Go Weekly #2: Go 1.23 Iterators

Why People are Angry over Go 1.23 Iterators

• Context:

  • Most languages provide standardized way to iterate over values stored in containers.

• Problem:

  • Go has for range for maps, slices, strings, arrays and channels but no generic mechanism for user-written containers.

  • Short list of some non-generic iterators:

    • runtime.CallersFrames: returns runtime.Frames iterates over stack frames; Frames has a Next method and a bool to check if there are more frames.

    • bufio.Scanner: iterates through an io.Reader; Scan method advances to the next value, Bytes method to return the value and Err to return error.

    • database/sql.Rows: iterates through the results of a query; also has Scan method.

  • Before Generics were introduced, no way to write an interface that described an iterator that would cover all of the use cases…

• Solution:

  • Go 1.22 proposal of adding package iter can range over integers and functions:

  type (
      Seq[V any]     func(yield func(V) bool) bool
      Seq2[K, V any] func(yield func(K, V) bool) bool
  )
  

  (Seq2 represents a sequence of paired values: key-value, index-value or value-error)

  • An iterator is a function that passes successive elements of a sequence to a callback function yield, it returns true if it should continue, false if it should stop.

  type (
      Yield[V any] func(V bool)
      Yield2[K, V any] func(K,V) bool
  )
  

  • Examples:

  func Backward[E any](s []E) func(func(int, E) bool) {
      return func(yield func(int, E) bool) {
          for i := len(s)-1; i >= 0; i-- {
              if !yield(i, s[i]) {
                  // Where clean-up code goes
                  return
              }
          }
      }
  }
  
  s := []string{"a", "b", "c"}
  for _, el in range Backward(s) {
      fmt.Print(el, " ")
  }
  // c b a
  

• Conclusion:

  • The for range is getting complex:
    • return true => continue
    • return false => break
  • Feels too functional than an imperative language
  • Still the purpose is good, let’s wait to see the adoption in community after awhile

• https://www.gingerbill.org/article/2024/06/17/go-iterator-design/
• https://github.com/golang/go/issues/61897
• https://en.wikipedia.org/wiki/Container_(abstract_data_type)