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Input variables passed for expression evaluation can be
repeating or non-repeating.For repeating variables, expression is
evaluated once for each instance of variable. Expression results
are returned as a collection. Input variables can repeat at different
levels, for example one of the input variables can be a list of strings
and another can be a list of list of strings. For handling the difference
in collection hierarchy, expression manager flattens these collection
variables into a two-dimensional array where each row represents
input parameters required to evaluate the expression. Expression
is evaluated on each row and result is organized according to the
hierarchy of the input variables.
All the collection variables being used for expression evaluation
are part of the single hierarchy. When examining a path from root
element to the deepest collection variable, all collection variables
falls on this path. For non-repeating variables, there is no such
restriction. If any of the input variable instance is null, the
result is set to null for this instance. Expression evaluation for
other instances is done in a normally. At any given level and index,
all variables contain same number of elements. If a variable does
not contain elements at a certain level, that variable is not compared
for that level.
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