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Proposal: Splatting #8987

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alrz opened this issue Feb 21, 2016 · 6 comments
Closed

Proposal: Splatting #8987

alrz opened this issue Feb 21, 2016 · 6 comments

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@alrz
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alrz commented Feb 21, 2016

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
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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
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alrz commented Feb 22, 2016

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

@DavidArno
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@alrz,

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

@bondsbw
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bondsbw commented Feb 22, 2016

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
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alrz commented Feb 22, 2016

@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
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alrz commented Apr 10, 2017

Moved to dotnet/csharplang#424

@alrz alrz closed this as completed Apr 10, 2017
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