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digitalmars.D.bugs - [Issue 7483] New: Can't recursively call function with auto return

reply d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=7483

           Summary: Can't recursively call function with auto return
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: major
          Priority: P2
         Component: DMD
        AssignedTo: nobody puremagic.com
        ReportedBy: andrej.mitrovich gmail.com



10:59:16 PST ---
module test;

auto test(int x)
{
    return test(1);
}

void main()
{
}

test.d(5): Error: forward reference to test

I was using tuple() returns and ran into this. Luckily I can work around it by
explicitly setting the return type, e.g. "Tuple!(bool, string, string) foo()".

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next sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=7483


dawg dawgfoto.de changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |dawg dawgfoto.de



Your example function is an infinite recursion. Both for determining the return
type as well as hypothetically at runtime.

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prev sibling next sibling parent d-bugmail puremagic.com writes:
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02:48:48 PST ---


 Your example function is an infinite recursion. Both for determining the return
 type as well as hypothetically at runtime.


I've rushed too quickly and posted a silly example.

My use-case was code like this:

auto foo(bool check)
{
    if (check)
    {
        check = false;
        return foo(check);
    }
    else
    {
        return 1;
    }
}

I don't know whether this would be too difficult for the compiler to figure out
on its own. But the basic idea is: if there's at least one known type for a
return expression then the function's return type becomes that type and there's
no longer a forward reference error. If there are any other return expressions
they all must have the same type (just like usual functions).

E.g. this would be legal:
auto foo(int state)
{
    if (state == 1)
    {
        state++;
        return foo(state);
    }
    else
    if (state == 2)
    {
        return tuple(0, 0);
    }
    else
    {
        return tuple(0, 0);
    }
}

The return type is std.typecons.Tuple!(int,int).Tuple for the last two return
expressions, the first return is not taken into account since it's a recursive
call, and all other return expression types match.

This one would be illegal since all the return types don't match:

auto foo(int state)
{
    if (state == 1)
    {
        state++;
        return foo(state);
    }
    else
    if (state == 2)
    {
        return tuple(0, 0);
    }
    else
    {
        return tuple(0, 0, 0);
    }
}

The OP sample would still be invalid since you can't figure out the return type
at all in that case.

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Feb 12 2012
prev sibling next sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=7483


Stewart Gordon <smjg iname.com> changed:

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



http://www.d-programming-language.org/function.html
"If there are multiple ReturnStatements, the types of them must match exactly.
If there are no ReturnStatements, the return type is inferred to be void."

But it doesn't comment on whether this is meant to work.  But since it isn't a
forward reference, the error message doesn't make sense in any case.

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prev sibling next sibling parent d-bugmail puremagic.com writes:
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Yeah, it should be possible to infer recursive return types.

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prev sibling parent d-bugmail puremagic.com writes:
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dawg dawgfoto.de changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|major                       |enhancement


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