8.3 KiB
- Arguments
assert_equal_typemacro in testing- use an enum for builtin identifiers
- dont create new nils or builtins, but store one of each globally
- source code locations for errors
- Build-in forms
- Types
Arguments
In Emacs lisp, keyword arguments must be passed in the same order as they were
defined in, so ((lambda (a &key b c) (+ a b c)) 1 3 :b 2) will preduce an
error, because b was passed as last argument but was defined as second argument.
((lambda (a b) (+ a b)) 1 2) ;; 3 ((lambda (a &key b) (+ a b)) 1 :b 2) ;; 3 ((lambda (a &key b) (+ a b)) :b 2 1) ;; error
But if mulitiple keyword arguments are defined, their ordering does not matter.
((lambda (a &key b &key c &key d) (+ a b c d)) 1 :d 4 :c 3 :b 2) ;; 10
It is even possible to have a non keyword argument bewtween keyword arguments and when calling, switch the order.
((lambda (a &key b c &key d) (+ a b c d)) 1 :d 4 3 :b 2) ;; 10 ((lambda (a &key b c &key d) (+ a b c d)) 1 :d 4 :b 2 3) ;; error
However somehow it is not possible to flip the ordering at the first argument
((lambda (a &key b) (+ a b)) 2 :b 1) ;; 3 ((lambda (a &key b) (+ a b)) :b 2 1) ;; error ((lambda (&key a b) (+ a b)) :a 2 1) ;; 3 ((lambda (&key a b) (+ a b)) 1 :a 2) ;; error
So it seems, positional arguments have to be at the exact position they were defined in, but keyword arguments can appear in any order, but at the places where keyword arguments have been defined.
Idea
Every argument can be set using keys, but keyword arguments always come strictly after positional arguments
((lambda (a b c d)) 1 2 3 4) ;; ok ((lambda (a b c d)) 1 2 :c 2 :d 1) ;; ok ((lambda (a b c d)) 1 2 :d 1 :c 2) ;; ok ((lambda (a b c d)) :d 1 1 2 :c 2) ;; error, keyword arguments must come last ((lambda (a b c d)) 1 2 :c 2 :d 1 :a 2) ;; error, 'a' was passed two times ((lambda (a b c d)) 1 2 :d 1) ;; error, c was not passed, but does not have a default value ((lambda (a b c :default 10 d)) 1 2 :d 1) ;; okay again
The drawback then is that you can never pass a keyword as a positional argument, because it will always try to assign the next argument to the variable defined by the keyword. You could still pass it as a keyword argument though. This is especially annoying because we want to use keywords as type identifiers.
(= (type "hello") :string)
((lambda (i-wanna-get-a-kw)) :hey) ;; error, unmatched value for keyword argument ((lambda (i-wanna-get-a-kw)) :i-wanna-get-a-kw :hey) ;; this would work again
Other Idea is to mark the place in the parameter list where keyword argumetns start and arguments before that are positional and will not be stuffed into key-value pairs and arguments after that will be keyword arguments
<lambda list> -> <positional argument>*
[:keys ( <keyword arguments> [:defaults-to <value>])*]
[(:rest <rest argument>)|(:body <body argument>)]
(= (type "hello") :string) ;; this will work again because (=) does
;; not use keyword arguments
((lambda (i-wanna-get-a-kw)) :hey) ;; would work because of the same reason
(define fun
(lambda (:keys a b c d)
(+ a b c d)))
(fun 1 2 3 4) ;; okay
(fun 1 2 :d 3 :c 4) ;; okay
(fun 1 2 :d 3 :c 4 :a 2) ;; error, 'a' was passed two times
(fun :c 1 :b 2 :d 3 :a 4) ;; okay
(fun :c 1 :b 2 :d 3 4) ;; error, positional argument after keyword
;; and fun does not accept rest parameter
(define fun2
(lambda (:quoted op a b :keys c d :rest r)
(print r)
(op a b c d)))
>> (fun2 * 1 2 :c 3 :d 4 "this" "can" :be "whatever")
("this" "can" :be "whatever")
24
>> (define print-before-eval (lambda (:body expr)
(print expr)
(eval expr)))
>> (define r (print-before-eval (+ 1 2 3)))
(+ 1 2 3)
>> r
6
Wait this does not work either because you won't be able to pass keywords into rest.
