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Persistent variables cannot appear on the left-hand side of a declaration in the prelude. 

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For historical reasons, if the persistent variable does not exist before it is used, the default value is 0. 

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Examples

The following declaration would add 3 to the value stored in the entity counter ent:fizz, storing the result in x:

pre {
  x = ent:fizz + 3
}

Deprecated Operators

KRL includes special built-in functions for accessing individual places in a trail. These are deprecated and should not be used. 

• history. This takes an expression and a persistent variable as arguments. If the expression evaluates to n, then the history function will return the n^th place on the trail. The most recent place is at history location 0. If n is larger than the length of the trail, the history function returns the empty string. 

pre {
  x = history 1 ent:another_trail;
}

• current. This takes a persistent variable as its argument. The current function performs the same operation as the history function with a location of 0. That is, it returns the most recent place on the trail.

pre {
  x = current ent:another_trail;fizz + 3
}