purescript lazy list

gistfile1.hs

module Main where

import Control.Monad.Eff

import Debug.Trace

foreign import data Random :: !

foreign import data Lazy :: * -> *

foreign import defer """
  function Defer (thunk) {
    this.thunk = thunk;
    return this;
  }
  
  function defer(thunk){
    return new Defer(thunk);
  }

  Defer.prototype.force = function () {
    var value = this.thunk();
    delete this.thunk;
    this.force = function () {
      return value;
    };
    return value;
  };
""" :: forall a. (Unit -> a) -> Lazy a

-- | Force evaluation of a `Lazy` value.
foreign import force """
  function force(l){
    return l.force();
  }
""" :: forall a. Lazy a -> a

data LList a = Empty | Cons a (Lazy (LList a))

infixr 6 :::
(:::) :: forall a. a -> (Lazy (LList a)) -> LList a
(:::) = Cons

instance showLlist :: (Show a) => Show (LList a) where
  show Empty = ""
  show (Cons x y) = (show x) ++ comma ++ tail
    where
    tail = show $ force y
    comma = case tail of
        Empty -> ""
        Cons _ _ -> ","

main = print $ (1 :::  defer (\_ -> Empty))