Expressions, the kind you create using the quote function , come in four flavors: a primitive value, a name, a function call or a control structure, and a pairlist. Function calls include operators such as the arithmetic or logical operators because these are function calls as well in R, and control structures can be considered just a special kind of function calls—they only really differ from function calls in the syntax you use to invoke them.
class(quote(1))## [1] "numeric"class(quote("foo"))## [1] "character"class(quote(TRUE))## [1] "logical"class(quote(x))## [1] "name"class(quote(f(x)))## [1] ...
