Standard ML / CML wrong operator - operand error

I am trying to 开发者_运维问答implement a concurrent list using CML extensions of Standard ML but i am running into errors that are probably to do with my being a newbie in Standard ML. I have implemented the clist as having an input and output channel and I store the list state in a loop. However my code does not compile and gives errors below

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 with initial contents l *)
  val cnil = 
    let
      val req = channel()
      val reply = channel()
      fun loop l = case recv req of
          CONS x =>
            (loop (x::l))
        | 
          HEAD => (send(reply, l); loop l)
    in
      spawn(fn () => loop nil);
      CLIST {reqCh=req,replyCh=reply}
    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

This is the signature file

signature CLIST =
  sig
    type 'a clist

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

Errors I am getting:

clist.sml:21.4-21.35 Error: operator and operand don't agree [circularity]
  operator domain: {replyCh:'Z list chan, reqCh:'Z list request chan}
  operand:         {replyCh:'Z list chan, reqCh:'Z request chan}
  in expression:
    CLIST {reqCh=req,replyCh=reply}

So your problem is in your definition of clist

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

This is saying that the request channel takes in a request of 'a and replies with a 'a. This is not consistent with your implementation. When you send a CONS x request on the channel, you are saying add x of type 'a to the list, but when you send a HEAD request, you are saying give me back the entire list. Thus, a CONS request should take a 'a and a HEAD request should return a 'a list. You can fix your problem by changing your clist definition to

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

I would also suggest changing your definition of cnil to a unit -> 'a clist function, this way you can create distinct concurrent lists.

For example:

val l1 = Clist.cnil()  
val l2 = Clist.cnil() (*cons'ing onto l2 won't affect l1*)