§11.18. The value after and the value before

A point which has come up several times in recent chapters is that enumerated kinds of value have a natural ordering. For example, if we write:

Colour is a kind of value. The colours are red, orange, yellow, green, blue, indigo and violet.

...then we not only have seven possible values, we have put them into a sequence, in order of their naming. We can't perform arithmetic on colours, of course, but we can perform comparisons on them. Thus "red < yellow" is true, while "green >= violet" is not. (More on comparisons in the chapter on units, which also covers arithmetic.)

It's also sometimes useful to get at the sequence directly. First, the two ends:

first value of (name of kind) ... value

This phrase produces the first-created value of the given kind, which should be an enumeration. Example: if we have

Colour is a kind of value. The colours are red, orange, yellow, green, blue, indigo and violet.

then "first value of colour" is red.

last value of (name of kind) ... value

This phrase produces the last-created value of the given kind, which should be an enumeration. Example: if we have

Colour is a kind of value. The colours are red, orange, yellow, green, blue, indigo and violet.

then "last value of colour" is violet.

And now how to step forward and back:

(name of kind) after (enumerated value) ... value

This phrase produces the next-created value of the given kind, which should be an enumeration. Example: if we have

Colour is a kind of value. The colours are red, orange, yellow, green, blue, indigo and violet.

then "colour after orange" is yellow.

(name of kind) before (enumerated value) ... value

This phrase produces the previous-created value of the given kind, which should be an enumeration. Example: if we have

Colour is a kind of value. The colours are red, orange, yellow, green, blue, indigo and violet.

then "colour before blue" is green.

 ExampleEntropyAll objects in the game have a heat, but if not kept insulated they will tend toward room temperature (and at a somewhat exaggerated rate).

 ExampleThe Hang of ThursdaysTurns take a quarter day each, and the game rotates through the days of the week.