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d-bugmail puremagic.com writes:

http://d.puremagic.com/issues/show_bug.cgi?id=7265

           Summary: Function Literals where a keyword was omitted should
                    be delegate even if inference.
           Product: D
           Version: D2
          Platform: Other
        OS/Version: Windows
            Status: NEW
          Severity: regression
          Priority: P2
         Component: DMD
        AssignedTo: nobody puremagic.com
        ReportedBy: zan77137 nifty.com


--- Comment #0 from SHOO <zan77137 nifty.com> 2012-01-10 09:30:54 PST ---
This code should be work (on git head):

-------
import std.stdio;

void main()
{
    auto dg = { writeln("delegate"); };
    static assert(is(typeof(dg) == delegate));
}
-------
main.d(6): Error: static assert  (is(void function() == delegate)) is false
-------

This is being documented as follows:

 If the keywords function or delegate are omitted, it defaults to being a
delegate.


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timon.gehr gmx.ch changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |timon.gehr gmx.ch
          Component|DMD                         |websites
           Severity|regression                  |normal


--- Comment #1 from timon.gehr gmx.ch 2012-01-15 11:56:18 PST ---
This is a case of outdated documentation.
From TDPL, p150:
"If both function and delegate are absent from the literal, the compiler
automatically detects which one is necessary".

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--- Comment #2 from SHOO <zan77137 nifty.com> 2012-01-16 03:48:12 PST ---
(In reply to comment #1)
 This is a case of outdated documentation.
 From TDPL, p150:
 "If both function and delegate are absent from the literal, the compiler
 automatically detects which one is necessary".


It is an example unlike the specifications(TDPL, p150).

The case is the following situation:

-------
import std.stdio, std.traits;

void main()
{
    void function() fn = { writeln("function"); };
    static assert(isFunctionPointer!(typeof(fn)));

    void delegate() dg = { writeln("delegate"); };
    static assert(isDelegate!(typeof(dg)));
}
-------

As noted above, the specifications are applied only when there is a necessary
type.
When there is not a required type, in the case like this time, it should infer
the type of the default.

This resembles the following examples:
-------
import std.traits;

void main()
{
    byte a = 1; // typeof(a) is byte
    auto b = 1; // typeof(b) is int

    // int cannot implicitly cast to byte.
    static assert (isImplicitlyConvertible!(typeof(b), typeof(a))); // false.
}
-------

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timon.gehr gmx.ch changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|DMD                         |websites


--- Comment #3 from timon.gehr gmx.ch 2012-01-16 08:44:22 PST ---
Everything behaves as specified in TDPL. This is not a compiler bug.

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Andrei Alexandrescu <andrei metalanguage.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |andrei metalanguage.com


--- Comment #4 from Andrei Alexandrescu <andrei metalanguage.com> 2012-01-16
09:25:15 PST ---
(In reply to comment #3)
 Everything behaves as specified in TDPL. This is not a compiler bug.


Yah, I just tested this:

import std.stdio, std.traits;
void main()
{
    auto fn = { writeln("function"); };
    static assert(isFunctionPointer!(typeof(fn)));
    int x;
    auto dg = { writeln("delegate because it prints ", x); };
    static assert(isDelegate!(typeof(dg)));
}

So the literal is properly classified as a function or delegate depending on it
needing a frame pointer or not.

There's one remaining question I have. I rewrote the example like this:


import std.stdio, std.traits;

void main()
{
    auto fn = { writeln("function"); };
    static assert(is(typeof(fn) == function));

    int x;
    auto dg = { writeln("delegate because it prints ", x); };
    static assert(is(typeof(dg) == delegate));
}

The second static assert works, but the first doesn't. Why?

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--- Comment #5 from SHOO <zan77137 nifty.com> 2012-01-16 10:07:04 PST ---
(In reply to comment #4)
 (In reply to comment #3)
 Everything behaves as specified in TDPL. This is not a compiler bug.


 Yah, I just tested this:
 
 import std.stdio, std.traits;
 void main()
 {
     auto fn = { writeln("function"); };
     static assert(isFunctionPointer!(typeof(fn)));
     int x;
     auto dg = { writeln("delegate because it prints ", x); };
     static assert(isDelegate!(typeof(dg)));
 }
 
 So the literal is properly classified as a function or delegate depending on it
 needing a frame pointer or not.
 


