标准ml值限制误差

Question

您好，我需要帮助理解为什么我在这段代码中遇到值限制错误，以及如果可能的话如何解决它。

特别是在 val cnil 中，我试图创建一个空的 CLIST 结构来与签名匹配，但我不断收到此值限制错误。

感谢您的帮助

structure Clist : CLIST = 
struct
  open CML

  datatype 'a request = CONS of 'a | HEAD

  datatype 'a clist = CLIST of { reqCh : 'a request chan, replyCh : 'a chan }

  (* create a clist *)
  val cnil =
    let
      val reqCh = channel()
      val replyCh = channel()
      fun loop l = case recv reqCh of
          CONS x =>
            (loop (x::l))
        | 
          HEAD => (let fun head (h::t) = h | head [] = Empty in send(replyCh, head(l)) end ; loop l)
    in
      spawn(fn () => loop nil);
      CLIST {reqCh = channel(), replyCh = channel() }
    end


  fun cons x (CLIST {reqCh, replyCh})=  
    (send (reqCh, CONS x); CLIST {reqCh = reqCh, replyCh = replyCh})

  fun hd (CLIST {reqCh, replyCh}) = (send (reqCh, HEAD); recv replyCh)  
end

，这是签名文件

signature CLIST =
  sig
    type 'a clist

    val cnil : 'a clist
    val cons : 'a -> 'a clist -> 'a clist
    val hd : 'a clist -> 'a
  end

，这是我收到的错误：

clist.sml:10.7-22.5 Warning: type vars not generalized because of
   value restriction are instantiated to dummy types (X1,X2,...)
clist.sml:1.1-29.4 Error: value type in structure doesn't match signature spec
    name: cnil
  spec:   'a ?.Clist.clist
  actual: ?.X1 ?.Clist.clist

Answer 1

您的代码有两个问题。一是值限制错误，您可以通过更改

val cnil : 'a clist

to

val cnil : unit -> 'a clist

和

val cnil =

to

fun cnil () =

来修复第二个问题是 cnil 似乎没有做任何事情 - 也许是因为 head 函数返回 Empty 而不是引发 Empty 并且您使其变得多余？ Basis 库中已经有一个 hd 函数可以执行此操作。 clist 类型也不需要是数据类型。我想你想要的是这样的：

structure Clist : CLIST =
struct
  open CML

  datatype 'a request = CONS of 'a | HEAD

  type 'a clist = { reqCh : 'a request chan, replyCh : 'a chan }

  (* create a clist *)
  fun cnil () =
    let
      val clist as {reqCh, replyCh} = {reqCh = channel(), replyCh = channel()}       
      fun loop l = case recv reqCh of
          CONS x =>     
            (loop (x::l))
        |                                     
          HEAD => (send(replyCh, hd l); loop l)
    in                        
      spawn(fn () => loop nil);
      clist
    end


  fun cons x (clist as {reqCh, replyCh})=
    (send (reqCh, CONS x); clist)

  fun hd (CLIST {reqCh, replyCh}) = (send (reqCh, HEAD); recv replyCh)
end