr 19 2007
next sibling parent reply Gregor Richards <Richards codu.org> writes:
bobef wrote:

 It would be nice if there was some character like $ or something, if used as
function argument, the default value for this argument to be used. What I mean
is this:
 
 void myfun(int a,char[] b,bool d=false,int e=55){}
 
 void main()
 {
     myfun(5,"5",$,$); // which should act like myfun(5,"5",false,55);
 }


Probably a better example of why this would be helpful:

void myfunction(int a, char[] b, Foo veryComplicatedThing = default, 
bool d = true) {}

void main()
{
     myfun(5,"5",$,false);
}
Apr 19 2007
parent Daniel Keep <daniel.keep.lists gmail.com> writes:
Gregor Richards wrote:

 bobef wrote:

 It would be nice if there was some character like $ or something, if
 used as function argument, the default value for this argument to be
 used. What I mean is this:

 void myfun(int a,char[] b,bool d=false,int e=55){}

 void main()
 {
     myfun(5,"5",$,$); // which should act like myfun(5,"5",false,55);
 }


 
 Probably a better example of why this would be helpful:
 
 void myfunction(int a, char[] b, Foo veryComplicatedThing = default,
 bool d = true) {}
 
 void main()
 {
     myfunction(5,"5",$,false);
 }


We already have the perfect keyword for this: default.

void main()
{
    myfunction(5, "5", default, false);
}

A little more verbose, but pretty self-explanatory as to what it does.
Also, keep in mind that "$" already means array length inside of a
slice; I think it would be a bad idea to overload it with a completely
different meaning.

On a side note, you can do something vaguely similar using templated
functions and "typedef int DefaultType; const Default =
DefaultType.ini;" like so:

const int e_default = 55;

void myfunction(T)(T _e=Default)
{
    static if( is( T == DefaultType ) )
       e = e_default;
    else
       e = e;
}

Or somesuch; been a while since I did that :)

	-- Daniel

-- 
int getRandomNumber()
{
    return 4; // chosen by fair dice roll.
              // guaranteed to be random.
}

http://xkcd.com/

v2sw5+8Yhw5ln4+5pr6OFPma8u6+7Lw4Tm6+7l6+7D
i28a2Xs3MSr2e4/6+7t4TNSMb6HTOp5en5g6RAHCP  http://hackerkey.com/
Apr 19 2007
prev sibling parent reply "David B. Held" <dheld codelogicconsulting.com> writes:
bobef wrote:

 It would be nice if there was some character like $ or something, if used as
function argument, the default value for this argument to be used. What I mean
is this:
 
 void myfun(int a,char[] b,bool d=false,int e=55){}
 
 void main()
 {
     myfun(5,"5",$,$); // which should act like myfun(5,"5",false,55);
 }


While some languages provide named parameters, they tend to be langs 
which encourage functions with many arguments.  If you find you are 
creating functions like this, you should first ask yourself if this is 
the best way.  If you decide that it is, one pattern to use is analogous 
to the Builder pattern:

struct myFunArgs
{
     int a;
     char[] b;
     bool d = false;
     int e = 55;
}

void myfun(myFunArgs args) { ... }

myFunArgs args;
args.a = 5; args.b = "5";
myfun(args);

This is the *very* Poor Man's named parameters mechanism, but if you 
have enough parameters to need named ones, then this probably isn't at 
all burdensome (and may increase clarity).

Another alternative, if you are creating a library function that you 
want to be *very* friendly, you can create a variadic type-safe template 
function and parse the args inside the function (though this doesn't 
work very well with args of the same type).  Of course, you can use the 
Perl convention of name, value pairs to make it completely general, but 
that's not very elegant or idiomatic D style.

Dave
Apr 19 2007
parent Leandro Lucarella <llucax gmail.com> writes:
David B. Held, el 19 de abril a las 23:42 me escribiste:

 bobef wrote:

It would be nice if there was some character like $ or something, if used as
function argument, the default value for this argument to be used. What I mean
is this:
void myfun(int a,char[] b,bool d=false,int e=55){}
void main()
{
    myfun(5,"5",$,$); // which should act like myfun(5,"5",false,55);
}


 
 While some languages provide named parameters, they tend to be langs which
encourage functions with many arguments.  If you find you are creating
functions like this, you 
 should first ask yourself if this is the best way.  If you decide that it is,
one pattern to use is analogous to the Builder pattern:
 
 struct myFunArgs
 {
     int a;
     char[] b;
     bool d = false;
     int e = 55;
 }
 
 void myfun(myFunArgs args) { ... }
 
 myFunArgs args;
 args.a = 5; args.b = "5";
 myfun(args);
 
 This is the *very* Poor Man's named parameters mechanism, but if you have
enough parameters to need named ones, then this probably isn't at all
burdensome (and may increase 
 clarity).
 
 Another alternative, if you are creating a library function that you want to
be *very* friendly, you can create a variadic type-safe template function and
parse the args 
 inside the function (though this doesn't work very well with args of the same
type).  Of course, you can use the Perl convention of name, value pairs to make
it completely 
 general, but that's not very elegant or idiomatic D style.


Another way is to use partial function aplication:

void myfun(int a,char[] b,bool d=false,int e=55){}

void partial_myfun(int a,char[] b,int e=55)
{
	myfun(a, b, false, e);
}

partial_myfun(5, "5", 65); // myfun(5, "5", false, 65);

I think template black-magic could help to make this a little shorter to
use (maybe boost has something like this?) and well, with a little
reflection (for default value extraction), you can have a complete
automated partial function application, I think.

-- 
LUCA - Leandro Lucarella - Usando Debian GNU/Linux Sid - GNU Generation
------------------------------------------------------------------------
E-Mail / JID:     luca lugmen.org.ar
GPG Fingerprint:  D9E1 4545 0F4B 7928 E82C  375D 4B02 0FE0 B08B 4FB2 
GPG Key:          gpg --keyserver pks.lugmen.org.ar --recv-keys B08B4FB2
------------------------------------------------------------------------
La esperanza es una amiga que nos presta la ilusión.
Apr 20 2007