[Issue 538] Can't return an expression tuple from a function

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=538





------- Comment #4 from wbaxter at 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).


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