4.6.5 Second method for decode-total-seconds

The decode-total-seconds method is called as follows:

? decode-total-seconds(*your-time-of-day*.total-seconds);

If we envision calling decode-total-seconds frequently to see the hours, minutes, and seconds stored in a <time-of-day> instance, we can make it possible to decode <time-of-day> instances, as well as integers. For example, we can make it possible to make this call:

? decode-total-seconds(*your-time-of-day*);

We can implement this behavior easily, by defining another method for decode-total-seconds, which takes a <time-of-day> instance as its argument:

define method decode-total-seconds 
    (time :: <time-of-day>)
 => (hours :: <integer>, minutes :: <integer>, seconds :: <integer>)
  decode-total-seconds(time.total-seconds);
end method decode-total-seconds;)

Figure 4.1 shows the two methods for the decode-total-seconds generic function.

Figure 4.1 The `decode-total-seconds` generic function and its methods.
Generic function `decode-total-seconds`
`// Method on <integer> define method deceode-total-seconds (total-seconds :: <integer>) => (hours :: <integer>, minutes :: <integer>, seconds :: <integer>) let(total-minutes, seconds) = truncate/(total-minutes, 60); values(hours, minutes, seconds); end method decode-total-seconds;`
`// Method on <time-of-day> define method deceode-total-seconds (time :: <time-of-day>) => (hours :: <integer>, minutes :: <integer>, seconds :: <integer>) decode-total-seconds(time.total-seconds); end method decode-total-seconds;`

Generic function decode-total-seconds

// Method on <integer>
define method deceode-total-seconds
    (total-seconds :: <integer>)
 => (hours :: <integer>, minutes :: <integer>, seconds :: <integer>)
  let(total-minutes, seconds) = truncate/(total-minutes, 60);
  values(hours, minutes, seconds);
end method decode-total-seconds;

// Method on <time-of-day>
define method deceode-total-seconds
    (time :: <time-of-day>)
 => (hours :: <integer>, minutes :: <integer>, seconds :: <integer>)
  decode-total-seconds(time.total-seconds);
end method decode-total-seconds;

Looking at Figure 4.1, we analyze what happens in this call:

? decode-total-seconds(*your-time-of-day*);

1. The argument is an instance of <time-of-day>, so the method on <time-of-day> is called.

2. The body of the method on <time-of-day> calls decode-total-seconds on an instance of <integer>, the value of the total-seconds slot of the <time-of-day> instance. In this call, the argument is an integer, so the method on <integer> is called.

3. The method on <integer> returns three values to its caller — the method on <time-of-day>. The method on <time-of-day> returns those three values.

The purpose of the method on <time-of-day> is simply to allow a different kind of argument to be used. The method extracts the integer from the <time-of-day> instance, and calls decode-total-seconds with that integer.