5.1 DSSSL formal argument lists

DSSSL formal argument lists are defined by the following grammar:

<formal-argument-list> → <required-formal-argument>*
  [(#!optional <optional-formal-argument>*)]
  [(#!rest <rest-formal-argument>)]
  [(#!key <key-formal-argument>*) (#!rest <rest-formal-argument>?)]
<required-formal-argument> → <ieee-ident>
<optional-formal-argument> → <ieee-ident>
     | (<ieee-ident> <initializer>)
<rest-formal-argument> → <ieee-ident>
<key-formal-argument> → <ieee-ident>
     | (<ieee-ident> <initializer>)
<initializer> → <expr>

When a procedure is applied to a list of actual arguments, the formal and actual arguments are processed from left to right as follows:

• Variables in required-formal-arguments are bound to successive actual arguments starting with the first actual argument. It shall be an error if there are fewer actual arguments than required-formal-arguments.
• Next, variables in optional-formal-arguments are bound to any remaining actual arguments. If there are fewer remaining actual arguments than optional-formal-arguments, then variables are bound to the result of the evaluation of initializer, if one was specified, and otherwise to #f. The initializer is evaluated in an environment in which all previous optional formal arguments have been bound.
• If there is a rest-formal-argument, then it is bound to a list of all remaining actual arguments. The remaining actual arguments are also eligible to be bound to keyword-formal-arguments. If there is no rest-formal-argument and there are no keyword-formal-arguments, the it shall be an error if there are any remaining actual arguments.
• If #!key was specified in the formal-argument-list, there shall be an even number of remaining actual arguments. These are interpreted as a series of pairs, where the first member of each pair is a keyword specifying the argument name, and the second is the corresponding value. It shall be an error if the first member of a pair is not a keyword. It shall be an error if the argument name is not the same as a variable in a keyword-formal-argument, unless there is a rest-formal-argument. If the same argument name occurs more than once in the list of actual arguments, then the first value is used. If there is no actual argument for a particular keyword-formal-argument, then the variable is bound to the result of evaluating initializer if one was specified, and otherwise #f. The initializer is evaluated in an environment in which all previous formal key arguments have been bound.
• If #!rest was specified in the formal-argument-list after a #!key formal parameter, it is bound to the list of optional non-keyword arguments.

It shall be an error for an <ieee-ident> to appear more than once in a formal-argument-list.

Example:

((lambda (x y) x) 3 4 5 6)   ⇒(3 4 5 6)
((lambda (x y #!rest z) z)
 3 4 5 6)                    ⇒ (5 6)
((lambda (x y #!optional z #!rest r #!key i (j 1)) 
    (list x y z i: i j: j))
 3 4 5 i: 6 i: 7)            ⇒ (3 4 5 i: 6 j: 1)
((lambda (x y #!optional z #!key i (j 1) #!rest r) 
    (list x y z i: i j: j r))
 3 4 5 i: 6 i: 7 8 9)        ⇒ (3 4 5 i: 6 j: 1 (8 9))

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