Expressions are used to define filter logic for passing to a Dataset Scanner.

Expression$scalar(x) constructs an Expression which always evaluates to the provided scalar (length-1) R value.

Expression$field_ref(name) is used to construct an Expression which evaluates to the named column in the Dataset against which it is evaluated.

Expression$create(function_name, ..., options) builds a function-call Expression containing one or more Expressions.