lisp lambda lists
Table of Contents
Rules to assert control over the arguments to functions and input symbols for macros See:
- &rest
- &optional
- &key
- &body
1. &rest
- accumulate an arbitary number of arguments into a list
(defun foo (&rest restlist) restlist) (list (foo 1 2 3 4 5 6) (foo 1 2 3) (foo))
| (1 2 3 4 5 6) | (1 2 3) | NIL |
|---|
2. &optional
- used to signify optional arguments.
- default values can also be provided just as in &key-worded arguments
- the only differences between this and &key:
- need to insert arguments in order for &optional but that isn't necessary for &key arguments that can be inserted in any order with the relevant :keyword
(defun foo (&optional (a 1)) a) (list (foo 3) (foo))
| 3 | 1 |
|---|
- initial regular parameters can also be referred in the optional arg list
(defun foo (a &optional (b a)) (list a b)) (list (foo 1) (foo 1 2))
| 1 | 1 |
|---|---|
| 1 | 2 |
- a passage predicate similar to that in &key can be employed
(defun foo (&optional (a 1 a-p)) (list a a-p)) (list (foo 3) (foo))
| 3 | T |
|---|---|
| 1 | NIL |
3. &key
(defun foo (&key a b c) (list a b c)) (foo :a "c" :b "a" :c "b")
| c | a | b |
|---|
- if keyworded arg not provided, set to NIL
- preferably provide a default if there is a possibility of NIL being a reasonable argument to the lambda
(defun foo (&key (a 0) b) `(,a ,b)) (list (foo) (foo :a 1) (foo :a 2 :b 3))
| 0 | NIL |
|---|---|
| 1 | NIL |
| 2 | 3 |
- a more explicit approach could be binding the predicate on the passage of a variable to a supplied-p associated to each variable.
(defun foo (&key (a 0 a-p) (b 1 b-p)) `(,a ,a-p ,b ,b-p)) (list (foo) (foo :a 1) (foo :b 0) (foo :a 1 :b 0))
| 0 | NIL | 1 | NIL |
|---|---|---|---|
| 1 | T | 1 | NIL |
| 0 | NIL | 0 | T |
| 1 | T | 0 | T |