使用Lisp宏可以做什么，而使用一等函数则不能做什么? [英] What can you do with Lisp macros that you can't do with first-class functions?

问题描述

我想我了解Lisp宏及其在编译阶段的作用.

I think I understand Lisp macros and their role in the compilation phase.

但是在Python中，您可以将一个函数传递给另一个函数

But in Python, you can pass a function into another function

def f(filename, g):
  try:                                
     fh = open(filename, "rb") 
     g(fh)
  finally:
     close(fh) 

因此，我们在这里得到了懒惰的评估.我可以使用宏而不是使用函数作为一流对象怎么办?

So, we get lazy evaluation here. What can I do with macros and not with functions as first class objects?

推荐答案

首先Lisp也具有一流的功能，因此您也可以问:如果我已经拥有一流的，为什么我需要Lisp中的宏?功能".答案是一流的函数不允许您使用语法.

First of all Lisp has first-class functions too, so you could as well ask: "Why do I need macros in Lisp if I already have first-class functions". The answer to that is that first-class functions don't allow you to play with syntax.

在外观上，一流的函数使您可以编写f(filename, some_function)或f(filename, lambda fh: fh.whatever(x))，但不能编写f(filename, fh, fh.whatever(x)).尽管可以说这是一件好事，因为在最后一种情况下，fh的突然来源还不清楚.

On a cosmetic level, first-class functions allow you to write f(filename, some_function) or f(filename, lambda fh: fh.whatever(x)), but not f(filename, fh, fh.whatever(x)). Though arguably that's a good thing because in that last case it is a lot less clear where fh suddenly comes from.

更重要的是，函数只能包含有效的代码.因此，您无法编写将函数作为参数并反向"执行的高阶函数reverse_function，以致reverse_function(lambda: "hello world" print)将执行print "hello world".使用宏，您可以执行此操作.当然，这个特定示例非常愚蠢，但是在嵌入领域特定语言时，此功能非常有用.

More importantly functions can only contain code that is valid. So you can't write a higher-order function reverse_function that takes a function as an argument and executes it "in reverse", so that reverse_function(lambda: "hello world" print) would execute print "hello world". With a macro you can do this. Of course this particular example is quite silly, but this ability is enormously useful when embedding domain specific languages.

例如，您无法在python中实现常见的lisp的loop构造.地狱，如果它不是真正内置的，甚至不能在python中实现python的for ... in构造-至少不是使用该语法.当然，您可以实现for(collection, function)之类的东西，但是要漂亮得多.

For example you couldn't implement common lisp's loop construct in python. Hell, you couldn't even implement python's for ... in construct in python if it wasn't really built-in - at least not with that syntax. Sure you could implement something like for(collection, function), but that's a lot less pretty.

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