[swift-evolution] Proposal: Introduce User-defined "Dynamic Member Lookup" Types

C. Keith Ray keithray at mac.com
Tue Dec 12 11:26:53 CST 2017

in https://gist.github.com/lattner/b016e1cf86c43732c8d82f90e5ae5438

there is this statement:

"For this reason, the compiler only permits conformance of this protocol on the original type definition, not extensions"

and this example:

extension JSON : DynamicMemberLookupProtocol {



Calling Java from C is done like the following, how would one do it in Swift using this proposal?

( Example from https://www.ibm.com/developerworks/java/tutorials/j-jni/j-jni.html )

Java code:

  public class Sample2
    public static int intMethod(int n) {
        return n*n;
    public static boolean booleanMethod(boolean bool) {
         return !bool;

C Code:

#include <jni.h>

#ifdef _WIN32
#define PATH_SEPARATOR ';'
#define PATH_SEPARATOR ':'
int main()
  JavaVMOption options[1];
  JNIEnv *env;
  JavaVM *jvm;
  JavaVMInitArgs vm_args;
  long status;
  jclass cls;
  jmethodID mid;
  jint square;
  jboolean not;

  options[0].optionString = "-Djava.class.path=.";
  memset(&vm_args, 0, sizeof(vm_args));
  vm_args.version = JNI_VERSION_1_2;
  vm_args.nOptions = 1;
  vm_args.options = options;
  status = JNI_CreateJavaVM(&jvm, (void**)&env, &vm_args);

  if (status != JNI_ERR)
    cls = (*env)->FindClass(env, "Sample2");
    if (cls !=0)
      mid = (*env)->GetStaticMethodID(env, cls, "intMethod", "(I)I");
      if (mid !=0)
        square = (*env)->CallStaticIntMethod(env, cls, mid, 5);
        printf("Result of intMethod: %d\n", square);
      mid = (*env)->GetStaticMethodID(env, cls, "booleanMethod", "(Z)Z");
      if (mid !=0)
         not = (*env)->CallStaticBooleanMethod(env, cls, mid, 1);
         printf("Result of booleanMethod: %d\n", not);
   return 0;
    return -1;

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