Problem using exported function type with C types as parameters

EDIT: changed the title to better reflect the issue thanks to @Not_a_Golfer

I have been experimenting with Go and cannot figure out this problem.

The following works great if contained in the main package:

// Define a callback-function type, and define an invoker for it
type Callback func(i C.int, d C.double)
func Invoke(cb Callback, i C.int, d C.double) {
        cb(i, d)
}
//... then define my callback and pass it into the invoker
func foo(i C.int, d C.double) {
        fmt.Printf("i %d d %f\n", i, d)
}
func main() {
        Invoke(foo, 2, 2.2)
}

      

        
        
        
      

    

But when I define type Callback

and Invoke

in a package, import the package in main and call mypkg.Invoker(foo, 2, 2.2)

. I am getting the following build error:

src/hello/hello.go:9: cannot convert foo (type func(C.int, C.double)) to type mypkg.Callback

      

        
        
        
      

    

Edit: Here's the main code for the welcome package. I structured everything so that I can use the command go

to build / install:

package main
import "C"
import "mypkg"
import "fmt"
func foo(i C.int, d C.double) {
        fmt.Printf("i %d d %f\n", i, d)
}
func main() {
        mypkg.Invoke( mypkg.Callback(foo), 2, 2.2 )
}

      

        
        
        
      

    

Does anyone know if there is anything special for a type exported from a package? I use C types in the arguments because it will eventually be a wrapper around the C library, and I just wanted to make sure that the C types were not part of the problem my callback matched to the callback type.

Here is the package code:

package mypkg
import "C"
import "fmt"
type Callback func(i C.int, d C.double)
func Invoke(cb Callback, i C.int, d C.double) {
        fmt.Println("Calling cb\n")
        cb( i, d )
}