Proposal: Splatting

[alrz]

As it is proposed in #347 you might be able to use splatting in method arguments

public double Avg(int sum, int count) => count == 0 ? 0 : sum / count;
Console.WriteLine($"Avg: {Avg(Tally(myValues))}");

And unsplatting would be like:

var list = List<(string name, int age)>();
list.Add("John Doe", 66); 

In the first case it's rather too implicit and as discussed, there should be some kind of operator to make it more visible.

Console.WriteLine($"Avg: {Avg(...Tally(myValues))}");

I want to propose the ability to use this syntax on tuples themselves e.g.

var foo = (1,2);
var bar = (...foo, true);

// translates to
var bar = (foo.Item1, foo.Item2, true);

In this case the target expression must be of a tuple type (names will be preserved in the resultant tuple).

Same syntax can be used with anonymous types.

var foo = new { Name = "foo", Family = "bar" };
var bar = new { ...foo, Age = 0.5 };

// translates to
var bar = new { foo.Name, foo.Family, Age = 0.5 };

This is specifically useful in pipeline processing (like LINQ) to compose short-lived tuples with additional information added in each step.

[DavidArno]

I'd suggest the syntax for your first two examples would probably need to be:

public double Avg(int sum, int count) => count == 0 ? 0 : sum / count;
Console.WriteLine($"Avg: {Avg(...Tally(myValues))}"); // ... means map tuple to two params of Avg

And

    var list = List<(string name, int age)>();
    list.Add(("John Doe", 66)); // inner () needed to denote it's a tuple, not two separate params 

[alrz]

@DavidArno That's the exact syntax proposed in #347. The rest of examples are my suggestions.

[DavidArno]

@alrz,

Thanks. I've cross-referenced my suggestion there too.

[bondsbw]

If ... is used as an actual language syntax, it could completely change the meaning of many of the issues and proposals on in this forum. 😉

[alrz]

@bondsbw I'm not fond of the syntax here, however, this is what ES, C++ and Java are actually using for various purposes. "it could completely change the meaning of many of the issues and proposals on in this forum" yeah actually it's kind of a breaking change ;).

[alrz]

Moved to dotnet/csharplang#424