Haskell错误：“没有实例for（Enum [Int]） [英] Haskell Error: "No instance for (Enum [Int])

问题描述

我有以下代码：

  betaRest :: Int  - [Int]  - > Int 
 betaRest n prevDigits | n == 0 =（length prevDigits）
 |否则=（sum（mapRest（n-1））[0..9]））
 
 betaFirst :: Int-> Int 
 betaFirst n | n == 0 = 0 
 |否则=（betaRest（n-1）[1..9]）
  

错误，我不知道为什么。

1）没有从算术序列'0 .. 9'

2）没有从文字'0'产生的（Num [Int]）实例

Haskell认为用..运算符做的事情是枚举吗？但是为什么下面4行的行没有错误（用[1..9]）呢？

---

编辑：我想让代码这样做（程序上）：

  int betaRest ，int [] prevDigits）{
 if（n == 0）return prevDigits.length; 
 else {
 sum = 0; 
 foreach prevDigit in prevDigits {
 sum + = betaRest（n-1，[0..9]）; 
} 
 return sum; 
} 
} 
 
 int betaFirst（int n）{
 if（n == 0）return 0; 
 else return betaRest（n  -  1，[1..9]）; 
} 
  

因此，betaFirst（1）== 9和betaFirst（2）= = 90.是的，有人可能想建议一个公式来生成这个，但我要添加一个过滤器[0..9]，从而减少范围。

< betaRest 到映射。地图是（a - > a） - > [a] - > [a] 所以对于 [Int] 列表你传递它它想要一个 Int - Int 函数。但部分应用 betaRest 是 [Int] - > [0..9] 其类型为<$

c $ c>（Enum t，Num t）=> [t] ，并将其转换为 enumFromTo 0 9 应用程序。所以编译器以另一种方式计算你的错误：如果你为列表定义了特殊的 Num 和 Enum c $ c> [0..9] 成为int列表的列表，那么你的应用程序将是有意义的。

您要使用 inits 或 tails 函数。让我们知道你想要达到什么，以便我们可以帮助解决方案。

最小的修复将添加 prevDigits 作为 map 的参数，并使用lambda抽象来忽略未使用的 prevDigit ：

  |否则= sum（map（\prevDigit  - > betaRest（n-1）[0..9]）prevDigits）
  

I have the following code:

betaRest :: Int -> [Int] -> Int
betaRest n prevDigits | n == 0    = (length prevDigits)
                      | otherwise = (sum (map (betaRest (n - 1)) [0..9]))

betaFirst :: Int -> Int
betaFirst n | n == 0    = 0
            | otherwise = (betaRest (n - 1) [1..9])

It gives me the following errors, and I don't know why.

1) No instance for (Enum [Int]) arising from the arithmetic sequence '0 .. 9'

2) No instance for (Num [Int]) arising from the literal '0'

Does Haskell think that things made with the ".." operator are enumerations? But why isn't there an error for the line that's 4 lines below (with "[1..9]") it then?

---

Edit: What I want the code to do is like this (procedurally):

int betaRest(int n, int[] prevDigits) {
  if (n == 0) return prevDigits.length;
  else {
    sum = 0;
    foreach prevDigit in prevDigits {
      sum += betaRest(n - 1, [0..9]);
    }
    return sum;
  }
}

int betaFirst(int n) {
  if (n == 0) return 0;
  else return betaRest(n - 1, [1..9]);
}

Thus, betaFirst(1) == 9, and betaFirst(2) == 90. Yes, somebody may want to suggest a formula for generating this, but I'm going to add a filter of some sort to [0..9], thus reducing the range.

解决方案

You pass betaRest to map. Map is (a -> a) -> [a] -> [a] so for [Int] list you pass it it wants an Int -> Int function. But partially applied betaRest is [Int] -> Int.

As for [0..9] its type is (Enum t, Num t) => [t] and it's translated into enumFromTo 0 9 application. So compiler figured your error the other way around: if you define special Num and Enum instances for lists, so [0..9] becomes a list of lists of int, then your application will make sense.

But I think you want to use inits or tails function. Let us know what you want to achieve so we can help with solution.

A minimal fix to would be to add prevDigits as an argument to map and use a lambda abstraction to ignore unused prevDigit:

   | otherwise = sum (map (\prevDigit -> betaRest (n - 1) [0..9]) prevDigits)

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