Chapter 11: Phrases
11.15. Let and temporary variables

A variable, as we have seen, is a name for a value which changes, though always remaining of the same kind. For instance, if "target" is a number variable (or "number that varies") then it may change value from 2 to 4, but not from 2 to "fishknife".

To make complicated decisions, phrases often need to remember values on a temporary basis. We have already seen this for the counter in a "repeat" loop, which exists only inside that loop, and then is no longer needed. Now we define named values which exist only inside the current phrase. For instance:

let outer bull be 25;
let the current appearance be "reddish brown";
let the special room be Marley Wood;

creates temporary variables "outer bull", "current appearance" and "special room". The kinds of these are deduced from the values given, so that, for instance,

say "The outer bull scores [the outer bull in words] when you practice archery in [special room]."

produces

The outer bull scores twenty-five when you practice archery in Marley Wood.

Temporary variables made by "let" are only temporarily in existence while a phrase is being carried out. Their values often change: we could say

let x be 10;
change x to 11;

for instance, or indeed we could "let x be 10" and then "let x be 11". But although we are allowed to change the value, we are not allowed to change the kind of value. The name "x" must always have the same kind of value throughout the phrase to which it belongs, so the following will not be allowed:

let x be 45;
change x to "Norway";

(The difference between "let" and "change" is that "let" can create a new temporary variable, whereas "change" can only alter one which already exists: on the other hand, "change" can change many other things as well, whereas "let" applies only to temporary variables.)


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* Example  M. Melmoth's Duel
Three basic ways to inject random or not-so-random variations into text.

RB

"M. Melmoth's Duel"

Saint-Germain-des-Prés is a room. "Haunt of artists, of the coffee-drinking sort, and of cafés, of the artist-haunted sort, you once again find yourself outside M. Melmoth's hotel. Today [one of]the recently-fallen rain runs down the gutters of the 6th[or]sunlight glints even off the blackened windows of the Abbey[or]crowds of vulgar children play chase around the lampposts[at random], and you long to be indoors."

The Hôtel d'Alsace is inside from Saint-Germain-des-Prés. "Typical. Oscar writes you a letter announcing his own imminent demise - 'My wallpaper and I are fighting a duel to the death. One or other of us has got to go.' - and then you get there and he's out, no doubt procuring paint the colour of absinthe, if he isn't procuring the painter."

Tint is a kind of value. The tints are green, aquamarine and darkish purple.

The wallpaper is fixed in place in the Hôtel. The wallpaper has a tint. "In this light, the wallpaper has a distinctly [tint of the wallpaper] wash. [if the tint of the wallpaper is darkish purple]You particularly dislike purple.[end if]"

Before going to the Hôtel: now the wallpaper is a random tint.

After going from the Hôtel, say "You leave, shaking your head. But within twenty-four hours, you are back, as you always knew you would be."

Test me with "in / out / look / in / out / look".


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