SMLNJ报错：type constructor list参数不匹配，新手求助修复及实数转整数方法

Let's break down your problems step by step:

Fixing the Type Error #

The error Error: type constructor list given 0 arguments, wants 1 happens because in Standard ML, list is a parameterized type — it requires you to specify what kind of elements it holds (like int list or real list). Your code uses lista:list and result:list without defining their element types, which confuses the type checker.

There are a couple other issues in your code to fix too:

• The gcd function typically takes two integers, but you're passing an entire list and a real value. Assuming you want the greatest common divisor of the current list element and lcm_wi, you should pass the single element (not the whole list).
• N is an unbound variable here — unless you've defined it elsewhere, you should use length lista to get the size of the input list instead.

Here's a revised version of your code (assuming Rate is a real list, and we define a helper function for real-number GCD since standard gcd works with integers):

(* Helper function to calculate GCD of two real numbers *)
fun real_gcd (x: real, y: real): real =
    if Real.==(y, 0.0) then x
    else real_gcd (y, x - Real.fromInt(Real.floor(x/y)) * y);

fun without_i (num:int) (lista:real list) (result:real list) (lcm_wi:real) =
    if num >= length lista - 1 then result
    else let
        val b = List.nth(lista, num)
        val a = real_gcd(b, lcm_wi)
    in
        without_i (num+1) lista (lcm_wi :: result) ((b/a)*lcm_wi)
    end;

val before_i_rev = without_i 0 Rate [] 1.0;

Converting REAL to INT #

Standard ML provides several built-in functions to convert real values to int, each with different rounding behavior:

• Real.trunc: Truncates the decimal part (rounds towards zero) — e.g., Real.trunc 3.7 = 3, Real.trunc ~3.7 = ~3
• Real.floor: Rounds down to the nearest integer — e.g., Real.floor 3.7 = 3, Real.floor ~3.7 = ~4
• Real.ceil: Rounds up to the nearest integer — e.g., Real.ceil 3.2 = 4, Real.ceil ~3.2 = ~3
• Real.round: Rounds to the nearest integer (standard rounding) — e.g., Real.round 3.4 = 3, Real.round 3.5 = 4

Example usage:

val rounded = Real.round 5.6;    (* Evaluates to 6 *)
val truncated = Real.trunc 7.9;  (* Evaluates to 7 *)

Note: If the real value is outside the valid range for int, this will cause a runtime error, so make sure your values are within bounds before converting.

内容的提问来源于stack exchange，提问作者Celia