为什么我在此代码的输出中有意外的'#'？

Question

我正在尝试创建一个派生函数来区分使用数据类型,如下所示:

datatype Symex = RCOEFF of real
               | COEFF  of string
               | VAR    of string
               | POWER  of Symex * int
               | NEG    of Symex
               | PLUS   of Symex * Symex
               | MINUS  of Symex * Symex
               | MULT   of Symex * Symex
               | DIVIDE of Symex * Symex

这是*x ^ 3 + 4.0*x ^ 2 + b*x + c的示例

PLUS (MULT (COEFF ("a"),
            POWER (VAR ("x"), 3)),
      PLUS (MULT (RCOEFF (4.0),
                  POWER (VAR ("x"), 2)),
            PLUS (MULT (COEFF ("b"),
                        VAR ("x")),
                  COEFF ("c"))))

我的代码的一部分是

fun deriv (POWER(a, b)) = MULT(RCOEFF(Real.fromInt(b)), POWER(a, b-1))

但是当我计算

deriv(POWER(VAR "x", 3))

输出是

MULT(RCOEFF 3.0 , POWER(VAR # , 3))

为什么输出中有'#'？

请任何帮助将不胜感激!

Answer 1

SML/NJ对结构打印到控制台的深度有限制.如果达到此限制,则a #用于表示结构较大但未显示.

如果您认为限制太低,可以通过将存储的值更新为Control.Print.printDepth更适合您的值来更改限制.

Standard ML of New Jersey v110.69 [built: Mon Jun  8 14:15:08 2009]

- datatype 'a ls = Nil | Cons of 'a * 'a ls;
datatype 'a ls = Cons of 'a * 'a ls | Nil

- Cons(1, Cons(2, Cons(3, Cons(4, Nil))));
val it = Cons (1,Cons (2,Cons #)) : int ls

- Control.Print.printDepth;
val it = ref 5 : int ref

- Control.Print.printDepth := 100;
val it = () : unit

- Cons(1, Cons(2, Cons(3, Cons(4, Nil))));
val it = Cons (1,Cons (2,Cons (3,Cons (4,Nil)))) : int ls