sml - ML 中的多态列表

Question

我在 ML 中有这段代码：

local

fun unfolder( [] , n ) = []
    | unfolder( l::ls, n ) = (n, l) :: unfolder( ls, n )

in

fun flat list = unfolder(list, 1)  

end;

它给了我一个错误：

unexpected exception (bug?) in SML/NJ: EA [EA]
  raised at: ../../MLRISC/x86/mltree/x86.sml:417.32-417.34
             ../compiler/Basics/stats/stats.sml:198.40
             ../compiler/Basics/stats/stats.sml:198.40
             ../compiler/Basics/stats/stats.sml:198.40
             ../compiler/TopLevel/interact/evalloop.sml:44.55

但是当我改变its (n, l) to (n, l:int)它时，什么时候可以(n, l:'a)，有人可以解释为什么多态类型不起作用，提前谢谢

Answer 1

It is an internal bug in SML/NJ. The program works flawlessly if compiled with MLton, adding:

val _ =
  let val l = flat [1,2,3]
      fun printer (a,b) = Int.toString(a) ^ ", " ^ Int.toString(b)
  in
    print (printer (hd l) ^ "\n")
  end

Report it. Your example seem digestible in size - and probably has to do with polymorphism representation.