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
evaluates to the named column in the
Dataset against which it is evaluated.
Expression$create(function_name, ..., options) builds a function-call
Expression containing one or more
Expressions. Anything in
is not already an expression will be wrapped in
Expression$op(FUN, ...) is for logical and arithmetic operators. Scalar
... will be attempted to be cast to the common type of the
Expressions in the call so that the types of the columns in the
are preserved and not unnecessarily upcast, which may be expensive.