title: cannot use mutating member on immutable value: 'XXX' is a 'let' constant
body: <p>i am new to swift .i am  trying  with several way but i failed when try as suggest error then anther error occur . what is going on exactly here i don't know </p>
  
  <pre><code>  public struct ThermometerStruct {
            var temperature: Double = 0.0
        public mutating func registerTemperature(temperature: Double) {
          self.temperature = temperature
        }
      }
  
      let thermometerStruct = ThermometerStruct()
      thermometerStruct.registerTemperature(temperature : 56.0)
  </code></pre>
  
  <blockquote>
    <p>ERROR at line 14, col 5: cannot use mutating member on immutable value: 'thermometerStruct' is a 'let' constant
        thermometerStruct.registerTemperature(temperature : 56.0)
        ^~~~~~~~~~~~~~~~~
    <br>
    <br>
    INFO at line 13, col 5: change 'let' to 'var' to make it mutable
        let thermometerStruct = ThermometerStruct()
        ^~~
        var</p>
  </blockquote>