codehakase's logs

Notes mostly about software engineering and what I’m working on.

Enum Stringer Interface optimisation in Go

Posted at — Apr 8, 2021

The most idiomatic way of describing an enum type in Go is to use constants, often in conjunction with iota.

A pattern I’ve used to implement the Stringer interface for an enum type is to lookup the string representation in a slice. Consider this snippet from a project I’m working on:

// Status ...
type Status uint32

// List of possible status values
const (
  // The operation is known, but hasn't been decided yet
	Processing Status = iota
  // The operation will never be accepted
  // The operation was accepted

func (s Status) String() string {
  return []string{"processing", "rejected", "accepted"}[s]

The snippet above works correct but has a few glitches – passing Status(-2) or Status(300) will cause the String() method to panic, and when appending to the slice, the order of the enum constants has to be taken into consideration.

A way to optimise this is to use a map which provides O(1) access time rather than the linear O(n) time in the string slice version.

// Map of Status values back to their constant names for pretty printing.
var txStatuses = map[Status]string{
	Processing: "processing",
	Rejected:   "rejected",
	Accepted:   "accepted",

// String returns the ErrorCode as a human-readable name.
func (s Status) String() string {
	if s := txStatuses[s]; s != "" {
		return s
	return fmt.Sprintf("Unknown Status (%d)", uint32(s))
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