F＃不会匹配列表中的所有元素

Question

I wrote a recursive function, doMoves, that should match each element of a list and call the designated function. However, I ran a list [Draw; Draw] in which Draw function was only called once. I'm not sure why it doesn't match each element.

I've written a recursive function that should take the head of a list each step and match it with a function

let officiate (cards:Card list) (moves:Move list) (goal:int) = 
  let mutable heldCards = []
  let mutable deck = cards

  let doNothing = 
    heldCards

  let DrawCard = 
    lazy (
    heldCards<-List.append heldCards [deck.Head]
    List.length heldCards |> printfn "Length of Drawn heldCards: %d"
    deck<-deck.Tail)

  let rec doMoves movs = 
    match movs with
    | [] -> doNothing
    | x::xs -> 
      match x with
        | Draw -> DrawCard.Force()
      doMoves xs
  doMoves moves
  true

let moves = [Draw; Draw]
let cards = [(Jack,Clubs); (Num(8),Diamonds)]
let card = (Jack,Spades)
officiate cards moves 42

I expected the output to state

Length of Drawn heldCards: 1
Length of Drawn heldCards: 2

but I get the following:

Length of Drawn heldCards: 1

Answer 1

The error is because DrawCard is a value, not a function. To write a parameterless function in F# you need to specify unit as input. ie

  let drawCard () = //your implementation

I made an attempt to write what I think you are trying to do in a more functional manner:

 let officiate (cards:Card list) (moves:Move list) (goal:int) =

   let drawCard (heldCards: Card list) (deck: Card list) =
     match deck with
     | [] -> Error "Empty deck"
     | head::tail -> Ok (List.append heldCards [head], tail)

   let rec doMoves heldCards deck movs =
     printfn "Hand is %d, deck is %d." (List.length heldCards) (List.length deck)
     match movs with
     | [] -> heldCards, deck
     | x::xs ->
       match x with
         | Draw ->
             drawCard heldCards deck
             |> function
             | Ok (h, d) -> doMoves h d xs
             | Error s ->
                 printfn "An error occurred: %s" s
                 heldCards, deck
         | Discard c -> failwith "todo"
   doMoves [] cards moves

 let moves = [Draw; Draw]
 let cards = [(Jack,Clubs); (Num(8),Diamonds)]
 officiate cards moves 42