Unrealated Idea: Distinguish between functions and macros
The difference is really simple:
- Function
-
like a lambda, but no argument is automatically quoted
- Macro
-
like a lambda, but every argument is automatically quoted
This is important, because a macro should not introduce a new environment, but use the one it lives in.
>> (define print-before-eval (macro (:rest expr)
(print expr)
(eval expr))
the actual problem
We have some requirements to the arguments:
-
positional arguments (always required, no default value)
-
keyword arguments (always come after positionals), can be not passed if there is a default value
-
We want to be able to have a rest paramter, where everything is dumped in to that exceeds the other parameters
The problem is, we have no way of knowing if the current keyword we are reading is still part of the keyword block or part of the rest paramter.
Consider:
>> (define fun (lambda (a b :key c d :defaults-to 4 :rest) nil))
>> (fun 1 2 :c 3 :d 5)
(defun haha (par1 par2)
(+ 1 2))
What should this be evaluated to?
| a | b | c | d | rest | |
|---|---|---|---|---|---|
| v1 | 1 | 2 | 3 | 5 | nil |
| v2 | 1 | 2 | 3 | 4 | (:d 5) |
It is ambiguous. It is however important to note that this can only happen when there are optinal keyword arguments (keyword arguments with default values). Otherwise we could always know which variables are set and wich have not been set to see if we are in an errornious state.
Solution
We can resolve that problem by always making the the decision of chosing v1
between v1 and v1. This means, when we are expecting to read kwargs, and we
encounter an kwarg, try to apply it (this can fail, for two reasons:
-
The keyword has no following value
-
The keyword is not a keyword argument in the funciton
), if it fails treat it as part of the rest arguments, and read all the following arguments into the rest arguments, or if it works, use it as the keyword argument.
Implementation
typedef struct {
char* docstring;
char** positional_arguments;
struct {
char* identifier;
Ast_node* default_value; // will be nullptr if no defalut value was declared
}* keyword_arguments;
char* rest_argument; // will be nullptr if no rest argument is declared
Ast_Node_array_list body;
} lambda;
TODO
assert_equal_type macro in testing
DONE use an enum for builtin identifiers
CLOSED: [2018-10-11 Do 17:15]
TODO dont create new nils or builtins, but store one of each globally
TODO source code locations for errors
Build-in forms
DONE +
CLOSED: [2018-09-18 Di 12:14]
DONE -
CLOSED: [2018-09-18 Di 12:14]
DONE *
CLOSED: [2018-09-18 Di 12:14]
DONE /
CLOSED: [2018-09-18 Di 12:14]
TODO >
TODO <
TODO =
DONE if
CLOSED: [2018-09-18 Di 12:14]
DONE and (short circuiting)
CLOSED: [2018-10-05 Fr 22:21]
DONE or
CLOSED: [2018-10-05 Fr 22:21]
DONE not
CLOSED: [2018-10-05 Fr 22:21]
(not 1) ;; == (not . (1 . nil)) (not (expression 1 3)) ;; == (not . ((expression . (1 . (3 . nil))) . nil) . nil) (not) ;; == (not . nil) (not 1 2) ;; == (not . (1 . (2 . nil)))
t
DONE first (car)
CLOSED: [2018-10-08 Mo 20:28]
DONE rest (cdr)
CLOSED: [2018-10-08 Mo 20:28]
DONE pair (cons)
CLOSED: [2018-10-08 Mo 20:28]
TODO load (import)
DONE define
CLOSED: [2018-10-08 Mo 20:28]
TODO lambda
TODO progn
DONE eval
CLOSED: [2018-10-08 Mo 20:28]
(setq condition (quote (= a 10))) (setq a 10) (eval condition) (setq a 11) (eval condition)
DONE quote
CLOSED: [2018-10-08 Mo 20:28]
DONE list
CLOSED: [2018-10-08 Mo 21:06]
(quote (cons 2 (cons 3 (cons 4 ())))) ;; (cons 2 (cons 3 (cons 4 nil))) (quote (2 . (3 . (4 . ())))) ;; (2 3 4)
(list (cons 2 (cons 3 (cons 4 ())))) ;; ((2 3 4)) (list (quote(2 . (3 . (4 . ())))))
DONE print
CLOSED: [2018-10-08 Mo 20:28]
DONE read
CLOSED: [2018-10-08 Mo 20:28]
DONE type
CLOSED: [2018-10-08 Mo 21:30]
TODO info
Types
-
Symbol
-
Number
-
String
-
pair (cons-cell)
-
lambda function lambda
-
built-in function