Lambda syntax, etc

[Andrei Alexandrescu]

Denis Koroskin wrote:
> On Thu, 05 Feb 2009 12:32:15 +0300, Kagamin <spam at here.lot> wrote:
> 
>> bearophile Wrote:
>>
>>> C#2 has lambdas, and C#3 adds closures and more type inferencing, so 
>>> C#3+ supports the following syntaxes:
>>> (int i) => { return i % 3 == 1; } // C#2
>>> i => i % 3 == 1 // C#3
>>> i => { return i % 3 == 1; } // C#3, with statements too
>>> To define a delegate o delegate closure:
>>> Func<int> foo = i => { return i % 3 == 1; };
>>> Func<int> foo = i => i % 3 == 1;
>>> Func<int> bar = () => 2;
>>> But this isn't allowed:
>>> Func<void> bar = () => 2;
>>
>> Yeah, C# lambdas are the killer feature. Slick, readable, 
>> C-compatible. Anders knows his job. Let's face it: delegate literals 
>> suck a little, mixins as delegates suck a lot, the former is too 
>> verbose, the latter just sucks.
> 
> I don't like C# lambda syntax (although it is not half as bad as C++ 
> lambda syntax).
> 
> I believe D delegate syntax is superior due to its natural and 
> unambiguous syntax.
> But yes, it could be made shorter by improving type deduction:
> 
> int delegate(int) inc = (i) { i + 1; }
> 
> Which would be the same as
> 
> int delegate(int) inc = (int i) { return i + 1; }

What if you wanted to just execute one expression and return void? This 
is relevant when e.g. large objects are involved that shouldn't be 
copied unwittingly.

> where i's type is deduced from inc's type and the only expression (i + 
> 1) made a return value:
> 
> auto x = inc(5); // yields 6
> 
> Here is an another example:
> 
> void foo(void delegate(ref int i) inc);
> 
> Could be used as follows:
> 
> foo( (i) { ++i; } );
> 
> as opposed to
> 
> foo( (ref int i) { ++i; } );

Aha! So here you are using a void-returning function. Now what if there 
was another overload of foo in place:

void foo(int delegate(ref int i) inc);

Which foo is to be called? The one that infers a return type of int or 
the one that assumes the code just returns void?

> I can put this enhancement request into bugzilla if anyone likes it.

It would be great to add the parameter type deduction stuff; that is 
already talked about and doesn't seem to have many issues. It does have 
one, which I'm sure people here will see rather quickly.


Andrei