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Start of Chapter 11: Phrases |
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Back to §11.14. Phrase options |
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Inform 7 Home Page / Documentation
§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.
We can also make temporary variables using "let":
let (a name not so far used) be (value)
let (a temporary named value) be (value)
This phrase creates a new temporary variable, starting it with the value supplied. The variable lasts only for the present block of phrases, which certainly means that it lasts only for the current rule. Examples:
let outer bull be 25;
let the current appearance be "reddish brown";
let the special room be Marley Wood;
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.
The variable name should be a new one; if it's the name of an existing one, then the kinds must agree. So:
is a legal combination, because the second "let" simply changes the value of the existing "outer bull" variable to a different number.
let (a name not so far used) be (name of kind)
This phrase creates a new temporary variable of the given kind. The variable lasts only for the present block of phrases, which certainly means that it lasts only for the current rule. Example:
The variable created holding the default value for that kind - in this case, the number 0. A handful of very obscure kinds have no default values, and then a problem message is produced. Inform also disallows:
because temporary variables aren't allowed to have kinds more specific than "object". (This is a good thing: suppose there are no vehicles in the world?) It's quite safe in such cases to use
instead, which creates it as the special object value "nothing".
Temporary variables made by "let" are only temporarily in existence while a phrase is being carried out. Their values often change: we could say
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:
(The difference between using "let" and "now" here is that "let" can create a new temporary variable, whereas "now" can only alter things already existing: on the other hand, "now" can change many other things as well, whereas "let" applies only to temporary variables.)
