Compiler can't deduce type for template struct Pair when using it with enum argument.  There is an example

import std.stdio;

enum Category { first, second, third };

struct Pair(F, S)
{
	F first;
	S second;
	
	this(F f, S s)
	{
		first = f;
		second = s;
	}
}


void main()
{
	auto p = Pair(Category.first, "first"); //It fails
	
	writeln(p);
}

Is it not working for some reason or I'm doing something wrong or is it just lack of implementation? How I could make this working without explicit specifying of types?

On Friday, 27 June 2014 at 06:04:20 UTC, Uranuz wrote:
> Compiler can't deduce type for template struct Pair when using it with enum argument.  There is an example
>
> import std.stdio;
>
> enum Category { first, second, third };
>
> struct Pair(F, S)
> {
> 	F first;
> 	S second;
> 	
> 	this(F f, S s)
> 	{
> 		first = f;
> 		second = s;
> 	}
> }
>
>
> void main()
> {
> 	auto p = Pair(Category.first, "first"); //It fails
> 	
> 	writeln(p);
> }
>
> Is it not working for some reason or I'm doing something wrong or is it just lack of implementation? How I could make this working without explicit specifying of types?

is this a solution for your problem?


enum Category { first, second, third };

struct Pair
{
	Category cat;
	string second;
	this(Category cat, string second){
		this.cat = cat, this.second = second;
	}
}
void main(){
        auto p = Pair(Category.first, "first");
        writeln(p);
}

On Friday, 27 June 2014 at 06:12:57 UTC, pgtkda wrote:
How I could make this
>> working without explicit specifying of types?

sorry, i should read better

On Friday, 27 June 2014 at 06:14:48 UTC, pgtkda wrote:
> On Friday, 27 June 2014 at 06:12:57 UTC, pgtkda wrote:
> How I could make this
>>> working without explicit specifying of types?
>
> sorry, i should read better

Ok. Maybe it was discussed already somewhere, but I am not god in searching in English. Is there any directions about it? How could I work around it? Should I mail some proposal or bug report for it?

On Friday, 27 June 2014 at 06:21:11 UTC, Uranuz wrote:
> On Friday, 27 June 2014 at 06:14:48 UTC, pgtkda wrote:
>> On Friday, 27 June 2014 at 06:12:57 UTC, pgtkda wrote:
>> How I could make this
>>>> working without explicit specifying of types?
>>
>> sorry, i should read better
>
> Ok. Maybe it was discussed already somewhere, but I am not god in searching in English. Is there any directions about it? How could I work around it? Should I mail some proposal or bug report for it?

I think, D is a typesafe language, therefore you can't use variables with no type declaration.

One thing you can search for, are templates but even there you have to define a type:

import std.stdio;

enum Category : string { first = "first"}

template Pair(T)
{
	T t;
	T cat;
}


void main()
{
	alias Pair!(string) a;
	a.cat = Category.first;
	a.t = "first";

	writeln(a.cat, " . ", a.t);
}

> I think, D is a typesafe language, therefore you can't use variables with no type declaration.
>
> One thing you can search for, are templates but even there you have to define a type:
>
> import std.stdio;
>
> enum Category : string { first = "first"}
>
> template Pair(T)
> {
> 	T t;
> 	T cat;
> }
>
>
> void main()
> {
> 	alias Pair!(string) a;
> 	a.cat = Category.first;
> 	a.t = "first";
>
> 	writeln(a.cat, " . ", a.t);
> }

Ok. I know that D is typesafe language, but I'm not going to do some implicit type casts in there, because type of Category.first is Category itself but not string or something. In this example `a.cat = Category.first;` tries to make implicit cast (I don't remember is it allowed or not)

Seems that I found answer myself. As far as I understand type inference is working only for template functions but not struct or class templates. This is why this not working and enum is not responsible for that.

I don't know why I use D enough long but I did not remember this fact.

There is proposal exists for this topic
http://wiki.dlang.org/DIP40

On Fri, Jun 27, 2014 at 06:04:18AM +0000, Uranuz via Digitalmars-d-learn wrote:
> Compiler can't deduce type for template struct Pair when using it with enum argument.  There is an example
> 
> import std.stdio;
> 
> enum Category { first, second, third };
> 
> struct Pair(F, S)
> {
> 	F first;
> 	S second;
> 
> 	this(F f, S s)
> 	{
> 		first = f;
> 		second = s;
> 	}
> }
> 
> 
> void main()
> {
> 	auto p = Pair(Category.first, "first"); //It fails
> 
> 	writeln(p);
> }
> 
> Is it not working for some reason or I'm doing something wrong or is it just lack of implementation? How I could make this working without explicit specifying of types?

Try this:

	struct Pair(F, S)
	{
		F first;
		S second;
	}

	auto pair(F,S)(F f, S s)
	{
		return Pair!(F,S)(f,s);
	}

	void main()
	{
		auto p = pair(Category.first, "first");
	}


T

-- 
Don't throw out the baby with the bathwater. Use your hands...

On Friday, 27 June 2014 at 07:43:27 UTC, Uranuz wrote:
> I don't know why I use D enough long but I did not remember this fact.

Sometimes we get spoiled by all the amazing/nifty things that do work, and expect comparable things like this to Just Work.  To be honest, at first I didn't see any issue in what you were doing either...

One thing you could do in the meantime is to use an instantiator function.  This works just fine:


auto pair(F, S)(F f, S s)
{
    return Pair!(F, S)(f, s);
}


void main()
{
    auto p = pair(Category.first, "first");
    writeln(p);
}