[suzuki]

Now the abstract sequence type of "LINQ in Go" https://github.com/nukata/linq-in-go can be written as

  type Enumerator(type T) func(yield func(element T))

and the "Select" method can be written as

  func (loop Enumerator(T)) Select(f func(T) T) Enumerator(T) {
        return func(yield func(T)) {
                loop(func(element T) {
                        value := f(element)
                        yield(value)
                })
        }
  }

You can call this method with type-safety as follows. Yay!

  func main() {
        squares := Range(1, 3).Select(func(x int) int { return x * x })
        squares(func(num int) {
                fmt.Println(num)
        })
  }
  // Output:
  // 1
  // 4
  // 9

See https://go2goplay.golang.org/p/b0ugT68QAy2 for the complete code.

And, for generality, you should write the method actually as follows.

  func (loop Enumerator(T)) Select(type R)(f func(T) R) Enumerator(R) {
        return func(yield func(R)) {
                loop(func(element T) {
                        value := f(element)
                        yield(value)
                })
        }
  }

However, you will get the error message then:

  type checking failed for main
  prog.go2:17:33: methods cannot have type parameters

According to https://go.googlesource.com/proposal/+/refs/heads/master/des... this seems an intended restriction:

> Although methods of a generic type may use the type's parameters, methods may not themselves have additional type parameters. Where it would be useful to add type arguments to a method, people will have to write a suitably parameterized top-level function.

> This is not a fundamental restriction but it complicates the language specification and the implementation.

For now, we have to write it as https://go2goplay.golang.org/p/mGOx3SWiFXq and I feel it rather inelegant. Good grief