Hi D gurus,

is there a way to obtain parameter names within the function body? I am particularly interested in variadic functions. Something like:

void myfun(T...)(T x){
    foreach(i, arg; x)
        writeln(i, " : ", arg);
}

void main(){
    myfun(a = 2, b = "two", c = 2.0);
}

// should print
a : 2
b : two
c : 2.0

Thanks in advance

Loving the mixins and tuples

On 3/16/16 4:24 PM, data pulverizer wrote:
> Hi D gurus,
>
> is there a way to obtain parameter names within the function body? I am
> particularly interested in variadic functions. Something like:
>
> void myfun(T...)(T x){
>      foreach(i, arg; x)
>          writeln(i, " : ", arg);
> }
>
> void main(){
>      myfun(a = 2, b = "two", c = 2.0);
> }

This isn't valid code. The name of the parameters is x[0], x[1], and x[2].

You could do something like:

myfun("a", 2, "b", "two", "c", 2.0);

and process it properly.

-Steve

On Wednesday, 16 March 2016 at 20:24:38 UTC, data pulverizer wrote:
> Hi D gurus,
>
> is there a way to obtain parameter names within the function body? I am particularly interested in variadic functions. Something like:
>
> void myfun(T...)(T x){
>     foreach(i, arg; x)
>         writeln(i, " : ", arg);
> }
>
> void main(){
>     myfun(a = 2, b = "two", c = 2.0);
> }
>
> // should print
> a : 2
> b : two
> c : 2.0
>
> Thanks in advance
>
> Loving the mixins and tuples

I try to anticipate the reason why you want this. As said in a previous answer you can access to an individual element by using the array syntax but also _param_<X>, with X the index of the parameter:

void myfun(T...)(T x)
{
    import std.traits; import std.stdio;
    writeln(ParameterIdentifierTuple!(myfun!T));
    writeln(_param_0);
    writeln(_param_1);
    writeln(_param_2);
}

void main()
{
    int a=1,b=2,c=3;
    myfun(a,b,c);
}

On Wednesday, 16 March 2016 at 20:24:38 UTC, data pulverizer wrote:
> Hi D gurus,
>
> is there a way to obtain parameter names within the function body? I am particularly interested in variadic functions. Something like:
>
> void myfun(T...)(T x){
>     foreach(i, arg; x)
>         writeln(i, " : ", arg);
> }
>
> void main(){
>     myfun(a = 2, b = "two", c = 2.0);
> }
>
> // should print
> a : 2
> b : two
> c : 2.0
>
> Thanks in advance
>
> Loving the mixins and tuples

You can do it precisely like that if the variables/symbols you pass as (template) arguments are properly declared first.

http://dpaste.dzfl.pl/0b452efeaaab


void printVars(Args...)()
if (Args.length > 0)
{
    import std.stdio : writefln;

    foreach (i, arg; Args) {
        writefln("%s\t%s:\t%s", typeof(Args[i]).stringof, Args[i].stringof, arg);
    }
}

void main() {
    int abc = 3;
    string def = "58";
    float ghi = 3.14f;
    double jkl = 3.14;

    printVars!(abc,def,ghi,jkl)();
}

On Wednesday, 16 March 2016 at 20:53:42 UTC, JR wrote:
> On Wednesday, 16 March 2016 at 20:24:38 UTC, data pulverizer wrote:
>> Hi D gurus,
>>
>> is there a way to obtain parameter names within the function body? I am particularly interested in variadic functions. Something like:
>>
>> void myfun(T...)(T x){
>>     foreach(i, arg; x)
>>         writeln(i, " : ", arg);
>> }
>>
>> void main(){
>>     myfun(a = 2, b = "two", c = 2.0);
>> }
>>
>> // should print
>> a : 2
>> b : two
>> c : 2.0
>>
>> Thanks in advance
>>
>> Loving the mixins and tuples
>
> You can do it precisely like that if the variables/symbols you pass as (template) arguments are properly declared first.
>
> http://dpaste.dzfl.pl/0b452efeaaab
>
>
> void printVars(Args...)()
> if (Args.length > 0)
> {
>     import std.stdio : writefln;
>
>     foreach (i, arg; Args) {
>         writefln("%s\t%s:\t%s", typeof(Args[i]).stringof, Args[i].stringof, arg);
>     }
> }
>
> void main() {
>     int abc = 3;
>     string def = "58";
>     float ghi = 3.14f;
>     double jkl = 3.14;
>
>     printVars!(abc,def,ghi,jkl)();
> }

That's brilliant! Thanks JR

On Wednesday, 16 March 2016 at 20:43:09 UTC, jkpl wrote:
> I try to anticipate the reason why you want this. [...]

