Vala syntax for delegates?

Question

first, some definitions from /usr/share/vala/vapi/libarchive.vapi

[CCode (cprefix="ARCHIVE_", lower_case_cprefix="archive_", cheader_filename = "archive.h")]
namespace Archive {...
[CCode (instance_pos = 1.9, cname="archive_open_callback")]
    public delegate int OpenCallback (Archive archive);
    ...
    [CCode (cname = "struct archive", free_function="archive_write_finish")]
    public class Write : Archive {...
            public Result open (
                    [CCode (delegate_target_pos = 0.9)] OpenCallback ocb,
                    [CCode (delegate_target_pos = 0.9)] WriteCallback rcb,
                    [CCode (delegate_target_pos = 0.9)] CloseCallback ccb
            );
    }
}

then my code

int mOpenCallback (Write archive)
{
    return Result.OK;
}

void foo(){...
Write archive = new Write();
archive.open(mOpenCallback,mWriteCallback,mCloseCallback);
...}

results : error: Assignment: Cannot convert from mOpenCallback' toArchive.OpenCallback'

what's wrong with it ?

then I tried this instead

namespace Archive {
int OpenCallback (Write archive)
{
    return Result.OK;
}
}

void foo(){...
Write archive = new Write();
archive.open(OpenCallback,mWriteCallback,mCloseCallback);
...}

but it outputs : Archive' already contains a definition forOpenCallback'

void foo(){...
Write archive = new Write();
Archive.OpenCallback t = mOpenCallback;
archive.open(t,mWriteCallback,mCloseCallback);
...}

outputs error: Assignment: Cannot convert from foo.mOpenCallback' toArchive.OpenCallback'

Answer 1

mOpenCallback has takes an argument of type Write, not Archive, so it has a different type from the delegate OpenCallback. If you expect Write, you should perform the cast from Archive to write inside your call back.