Using Persistents in Preludes

Persistent variables can be used inside prelude expressions like any other variable.

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

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

This means that the isnull() operator, for example, is never true for an entity variable.

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

The following are deprecated and should not be used.

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;
}