I use something *kinda* sort of similar in my toy project to print all fields of a struct, for debugging purposes when stuff goes wrong. Getting the names of the member variables is crucial then.

http://dpaste.dzfl.pl/748c4dd97de6

On Wednesday, 16 March 2016 at 21:05:43 UTC, JR wrote:
> On Wednesday, 16 March 2016 at 20:43:09 UTC, jkpl wrote:
>> I try to anticipate the reason why you want this. [...]
>
> I use something *kinda* sort of similar in my toy project to print all fields of a struct, for debugging purposes when stuff goes wrong. Getting the names of the member variables is crucial then.
>
> http://dpaste.dzfl.pl/748c4dd97de6

That's a nice learning piece. I think "with" is cool, reminds me of a nice R feature.

On Wednesday, 16 March 2016 at 20:53:42 UTC, JR wrote:
>
>
> void printVars(Args...)()
> if (Args.length > 0)
> {
>     import std.stdio : writefln;
>
>     foreach (i, arg; Args) {
>         writefln("%s\t%s:\t%s", typeof(Args[i]).stringof, Args[i].stringof, arg);
>     }
> }
>
> void main() {
>     int abc = 3;
>     string def = "58";
>     float ghi = 3.14f;
>     double jkl = 3.14;
>
>     printVars!(abc,def,ghi,jkl)();
> }

Interesting, any idea if it is possible to do assignment within template.. Either:

printVars!(int abc=5,string def="58")();
or something like
printVars!("abc","def",ghi)(5,"58");

On Thursday, 17 March 2016 at 11:52:13 UTC, Edwin van Leeuwen wrote:
> On Wednesday, 16 March 2016 at 20:53:42 UTC, JR wrote:
>>
>>
>> void printVars(Args...)()
>> if (Args.length > 0)
>> {
>>     import std.stdio : writefln;
>>
>>     foreach (i, arg; Args) {
>>         writefln("%s\t%s:\t%s", typeof(Args[i]).stringof, Args[i].stringof, arg);
>>     }
>> }
>>
>> void main() {
>>     int abc = 3;
>>     string def = "58";
>>     float ghi = 3.14f;
>>     double jkl = 3.14;
>>
>>     printVars!(abc,def,ghi,jkl)();
>> }
>
> Interesting, any idea if it is possible to do assignment within template.. Either:
>
> printVars!(int abc=5,string def="58")();
> or something like
> printVars!("abc","def",ghi)(5,"58");

What would the use-cases for those be?

I don't think the first is valid grammar, and I'm not sure what you want the second to do. Resolve symbols by string literals of their names? That might need a string mixin as they wouldn't be in scope when in the called template function, but I've never tried it.

You *can* cook up something that modifies the values of variables you pass in -- like modifyVars!(abc,def,ghi)("asdf", 123, 3.14) -- but you just might be better off with runtime ref parameters then.

On Thursday, 17 March 2016 at 13:53:00 UTC, JR wrote:
>>
>> Interesting, any idea if it is possible to do assignment within template.. Either:
>>
>> printVars!(int abc=5,string def="58")();
>> or something like
>> printVars!("abc","def",ghi)(5,"58");
>
> What would the use-cases for those be?
>
> I don't think the first is valid grammar, and I'm not sure what you want the second to do. Resolve symbols by string literals of their names? That might need a string mixin as they wouldn't be in scope when in the called template function, but I've never tried it.

Both use cases are when you want a named parameter, without having to assign it first. I know the first is not valid grammar, was just wondering if you might be smarter than me and see a way to make it valid :)

Second one is another possible alternative that I have been thinking about.

Basically, say I want to have the named (optional) parameters x and y. In your initial example I would be required to do:

```
int x = 1;
string y = "2";
doSomethingWithNamedPars!(x,y)();
```

I just hoped to shorten that to a one liner similar to:

```
doSomethingWithNamedPars!(x=1,y="2")();
```

or alternatively

```
doSomethingWithNamedPars!("x","y")(1,"2");
```

(where doSomethingWithNamedPars's behaviour depends on which named parameters it is passed)

Just as a reference, my current approach (in ggplotd) is with named tuples, but that is slightly more verbose than I had hoped:

```
doSomethingWithNamedPars( Tuple!(int, "x", string, "y")( 1, 2 ) );
```