List 无法从算术序列推断枚举a

我是Haskell的新手，在尝试编译此代码（计算行列式的一部分）时遇到了无法解决的错误：

---

谢谢您的帮助。

您可能想交换

foldr

调用的参数，因此：

致：

（一些额外的修改）

出现错误的原因是，作为

foldr

的结果，您希望出现

[[a]]]

。因此，Haskell认为

[0..genericLength x-1]

也应该有

[[a]]]

类型。由于它内部有一个

。

结构，因此它派生出

0

和

genericLength x-1

应该具有类型

[a]

，但它不知道如何枚举该类型。然而，上述所有推理都基于传递给

foldr

的错误参数

我也认为我们根本不需要

genericLength

。我们可以将

length

与

Int

一起使用，因此：

--pivotzeile = 1
--pivotspalte 0 based!
genLaplaceMatrix :: (Num a, Enum a) => [[a]] -> Int -> [[a]]
genLaplaceMatrix [[]] _ = error "Cannot calculate LaplaceMatrix from [[]]"
genLaplaceMatrix x pivotSpalte 
    | pivotSpalte < 0                         = error "invalid column"
    | pivotSpalte > ((genericLength x) -1)    = error "invalid column"
    | otherwise                       = foldr (\y -> (conVecs y:)) [] [0..length x-1]
    where
      leftPart = take pivotSpalte
      rightPart lP z = takeRight (leftPart lP) z
      conVecs col = concatVectors (leftPart x!!col) (rightPart (leftPart x!!col) (x!!col) )

---

（假设你计算了行列式）这是正确的答案。

（我最近问了一个类似的问题，但删除了它，因为问题的表述不正确，讨论偏离了主题。）请不要删除问题。格式可以修改。下次我会记住这一点；但问题也有了一些变化，这就是为什么我认为最好打开一个新的话题。@FabianSchneider:你为什么要使用

genericLength

？

Int

的最小范围为-2^29到+2^29，这似乎足够了。我自己通过在声明中使用[[a]]->Int->[[a]]解决了第二个错误。非常感谢：D

main.hs:29:76: error:
    • Could not deduce (Enum [a])
        arising from the arithmetic sequence ‘0 .. ((genericLength x) - 1)’
      from the context: (Num a, Enum a, Num b, Ord b)
        bound by the type signature for:
                   genLaplaceMatrix :: (Num a, Enum a, Num b, Ord b) =>
                                       [[a]] -> b -> [[a]]
        at main.hs:24:1-72
    • In the second argument of ‘foldr’, namely
        ‘[0 .. ((genericLength x) - 1)]’
      In the expression:
        foldr
          (\ y acc -> (conVecs y) : acc) [0 .. ((genericLength x) - 1)] []
      In an equation for ‘genLaplaceMatrix’:
          genLaplaceMatrix x pivotSpalte
            | pivotSpalte < 0 = error "invalid column"
            | pivotSpalte > ((genericLength x) - 1) = error "invalid column"
            | otherwise
            = foldr
                (\ y acc -> (conVecs y) : acc) [0 .. ((genericLength x) - 1)] []
            where
                leftPart = take pivotSpalte
                rightPart lP z = takeRight (leftPart lP) z
                conVecs col
                  = concatVectors
                      (leftPart x !! col) (rightPart (leftPart x !! col) (x !! col))
main.hs:29:77: error:
    • Could not deduce (Num [a]) arising from the literal ‘0’
      from the context: (Num a, Enum a, Num b, Ord b)
        bound by the type signature for:
                   genLaplaceMatrix :: (Num a, Enum a, Num b, Ord b) =>
                                       [[a]] -> b -> [[a]]
        at main.hs:24:1-72
    • In the expression: 0
      In the second argument of ‘foldr’, namely
        ‘[0 .. ((genericLength x) - 1)]’
      In the expression:
        foldr
          (\ y acc -> (conVecs y) : acc) [0 .. ((genericLength x) - 1)] []

main.hs:31:23: error:
    • Couldn't match expected type ‘Int’ with actual type ‘b’
      ‘b’ is a rigid type variable bound by
        the type signature for:
          genLaplaceMatrix :: forall a b.
                              (Num a, Enum a, Num b, Ord b) =>
                              [[a]] -> b -> [[a]]
        at main.hs:24:21
    • In the first argument of ‘take’, namely ‘pivotSpalte’
      In the expression: take pivotSpalte
      In an equation for ‘leftPart’: leftPart = take pivotSpalte
    • Relevant bindings include
        pivotSpalte :: b (bound at main.hs:26:20)
        genLaplaceMatrix :: [[a]] -> b -> [[a]] (bound at main.hs:25:1)

foldr (\y acc -> (conVecs y):acc ) [0..((genericLength x)-1)] []

foldr (\y -> (conVecs y:)) [] [0..genericLength x-1]

--pivotzeile = 1
--pivotspalte 0 based!
genLaplaceMatrix :: (Num a, Enum a) => [[a]] -> Int -> [[a]]
genLaplaceMatrix [[]] _ = error "Cannot calculate LaplaceMatrix from [[]]"
genLaplaceMatrix x pivotSpalte 
    | pivotSpalte < 0                         = error "invalid column"
    | pivotSpalte > ((genericLength x) -1)    = error "invalid column"
    | otherwise                       = foldr (\y -> (conVecs y:)) [] [0..length x-1]
    where
      leftPart = take pivotSpalte
      rightPart lP z = takeRight (leftPart lP) z
      conVecs col = concatVectors (leftPart x!!col) (rightPart (leftPart x!!col) (x!!col) )

> print $ det [[1,2],[3,4]]
-2