Gotchas in Go-lang

go_gotchas.go

// Interfaces assigned a value of `nil` are not equal to `nil`

package main

import "fmt"

type I interface {
}

type T struct {
	S string
}

func main() {
	var i I
	var t *T
	describe(i)
	// Output
	// i -> value: <nil>, type: <nil>, equal_nil?: true
	// the interface `i` is nil as it hasn't been assigned

	i = t
	describe(i)
	// Output
  // i -> value: <nil>, type: *main.T, equal_nil?: false
	// the intervace `i` still returns a value of nil, but it's not equal to nil!
  // Internally you should think of the value of `i` as a special object like this:
  // `i = (value: nil, concrete_type: *T)`
  // When you try to Printf('%v', i) it's actually printing `i.value`
  // At least this is how I'm currently thinking about it.
}

func describe(i I) {
	fmt.Printf(
		`
			i -> value: %v, type: %T, equal_nil?: %v
		`,
		i, i, i == nil)
}