了解常见Lisp中的引号 [英] Understanding sharp quote in common lisp

问题描述

在下面的实验中，我将REPL返回错误的地方缩写为&.添加了[num]，因此可以在讨论中参考.

In my experiments below I've abbreviated where the REPL returns an error, & added [num] so these can be referenced in discussion.

我对为什么调用存储在变量中的函数的尝试失败感到困惑.在我看来，语法比需要的复杂.

I'm a bit confused as to why my attempts to call a function stored in a variable are failing. It seems to me that the syntax is more complex than it needs to be.

为什么我既不发布(f 3)，也不发布(#'f 3)?不允许将尖引号作为表格的第一个元素?为什么在这里需要 funcall ?

Why can I issue neither (f 3) nor even (#'f 3)? Is sharp quote not allowed as the first element of a form? Why is funcall required here?

[235]> (setf f #'abs)               ; I'm ok with this
#<SYSTEM-FUNCTION ABS>

[236]> (abs 3)                      ; This is fine
3

[237]> (f 3)                        ; Err due to sep. fn namespace. OK.
-- Err[1]: "Undefined function f" --

[238]> (#'f 3)                      ; Don't get what this err is telling me...
-- Err[2]: "#'F is not a function name, try using a symbol instead"

[239]> (funcall #'f 3)              ; seems very long winded...!
3

这是否意味着系统功能与用户定义功能的区别?

Does this mean system functions are treated differently from user defined functions?

出于完整性考虑:

[240]> (funcall abs 3)
-- Err[3]: variable ABS has no value -- ; I get why this is an error.

[241]> (funcall #'abs 3)                ; Isn't this verbose... ?
3

我还没有进入ANSI Common Lisp中的符号一章，也许对您有所帮助.

I haven't got to the symbols chapter in ANSI Common Lisp yet, maybe that will help... thanks for any tips.

推荐答案

[235]> (setf f #'abs)               ; I'm ok with this
#<SYSTEM-FUNCTION ABS>

在上述类型中，将一个名为 f 的变量设置为一个函数对象-来自名为 abs 的函数.

Above kind of sets a variable named f to a function object - from the function called abs.

[236]> (abs 3)                      ; This is fine
3

上面调用了函数 abs .

[237]> (f 3)                        ; Err due to sep. fn namespace. OK.

上面:没有名为 f 的函数.

Above: there is no function named f.

-- Err[1]: "Undefined function f" --

[238]> (#'f 3)                      ; Don't get what this err is telling me...
-- Err[2]: "#'F is not a function name, try using a symbol instead"

以上:Common Lisp仅接受符号作为函数名称，符号作为宏名称，符号作为特殊运算符或lambda表达式作为cons形式的第一个元素.(function f)不是函数名称.

Above: Common Lisp accepts only symbols as function names, symbols as macro names, symbols as special operators or lambda expressions as the first element of a cons form. (function f) is not a function name.

这是否意味着系统功能与用户定义功能的区别?

Does this mean system functions are treated differently from user defined functions?

否.

[239]> (funcall #'f 3)              ; seems very long winded...!
3

上面用命名函数 f 中的函数对象调用函数 funcall .然后 funcall 会以 3 作为参数来调用此函数对象.

Above calls the function funcall with the function object from the named function f. funcall then calls this function object with 3 as the argument.

似乎缠绕很久

是的.

为什么我既不发行(f 3)也不发行(#'f 3)?不允许将尖引号作为表格的第一个元素吗?

Why can I issue neither (f 3) nor even (#'f 3)? Is sharp quote not allowed as the first element of a form?

因为 f 没有命名函数.它命名一个变量.#'f 也不是函数名称.我们必须使用函数名(实际上是一个符号).

Because f is not naming a function. It names a variable. #'f is also not a function name. We are required to use a function name (a symbol actually).

命名空间

公用Lisp(与其他Lisp方言一样)有两个用于函数和变量的命名空间.

Common Lisp (like some other Lisp dialects) has two namespaces for functions and for variables.

定义变量 foo :

CL-USER 54 > (defvar foo 3)
FOO

定义一个函数 foo :

CL-USER 55 > (defun foo (x) (* foo 10))
FOO

我们可以使用从变量 foo 获得的值调用函数 foo :

We can call the function foo with the value obtained from the variable foo:

CL-USER 56 > (foo foo)
30

如何从函数的全局名称获取函数对象:

How to get the function object from the global name of the function:

CL-USER 57 > (fdefinition 'foo)
#<interpreted function FOO 4060001CAC>

CL-USER 58 > (symbol-function 'foo)
#<interpreted function FOO 4060001CAC>

与上述相同，但用一个简短的符号表示:

Same as above, but with a short notation:

CL-USER 58a > #'foo
#<interpreted function FOO 4060001CAC>

CL-USER 59 > (function foo)            ; works also for local functions
#<interpreted function FOO 4230008AAC>

如何从全局变量获取值:

How to get a value from a global variable:

CL-USER 60 > (symbol-value 'foo)
3

或者只使用变量:

CL-USER 61 > foo
3

一些肯定:

正面:没有名称冲突.

我们可以写

(defun foo (list) (list list))

也不必写

(defun foo (lst) (list lst))

正面:更简单的编译

(let ((list 3))
  (list 1 list 3))

以上内容在Common Lisp中永远不会是一个错误.在Scheme中，这将是一个错误: 3不是函数.

Above will never be an error in Common Lisp. In Scheme it would be an error: 3 is not a function.

这篇关于了解常见Lisp中的引号的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！