Hmm...
Because it included a clear breaking change, I thought that a sample code was
wrong.

 There's one remaining question I have. I rewrote the example like this:
 
 
 import std.stdio, std.traits;
 
 void main()
 {
     auto fn = { writeln("function"); };
     static assert(is(typeof(fn) == function));
 
     int x;
     auto dg = { writeln("delegate because it prints ", x); };
     static assert(is(typeof(dg) == delegate));
 }
 
 The second static assert works, but the first doesn't. Why?


"fn" of the first case is a function pointer and is not a function. The code
outputs a compilation error definitely.

-----
import std.stdio, std.traits;

void main()
{
    static assert(is(typeof(main) == function)); // true, typeof(main) is
function
    static assert(is(typeof(&main) == function)); // false, typeof(&main) is
function pointer
}
-----

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--- Comment #6 from Andrei Alexandrescu <andrei metalanguage.com> 2012-01-16
10:15:03 PST ---
(In reply to comment #5)
 -----
 import std.stdio, std.traits;
 
 void main()
 {
     static assert(is(typeof(main) == function)); // true, typeof(main) is
 function
     static assert(is(typeof(&main) == function)); // false, typeof(&main) is
 function pointer
 }
 -----


I think fn in

auto fn = { ... };

should be the same thing - a name for a function. I guess that's not easy to
implement. Kenji, Walter?

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--- Comment #7 from Kenji Hara <k.hara.pg gmail.com> 2012-01-16 16:11:52 PST ---
 I think fn in
 
 auto fn = { ... };
 
 should be the same thing - a name for a function. I guess that's not easy to
 implement. Kenji, Walter?


Rewriting it to true function declaration is easy, but
fn is looks like a variable so peaple will try to
rebind it with assignment:

  fn = { writeln("another function literal"); };

and will get an error. It is inconsistent.

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Andrei Alexandrescu <andrei metalanguage.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |FIXED


--- Comment #8 from Andrei Alexandrescu <andrei metalanguage.com> 2012-01-16
16:36:05 PST ---
OK, thanks.

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--- Comment #9 from SHOO <zan77137 nifty.com> 2012-01-17 06:37:31 PST ---
By the way, the reason why I set this Issue with regression is that I cause a
difficult problem with the following cases:

------
import std.stdio, std.functional;

// Library code
struct Control {
    alias void delegate(Control o) Handler;
    Handler[] _handlers;

    void addHandler(H)(H hnd) {
        _handlers ~= cast(Handler)hnd;
    }

    void addHandler2(H)(H hnd) if (is(H == delegate)) {
        _handlers ~= cast(Handler)hnd;
    }

    // Workaround. It is settled if can handle either by toDelegate.
    void addHandler3(H)(H hnd) {
        _handlers ~= cast(Handler)toDelegate(hnd);
    }

    void call() {
        foreach (h; _handlers) {
            h(this);
        }
    }
}

// User code
void main() {
    int i;
    auto c = new Control;
    // OK. This literal is inferred delegate.
    c.addHandler( (Object o){ writeln(i); } );

    // Error. This literal is inferred function pointer.
//    c.addHandler( (Object o){ writeln("test"); } );

    // Error. This literal is inferred function pointer, too.
    // The constraint-if does not influence the type inference.
//    c.addHandler2( (Object o){ writeln("test2"); } );

    // Workaround.
    c.addHandler3( (Object o){ writeln("test3"); } );

    c.call();
}
------

When a library code has a definition such as Control, many breaking changes
befall user codes.
Actually, this way is used by DFL.

One of simple workaround is to use toDelegate with library codes.

I think that another solution is to let implicit conversion perform a role as
toDelegate.

I remember it has entered ML agenda several times. But I've forgotten the
conclusion.
Is there a person knowing the details?

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--- Comment #10 from timon.gehr gmx.ch 2012-01-17 10:11:05 PST ---
(In reply to comment #9)
 One of simple workaround is to use toDelegate with library codes.
 
 I think that another solution is to let implicit conversion perform a role as
 toDelegate.
 
 I remember it has entered ML agenda several times. But I've forgotten the
 conclusion.
 Is there a person knowing the details?


I think no conclusion has been reached yet. Andrei (and me too) wants implicit
conversions from function pointer to delegate to work, but Walter disagrees
with it.

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