Racket中的'（撇号）是什么？ [英] What is ' (apostrophe) in Racket?
问题描述
我对球拍中'标志的含义有点困惑。在我看来，同一个符号有不同的含义。看看下面的两个简单例子：
返回一个新分配的列表，其中包含vs作为其元素。
> （list 1 2 3 4）
'（1 2 3 4）
产生一个对应于没有它的词汇信息，源位置等，引用的对，矢量和框是不可变的。
> '（1 2 3 4）
'（1 2 3 4）
是：
'符号是否有2个含义（a 符号和列表）或是相同的数据类型和 list 实际上会返回一个引用的常量值？如果第二种情况是这种情况，为什么这么做：
> '（+（ - 2 13）11）
'（+（ - 2 13）11）
> （eval（list +（ - 2 13）11））
0
code>（eval'（+（ - 2 13）11））正常工作并正确评估为 0 ）
但这并不是：
> （list +（ - 2 13）11）
'（＃< procedure：+> -11 11）
> （eval'（＃< procedure：+> -11 11））
。阅读：坏语法`＃<'
> 是REPL的标志 - Read-Eval-Print Loop。
首先，您在REPL提示符处输入的任何表达式都是 read - 转换为某种内部抽象语法树表示形式。然后，这个输入表达式的内部表示被评估 - 即找到它的值。然后打印结果。
当我们输入
> （list 1 2 3 4）
输入表达式被读为嵌套结构，让我们写它作为
[LIST | [1 | [2 | [3 | [4 | NIL]]]]]
根据数据对的列表的常见表示形式， （这里显示一对 a 和 b 为 [a | b] ）。
然后上面的结构被评估，因为它的第一个元素是 LIST 它会调用带有指定参数的 list ，这会导致建立一个新的列表结构，它可以表示为
[1 | [2 | [3 | [4 | NIL]]]]
然后打印它，通常为（1 2 3 4）但球拍选择打印它为'（1 2 3 4）。顺便说一句，它不能被评估，因为 1 不能被称为。
接下来，引用的表达式'（1 2 3 4），它被读作（quote（1 2 3 4））。它被转换为
[QUOTE | [[1 | [2 | [3 | [4 |无]]]] NIL]]
当评估时（根据报价的评估规则），返回它收到的数据。我们表示为
[1 | [2 | [3 | [4 | NIL]]]]
这就是两者相似的原因。我们是否建立一个包含1,2,3和4的新列表;或者我们将它创建为读取过程的一部分，所以它被逐字返回 quote ;结果是一样的。
I am a little confused about the meaning of the ' sign in racket. It appears to me that the same sign has different meanings. Look at 2 simple examples below:
Returns a newly allocated list containing the vs as its elements.
> (list 1 2 3 4)
'(1 2 3 4)
Produces a constant value corresponding to datum (i.e., the representation of the program fragment) without its lexical information, source location, etc. Quoted pairs, vectors, and boxes are immutable.
> '(1 2 3 4)
'(1 2 3 4)
So my question is:
Does the ' sign has 2 meanings (a symbol and a list) or are these the same data type and list actually returns a quoted constant value? If the second is the case why does this work:
> '(+ (- 2 13) 11)
'(+ (- 2 13) 11)
> (eval (list + (- 2 13) 11))
0
(also (eval '(+ (- 2 13) 11)) works and evaluates correctly to 0)
But this does not:
> (list + (- 2 13) 11)
'(#<procedure:+> -11 11)
> (eval '(#<procedure:+> -11 11))
. read: bad syntax `#<'
Related maybe: What is ' (apostrophe) in Lisp / Scheme?
> is a sign of REPL - the Read-Eval-Print Loop.
First, whatever expression you've typed at the REPL prompt is read - converted to some internal abstract syntax tree representation. Then this internal representation of the typed-in expression is evaluated - i.e. its value is found. Then the result is printed.
When we type
> (list 1 2 3 4)
the typed-in expression is read as a nested structure, let's write it as
[LIST | [1 | [2 | [3 | [4 | NIL ]]]]]
according to the usual representation of lists as pairs of data and the rest of the list (here showing a pair of a and b as [a | b]).
Then the above structure is evaluated, and because its first element was LIST it causes the invocation of list with the arguments as specified, which causes a fresh list structure to be built, which can be represented as
[1 | [2 | [3 | [4 | NIL ]]]]
Then it is printed, usually as (1 2 3 4) but Racket chooses to print it as '(1 2 3 4). Incidentally, it can't be evaluated, because 1 can not be called.
Next, the quoted expression '(1 2 3 4), which is read as (quote (1 2 3 4)). It is converted into
[QUOTE | [ [1 | [2 | [3 | [4 | NIL ]]]] | NIL ]]
which, when evaluated (according to the evaluation rule for quote), returns the data it received. Which we represent as
[1 | [2 | [3 | [4 | NIL ]]]]
That's why the two are similar. Whether we build a new list containing 1, 2, 3, and 4; or we cause it to be created as part of read process, so it gets returned verbatim by quote; the result is the same.
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