[英]Can one use cons to do ((a . b) . (c . d)) and if not any other means or dot pair cannot have 2nd element like this?
;;; <- can one use cons to do ((a . b) . (c . d))?
(define x (cons a b)); nil -- should it be error
(define x (cons 'a 'b)); (a . b)
(define y (cons 'c 'd)); (c . d)
(define z00 (cons x y)) ; (((a . b) c . d) <- cannot use cons to do ((a . b) . (c . d))?
(define z01 (cons x 'y)) ; ((a . b) . y)
(define z10 (cons 'x y)) ; (x c . d)
(define z11 (cons 'x 'y)); (x . y))
(define z (list x y z00 z01 z10 z11))
; ((a . b) (c . d) ((a . b) c . d) ((a . b) . y) (x c . d) (x . y))
;;; ;;; and if not any other means or dot pair cannot have 2nd element like this?如果没有任何其他方式或点对不能有这样的第二个元素?
Yes, you can.是的你可以。 And the language has a wonderful predicate called equal?
语言有一个很棒的谓词叫做equal?
which will allow you to test this:这将允许您对此进行测试:
> (equal? (cons (cons 'a 'b) (cons 'c 'd))
'((a . b) . (c . d)))
#t
> (equal? '((a . b) . (c . d))
'((a . b) c . d))
#t
And you can even write a little display function which will confirm this:你甚至可以写一点显示 function 来证实这一点:
(define (display-thing thing)
(if (cons? thing)
(begin
(display "(")
(display-thing (car thing))
(display " . ")
(display-thing (cdr thing))
(display ")"))
(display thing)))
And now现在
> (display-thing (cons (cons 'a 'b) (cons 'c 'd)))
((a . b) . (c . d))
> (display-thing '((a . b) . (c . d)))
((a . b) . (c . d))
> (display-thing '((a . b) c . d))
((a . b) . (c . d))
What this should all be telling you is that ((a. b). (c. d))
and ((a. b) c. d)
are merely different ways of writing a structurally identical object .这应该告诉你的是((a. b). (c. d))
和((a. b) c. d)
只是编写结构相同的 object 的不同方式。
Pairs visualize differently based on their content.对根据其内容进行不同的可视化。 If the cdr
of a pair contains the empty list it is a proper list an dthe dot and extra empty list is not shown:如果一对的cdr
包含空列表,则它是一个正确的列表,并且不显示点和额外的空列表:
(cons 'a '())
'(a . ())
; ==> (a)
A pair that has pair as it's cdr
can be visualized as a list element without the .
具有 pair 作为cdr
的对可以被可视化为没有.
and extra parenthesis:和额外的括号:
(cons 'b '(a))
'(b . (a))
; ==> (b a)
(cons 'b '(a . c))
'(b . (a . c))
; ==> (b a . c)
These are just made so that we can have (1 2 3)
displayed instead of (1. (2. (3. ())))
which is how it really is made.这些只是为了让我们可以显示(1 2 3)
而不是(1. (2. (3. ())))
,这就是它的真正制作方式。
If you were to not have a pair or a empty list in the cdr
then it falls back to showing the dotted pair:如果您在cdr
中没有一对或空列表,那么它会退回到显示点对:
(cons 'a 'b)
'(a . b)
; ==> (a . b)
In your example '((a. b). (c. d))
because there is a pair after a dot (eg. the cdr
if the pair the visualization will remove the dot and one pair of parentheses and show it like ((a. b) c. d)
. This is the only acceptable correct way for a REPL to display this even though both your example and the display will be read in as the same structure.在您的示例'((a. b). (c. d))
因为在点之后有一对(例如cdr
,如果可视化将删除点和一对括号并将其显示为((a. b) c. d)
. 这是 REPL 显示此内容的唯一可接受的正确方法,即使您的示例和显示都将作为相同的结构读入。
There is a similar issue with numbers.数字也有类似的问题。 In code you can use 10
, #xa
and #o12
to get the number 10 and the value will have no idea what format is was read in as and only show the base 10 in the REPL.在代码中,您可以使用10
、 #xa
和#o12
来获取数字 10 并且该值将不知道读取的格式是什么,并且仅在 REPL 中显示以 10 为底的值。
;;; ```
;;; <- can one use cons to do ((a . b) . (c . d))?
(define x (cons a b)); nil -- should it be error
(define x (cons 'a 'b)); (a . b)
(define y (cons 'c 'd)); (c . d)
(define z00 (cons x y)) ; (((a . b) c . d) <- cannot use cons to do ((a . b) . (c . d))?
(define z01 (cons x 'y)) ; ((a . b) . y)
(define z10 (cons 'x y)) ; (x c . d)
(define z11 (cons 'x 'y)); (x . y))
(define z22 (cons '(f g) '(h i)))
(define z2c (cons (cons 'f 'g) (cons 'h 'i)))
(define fgc (cons ('f 'g))); should it be error (nil)
; actually not as 'f is an exoression (quote f) and f for some reason is nil it becomes nil from quote of nil abd so is the second one. now (nil . nil) is (nil)
(define z (list x y z00 z01 z10 z11 z22 z2c fgc))
; ((a . b) (c . d) ((a . b) c . d) ((a . b) . y) (x c . d) (x . y))
;;; ```
;;; ;;; and if not any other means or dot pair cannot have 2nd element like this?如果没有任何其他方式或点对不能有这样的第二个元素?
;;; ;;; possibly not可能不是
;;; as the cons join 2 pairs of dotted pairs and can generate one dotted pair
;;; ((a . b) . (c . d)) but the printing rule is reflected the list bias
;;; this new dotted pair will have the first element as (( a . b) ... print as
;;; ((a . b) ...
;;; the 2nd element it will consider whether it is an atom or another dotted pair
;;; (other possibilities like loop back or something else ... not sure)
;;; as (c . d) is a dotted pair the "printing" continues as a list would
;;;
;;; ((a . b) c ...
;;; however the second element of (c . d) is not a dotted pair but an atom and print as
;;; . d) will it becomes
;;; hence even though you form the binary tree head the dot pair would display as a partial list
;;; you can have a list of dotted pairs like z
;;; but not dotted pair of dotted pairs
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