Multiple Returns and Extra Parameters “LISP is unique in its capability to return more than one value (or no value at all) from a form. This neatly avoids the problems which other languages are prone to when more than one object is produced or affected by a function.” -Star Sapphire Common Lisp Refernce Manual

Returning Multiple Values Any mathematical function returns exactly one value, and that’s usually true in LISP The exception to this is values (values S-expression*) Takes an arbitrary number of return values and returns all of the separately

Floor, Ceiling >(floor 10.5) >(+ (floor 10.5) 7) 17 >(ceiling 3.2) >(+ (ceiling 3.4) (floor 2.8)) 6 Extra return values are ignored if they aren’t expected

Returning with Values >(defun print-and-return-nothing () (format t "Hi") (values)) PRINT-AND-RETURN-NOTHING >(print-and-return-nothing) Hi >(values) >(values 1 (car '(a b)) 2) 1 A 2

Receiving Multiple Values multiple-value-bind operates like let, except local variables are initialized by the multiple return values If there are more variables than values, those left over will be nil If there are more values than variables, the extra values are discarded

Multiple-Value-Bind Examples >(multiple-value-bind (x y z) (values 1 2 3) (list x y z)) (1 2 3) >(multiple-value-bind (x y z) (values 1 2) (list x y z)) (1 2 NIL)

Other Multiple Value Functions multiple-value-call applies a given function to the multiple values returned by some function >(multiple-value-call #'+ (values 1 2 3)) 6 Multiple-value-list collects the values returned into a list >(multiple-value-list (values 1 2 3)) (1 2 3)

Get-Decoded-Time Returns multiple values indicating the current data and time >(get-decoded-time) NIL 5 Seconds Minutes Hours Day Month Year Day of Week Daylight Savings Time Zones West of GMT

Unlimited Arguments Some functions take an indefinite number of arguments ( +, *, etc.) To indicate a parameter that may occur zero or more times, put &rest in front of it The variable following &rest will be bound to a list of the remaining arguments

&rest Example >(defun test-rest (a b &rest c) (list a b c)) TEST-REST >(test-rest 'a 'b 'c) (A B (C)) >(test-rest 5 6) (5 6 NIL) >(test-rest 'a 'b 'foo 'y 60) (A 56 (27 B FOO Y 60))

Optional Argument An optional argument is one that may or may not be present Indicated with &optional Can specify in three ways – name : defaults to nil – (name default) : defaults to default – (name default supplied-flag) : defaults to default, and supplied-flag is t if given, nil otherwise

&optional Example >(defun test-optional (a &optional b (c 3) (d 4 e)) (list a b c d e)) TEST-OPTIONAL >(test-optional 1) (1 NIL 3 4 NIL) >(test-optional 1 2) ( NIL) >(test-optional 1 2 6) ( NIL) >(test-optional ) ( T)

Keyword Arguments (1) A keyword is a symbol whose first character is colon ( : ) Can’t have values changed Predicate for keywords is keywordp Indicated with &key

Keyword Arguments (2) Three ways to specify keyword arguments – name : called with :name, default is nil – (name default) : defaults to default – (name default supplied-flag) : as with optional parameters In all of the above, name can be replaced with (:keyword name), where :keyword is used in the call, and name within the function definition

&key Example >(defun test-key (a &key b (c 3) (d 4 e) ((:key f) 5)) (list a b c d e f)) TEST-KEY >(test-key 1) (1 NIL 3 4 NIL 5) >(test-key 1 2) Error >(test-key 1 :b 2) ( NIL 5) >(test-key 1 :c 12) (1 NIL 12 4 NIL 5) >(test-key 1 :d 14) (1 NIL 3 14 T 5) >(test-key 1 :key 30) (1 NIL 3 4 NIL 30) >(test-key 1 :key 17 :d 14 :b 13 :c 15) ( T 17)

Combining &rest, &optional and &key Required arguments first &optional must precede &rest When combining, the results can be counterintuitive >(defun test-together (a &optional b (c 3) &rest d &key e) (list a b c d e)) TEST-TOGETHER >(test-together 10 :e 13 :e 14) (10 :E 13 (:E 14) 14) >(test-together :e 13 :e 14) ( (:E 13 :E 14) 13)