Haskell没有出现（小数a0）的实例 [英] Haskell No instance for (Fractional a0) arising

问题描述

我在Haskell中有以下代码：

  powmod base 1 m = mod base m 
 powmod base exp m |即使exp = mod（pow2 * pow2）m 
 |否则= mod（base * powmod base（exp-1）m）m 
 where 
 pow2 = powmod base（div exp 2）m 
 
 part1 jn 0 = 0 
 part1 jnk =（part1 jn（k-1））+（powmod 16（nk）r）/ r 
其中
r = 8 * k + j 
  

代码在ghci上没有任何问题。问题出现在我试图调用函数part1为：

  part1 4 10 4 
 
 
 
没有实例（分数a0）由于使用它'类型
变量 a0'是不明确的注意：有几个潜在的
实例：instance Integral a =>分数（GHC.Real.Ratio a）
  - 定义在 GHC.Real'实例分数双 - 定义在 GHC.Float'实例分数浮点 - 定义于 GHC.Float'
在 print'的第一个参数中，即`it'在
交互式GHCi命令的列表中：print it  
 
我不明白这个问题。任何帮助都会很棒。谢谢 
 
解决方案
这里的问题是一个非常常见的问题 - 混入 / code>与整合操作（如 mod ） - 这导致两个约束：一个用于 Integral 和一个用于小数（（/）） - 但是没有基本的数字类型，它们都是它们的一个实例 - 所以最终会遇到不可解析的约束条件，而GHC（i）不会找到匹配的实例并投诉。
 
 
 
如果您签名的话，其中大部分会变得很明显 - 是的，您不必如此，但正如您在这里看到的那样，无论如何这都是非常有价值的。
 
 
 
假设你想在 Integer 上工作，你可以得到： 
 
 
  part1： ：整数 - >整数 - >整数 - >整数
 part1 jn 0 = 0 
 part1 jnk =（part1 jn（k-1））+（powmod 16（nk）r）`div` r 
 where 
r = 8 * k + j 
  
注意我替换了（/）与 div ？ Haskell一旦包含预期类型就会指出这一点。
 
 
 
 也注意当然
  part1 :: Integral a => a  - > a  - > a  - > 
  
也可以工作
 
I have the following code in Haskell:
powmod base 1 m = mod base m
powmod base exp m | even exp  = mod (pow2 * pow2) m
                  | otherwise = mod (base * powmod base (exp - 1) m) m
     where
        pow2 = powmod base (div exp 2) m

part1 j n 0 = 0
part1 j n k = (part1 j n (k-1)) + (powmod 16 (n-k) r)/r
   where
     r = 8*k+j
The code is loaded without any problem on ghci. The problem comes when I try to call the function part1 as:
part1 4 10 4
I get:

  
No instance for (Fractional a0) arising from a use of it' The type
  variablea0' is ambiguous Note: there are several potential
  instances:   instance Integral a => Fractional (GHC.Real.Ratio a)
      -- Defined in GHC.Real'   instance Fractional Double -- Defined inGHC.Float'   instance Fractional Float -- Defined in GHC.Float'
  In the first argument ofprint', namely `it' In a stmt of an
  interactive GHCi command: print it
I don't understand this problem. Any help would be great. Thanks
解决方案
Your problem here is a very common one - it roots into you mixing (/) with Integral operations like mod - this leads to two constraints: one for Integral and one for Fractional (the (/)) - but there is no basic number type which is an instance of both of them - so you end up with unsolvable constraints and GHC(i) will not find a matching instance and complain.

Most of this becomes obvious if you signatures - yes you don't have to but as you can see here it's valuable to do so anyway.

Assuming you want to work over Integer you get this:
part1 :: Integer -> Integer -> Integer -> Integer
part1 j n 0 = 0
part1 j n k = (part1 j n (k-1)) + (powmod 16 (n-k) r) `div` r
   where
     r = 8*k+j
note that I replaced (/) with div? Haskell points you to this as soon as you include expected types.

also note that of course
part1 :: Integral a => a -> a -> a -> a
will work as well

                        
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