Why Can't I Assign to a Struct Field in a Go Map?

Assigning to Struct Fields in Map

The error "cannot assign to struct field in a map" arises when attempting to modify a field of a struct stored within a map. This limitation stems from the immutability of map keys and values in Go.

In the provided example, snapshots := make(map[string] Snapshot, 1) creates a map with keys of type string and values of type Snapshot, where Snapshot is a struct. To modify the Users slice within a Snapshot value, you must follow proper procedure.

The following approach ensures that the Users slice is modified correctly:

func main() {
  snapshots := make(map[string]Snapshot, 1)
  snapshots["test"] = Snapshot{
    Key:   "testVal",
    Users: make([]Users, 0),
  }
  
  // Get a copy of the 'Users' slice
  users := snapshots["test"].Users

  // Append user to the copy
  users = append(users, user)

  // Reassign the map entry
  snapshots["test"].Users = users
}

By obtaining a copy of the 'Users' slice, you can modify and then reassign the modified copy to the map entry. This approach adheres to the immutability of the map.

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