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digitalmars.D.bugs - [Issue 538] New: Can't return an expression tuple from a function

reply d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=538

           Summary: Can't return an expression tuple from a function
           Product: D
           Version: 0.174
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: normal
          Priority: P2
         Component: DMD
        AssignedTo: bugzilla digitalmars.com
        ReportedBy: wbaxter gmail.com


You can't return an expression tuple from a function.
Right now, if you try to return one:

  template Tuple(T...) { alias T Tuple; }
  alias Tuple!(int,int) TType;

  TType foo()
  {
      TType x;  x[0] = 1; x[1] = 2;
      return x;
  }

You get the error: 
  "Error: cannot implicitly convert expression
(tuple((_x_field_0),(_x_field_1))) of type (int, int) to (int, int)"


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Nov 16 2006
next sibling parent d-bugmail puremagic.com writes:
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------- Comment #1 from brunodomedeiros+bugz gmail.com  2006-11-18 09:13 -------
Not a bug. "A Tuple is not a type" (from
http://www.digitalmars.com/d/template.html), so (unless stated otherwise) it
cannot be used as the return value of a function. And it is not stated
otherwise, the only special uses for tuples are "A Tuple can be used as an
argument list to instantiate another template, or as the list of parameters for
a function.".


-- 
Nov 18 2006
prev sibling next sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=538


wbaxter gmail.com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|normal                      |enhancement




------- Comment #2 from wbaxter gmail.com  2006-11-18 10:03 -------
I'll make it an enhancement.


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Nov 18 2006
prev sibling next sibling parent d-bugmail puremagic.com writes:
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------- Comment #3 from brunodomedeiros+bugz gmail.com  2006-11-18 11:11 -------
Hum, you are quite more correct in your reasoning that I thought. When I made
the previous post I had only read the new spec changes, which indicate what I
said in the previous post. However upon reading
http://www.digitalmars.com/d/tuple.html it seems the spec is incomplete. Tuples
can indeed be used for more than just declaring function parameters.


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Nov 18 2006
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------- Comment #4 from wbaxter gmail.com  2006-11-18 12:49 -------
In any event, you are correct that it does not say anywhere that you should be
able to return an expression tuple from a function.

On the other hand you can convert a struct to an expression tuple and
initialize it's values by setting elements of the tuple.  So there really
shouldn't be much difference at the metal level between returning a  struct{int
x, int y}  and a Tuple!(int,int).  It should be doable as long as it's an
expression tuple.

I tried for a while to write a simple list picking template.  I got it working
for type tuples:

template Pick(TList...) 
{
    template Indexes(int I, IList...)
    {
        static if (IList.length==0)
        {
            alias TList[I] Indexes;
        }
        else
        {
            alias Tuple!(TList[I], Indexes!(IList)) Indexes;
        }
    }

    template PickArgs(IList...) {
        alias With!(IList)  DList;
        DList PickArgs(TList arg) {
            DList darg;
            return darg;
        }
    }
}

    alias Tuple!(int,float,char[],long) ArgTSet;
    alias Tuple!(0,3,1) IdxSet;
    alias Pick!(ArgTSet) P;
    alias P.Indexes!(IdxSet) sublist;
    writefln(typeid(sublist));

Output:  int, long, float

But I couldn't figure out any way to do the equivalent with an Expression tuple
without having return values.   Hmm, maybe it was just a bug that was causing
the problem.

Here's my test for expression tuples:

    alias Tuple!(3,2.3,"hi",9) args;
    alias Pick!(args) P2;
    alias P2.Indexes!(IdxSet) subargs;
    writefln(subargs);

It fails to compile with a "tuple TList is used as a type".  But looking at it
now I think that may just be a compiler error.  I think maybe that should work.
 TList shouldn't have to be a type to define that alias.  (Otherwise alias
Tuple!(3,2.3,"hi",9) should fail too).


-- 
Nov 18 2006
prev sibling next sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=538


smjg iname.com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |smjg iname.com
           Keywords|                            |diagnostic




------- Comment #5 from smjg iname.com  2007-01-02 11:56 -------
That this doesn't work is one thing, but the error message ("Error: cannot
implicitly convert expression [...] of type (int, int) to (int, int)") is
certainly absurd.


-- 
Jan 02 2007
prev sibling next sibling parent d-bugmail puremagic.com writes:
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Andrei Alexandrescu <andrei metalanguage.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
                 CC|                            |andrei metalanguage.com
         Resolution|                            |WONTFIX


--- Comment #6 from Andrei Alexandrescu <andrei metalanguage.com> 2010-11-26
11:42:37 PST ---
This will not be fixed. D1 is frozen and D2 has Tuple.

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Nov 26 2010
prev sibling next sibling parent d-bugmail puremagic.com writes:
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Leandro Lucarella <llucax gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |llucax gmail.com


--- Comment #7 from Leandro Lucarella <llucax gmail.com> 2010-11-26 14:13:03
PST ---
Is the error message improved? It's really misleading...

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Nov 26 2010
prev sibling parent d-bugmail puremagic.com writes:
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nfxjfg gmail.com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |REOPENED
                 CC|                            |nfxjfg gmail.com
         Resolution|WONTFIX                     |


--- Comment #8 from nfxjfg gmail.com 2010-11-26 14:58:06 PST ---
There's no reason to close this as WONTFIX. It's an backward compatible
enhancement requests, and some of these have been answered by Walter in the
past.

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