Java FFI docs say that arrays of objects map to Fantom List of appropriate type, but I can't get it to work. Tried variations on nullable, but no luck...

An example:

Ffi.fan

class Ffi
{
  native Str passes()
  native Str?[]? fails()

  Void main() { echo(passes); echo(fails); }
}

FfiPeer.java

package fan.klk;
public class FfiPeer
{
  public static FfiPeer make(fan.klk.Ffi self) { return new FfiPeer(); }

  public String passes(fan.klk.Ffi self) { return "works"; }

  public String[] fails(fan.klk.Ffi self) { String[] ss = {"f","a","i","l","s"}; return ss; }
}

build.fan

using build
class Build : BuildPod
{
  new make()
  {
    podName = "klk"
    summary = "test FFI returning String[]"
    version = Version("1.0")
    depends = ["sys 1.0", "build 1.0"]
    srcDirs = [`./`]
    javaDirs= [`./`]
  }

}

building that and running, yields:

>fan klk::Ffi works sys::Err: java.lang.NoSuchMethodError: fan.klk.FfiPeer.fails(Lfan/klk/Ffi;)Lfan/ sys/List;

klk::Ffi.fails (Ffi.fan)
klk::Ffi.main (Ffi.fan:8)
java.lang.reflect.Method.invoke (Unknown)
fan.sys.Method.invoke (Method.java:552)
fan.sys.Method$MethodFunc.callOn (Method.java:230)
fan.sys.Method.callOn (Method.java:139)
fanx.tools.Fan.callMain (Fan.java:137)
fanx.tools.Fan.executeType (Fan.java:102)
fanx.tools.Fan.execute (Fan.java:38)
fanx.tools.Fan.run (Fan.java:250)
fanx.tools.Fan.main (Fan.java:288)

Did you try returning fan.sys.List instead of String[] in FfiPeer.java?

Yes, changing the peer method to

public fan.sys.List fails(fan.klk.Ffi self) { return null; }

works, but leaves me needing to manually create and fill the fan.sys.List from the Java array. Maybe I'm being dense -- the fandoc I quoted sounds as though there's an automatic mapping for that, and I'm not seeing how to trigger it.

One approach that works for me is (assuming you import fan.sys.*):

public List fails(fan.klk.Ffi self) { 
  String[] ss = {"f","a","i","l","s"}; 
  return new List(Type.of(ss[0]), ss); 
}

and then in the Fantom code:

native Str[] fails

The non-list/array types seem to convert automatically. Also the primitive-array types. But I also fail to have an object-list/array type convert automatically, without something like the above.

Kevin,

A native method is a different beast than Java FFI. So if a native method returns Str[], then your implementation must return the exact Fantom representation of that type which is List(Sys.StrType, ...).

Think about it like this:

• a class with native methods must look exactly like any other Fantom class, there is never any translation
• a Java FFI class on the other hand might do all sorts of translations, boxing, or unboxing

@Peter, thanks, that's the invocation I needed; manually constructing the wrapper list under the covers. That works.

@Brian, in the Natives overview, there's a sentence saying "see Java FFI to map between Fantom and Java types", and I was trying to language-lawyer my way into having my cake and eating it too. :-)

Thanks for the good comments.