如何进行模式匹配，执行一个功能，然后在执行的功能上进行模式匹配，以执行另一个功能 [英] how to pattern match, execute a function then pattern match on the executed function to execute another function

问题描述

我想编写一个函数，将数组中的元素从一个位置复制到另一个位置. copy_obj是执行此操作的函数.现在，我得到了一个表示要复制的元素位置的指针列表，因此，我需要在列表中的每个元素上应用函数copy_obj以及我应该开始复制的空闲位置的地址.在我的代码中是f.

I'm suppose to write a function that copies elements in a array from one place to the other. copy_obj is the function doing that. now I am given a list of pointers that represents the locations of elements i need to copy therefore I need to apply the function copy_obj on each elements in the list with the address of the free location where I should start to copy. In my code it is f.

考虑到函数copy_obj返回一对地址，并且其中一个是free的更新值，我需要使用它来递归地调用列表中其他元素上的函数.

Considering that the function copy_obj returns a pair of addresses, and one is the updated value of free, I need to use to call recursively the function on other elements on the list.

下面是我编写的代码，它可以编译，但我在| h::tl -> copy_obj f h处警告说此表达式应具有类型单位"

below is the code I wrote, it compiles but I'am having a warning at | h::tl -> copy_obj f h saying this "expression should have type unit"

我该怎么办?

let rec funct f myList =
  match myList with
  | [] -> f
  | h::tl-> 
      copy_obj f h;
      match (copy_obj f h) with
      | (free_n,addr_n) -> funct free_n tl   

推荐答案

您似乎是另一位写Copy GC的学生. :-)

You seem to be another student writing Copy GC. :-)

表达式e1; e2将按顺序执行e1和e2，通常e1的返回类型应为unit，这意味着不返回任何特殊内容".如果e1是其类型不是unit的表达式，则OCaml编译器会发出警告:表达式应具有类型单位"，因为您可能会通过e1计算出有意义的内容，但将其丢弃.有时这很好地表明了可能的错误.

Expression e1; e2 is to execute e1 and e2 sequentially, and normally e1's return type is expected to be unit, which means "returns nothing special". If e1 is an expression whose type is other than unit, OCaml compiler emits a warning: "expression should have type unit", since it is possible that you compute something meaningful by e1 but you throw it away. It is sometimes a good indication of possible bugs.

在您的情况下，copy_obj f h返回一个元组，可能是(int * int)，而不是unit.因此，您得到了警告.如果确实可以放弃计算结果，则必须写ignore (copy_obj f h)，其中ignore : 'a -> unit.

In your case, copy_obj f h returns a tuple, probably (int * int), and it is not unit. Therefore you got the warning. If you really ok to discard the compuation result you must write ignore (copy_obj f h), where ignore : 'a -> unit.

您两次致电copy_obj f h，这似乎很奇怪.如果没有该函数的定义，我们将无法100％确定，但我想您不需要第一次调用:

You called copy_obj f h twice, which seems very strange. Without the definition of the function we cannot tell 100% sure but I guess you do not need the first call:

let rec funct f myList =
  match myList with
  | [] -> f
  | h::tl-> 
      match copy_obj f h with
      | (free_n,addr_n) -> funct free_n tl   

如果要实现Copy GC，并且copy_obj将对象h复制到可用插槽中且有副作用的可用插槽中，则在此调用两次函数将h存储两次.对于GC算法，这将是一个严重的错误.实际上，警告实际上会尽力为您提供帮助！！

If you are implementing Copy GC and if copy_obj copies an object h to a free slot somewhere available using side effect, calling the function twice here stores h twice. It would be a serious bug for a GC algorithm. And the warning actually tries to help you at this exact point!!

还有一件事. match不是解构您价值的唯一方法. let还可以提取元组的元素.通常我们这样写:

One more thing. match is not the only way to deconstruct your value. let can also extract the elements of your tuple. Normally we write like:

let rec funct f myList =
  match myList with
  | [] -> f
  | h::tl-> 
      let (free_n, addr_n) = copy_obj f h in
      funct free_n tl   

或者您也可以简单地写funct (fst (copy_obj f h)) tl.

Or you can simply write funct (fst (copy_obj f h)) tl.

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