The R Book

The Evaluation Environment of a Function

When a function is called or invoked a new evaluation frame is created. In this frame the formal arguments are matched with the supplied arguments according to the rules of argument matching (below). The statements in the body of the function are evaluated sequentially in this environment frame.

The first thing that occurs in a function evaluation is the matching of the formal to the actual or supplied arguments. This is done by a three-pass process:

• Exact matching on tags. For each named supplied argument the list of formal arguments is searched for an item whose name matches exactly.
• Partial matching on tags. Each named supplied argument is compared to the remaining formal arguments using partial matching. If the name of the supplied argument matches exactly with the first part of a formal argument then the two arguments are considered to be matched.
• Positional matching. Any unmatched formal arguments are bound to unnamed supplied arguments, in order. If there is a ... argument, it will take up the remaining arguments, tagged or not.
• If any arguments remain unmatched an error is declared.

Supplied arguments and default arguments are treated differently. The supplied arguments to a function are evaluated in the evaluation frame of the calling function. The default arguments to a function are evaluated in the evaluation frame of the function. In general, supplied arguments behave as if they are local variables initialized with the value ...