Standard Error ML / CML Statement - Operand Error

Question

I am trying to implement a parallel list using the CML extensions for standard ML, but I run into errors that are probably due to the fact that I was new to standard ML. I implemented the clister as having input and output channels, and I save the state of the list 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 a 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 get:

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}

Answer 1

So your problem is to determine clist

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

, 'a 'a. . CONS x , , x 'a , HEAD, , . , CONS 'a a HEAD 'a list. , clist

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

cnil unit -> 'a clist, .

:

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