Question

我在haskell中有我的isPrime函数，如下所示：

isPrime = \x -> length (filter (\y -> x `mod` y == 0) [2..(x - 1)]) == 0

它工作正常。

但当我即将优化这样的功能时：

isPrime = \x -> length (filter (\y -> x `mod` y == 0) [2..(floor(sqrt x))]) == 0

发生了一些错误，为什么会发生这种情况？

嗯，错误代码是：

Euler.hs:34:41:
    No instance for (Integral a0) arising from a use of ‘mod’
    The type variable ‘a0’ is ambiguous
    Relevant bindings include
      y :: a0 (bound at Euler.hs:34:34)
      x :: a0 (bound at Euler.hs:34:12)
      isPrime :: a0 -> Bool (bound at Euler.hs:34:1)
    Note: there are several potential instances:
      instance Integral Int -- Defined in ‘GHC.Real’
      instance Integral Integer -- Defined in ‘GHC.Real’
      instance Integral GHC.Types.Word -- Defined in ‘GHC.Real’
    In the first argument of ‘(==)’, namely ‘x `mod` y’
    In the expression: x `mod` y == 0
    In the first argument of ‘filter’, namely ‘(\ y -> x `mod` y == 0)

Euler.hs:34:49:
    No instance for (Eq a0) arising from a use of ‘==’
    The type variable ‘a0’ is ambiguous
    Relevant bindings include
      y :: a0 (bound at Euler.hs:34:34)
      x :: a0 (bound at Euler.hs:34:12)
      isPrime :: a0 -> Bool (bound at Euler.hs:34:1)
    Note: there are several potential instances:
      instance Eq a => Eq (GHC.Real.Ratio a) -- Defined in ‘GHC.Real’
      instance Eq () -- Defined in ‘GHC.Classes’
      instance (Eq a, Eq b) => Eq (a, b) -- Defined in ‘GHC.Classes’
      ...plus 23 others
    In the expression: x `mod` y == 0
    In the first argument of ‘filter’, namely ‘(\ y -> x `mod` y == 0)
    In the first argument of ‘length’, namely
      ‘(filter (\ y -> x `mod` y == 0) [2 .. (floor (sqrt x))])’

Euler.hs:34:52:
    No instance for (Num a0) arising from the literal ‘0’
    The type variable ‘a0’ is ambiguous
    Relevant bindings include
      y :: a0 (bound at Euler.hs:34:34)
      x :: a0 (bound at Euler.hs:34:12)
      isPrime :: a0 -> Bool (bound at Euler.hs:34:1)
    Note: there are several potential instances:
      instance Num Double -- Defined in ‘GHC.Float’
      instance Num Float -- Defined in ‘GHC.Float’
      instance Integral a => Num (GHC.Real.Ratio a)
        -- Defined in ‘GHC.Real’
      ...plus three others
    In the second argument of ‘(==)’, namely ‘0’
    In the expression: x `mod` y == 0
    In the first argument of ‘filter’, namely ‘(\ y -> x `mod` y == 0)

Euler.hs:34:55:
    No instance for (Enum a0)
      arising from the arithmetic sequence ‘2 .. (floor (sqrt x))’
    The type variable ‘a0’ is ambiguous
    Relevant bindings include
      x :: a0 (bound at Euler.hs:34:12)
      isPrime :: a0 -> Bool (bound at Euler.hs:34:1)
    Note: there are several potential instances:
      instance Enum Double -- Defined in ‘GHC.Float’
      instance Enum Float -- Defined in ‘GHC.Float’
      instance Integral a => Enum (GHC.Real.Ratio a)
        -- Defined in ‘GHC.Real’
      ...plus 7 others
    In the second argument of ‘filter’, namely
      ‘[2 .. (floor (sqrt x))]’
    In the first argument of ‘length’, namely
      ‘(filter (\ y -> x `mod` y == 0) [2 .. (floor (sqrt x))])’
    In the first argument of ‘(==)’, namely
      ‘length (filter (\ y -> x `mod` y == 0) [2 .. (floor (sqrt x))])’

Euler.hs:34:60:
    No instance for (RealFrac a0) arising from a use of ‘floor’
    The type variable ‘a0’ is ambiguous
    Relevant bindings include
      x :: a0 (bound at Euler.hs:34:12)
      isPrime :: a0 -> Bool (bound at Euler.hs:34:1)
    Note: there are several potential instances:
      instance RealFrac Double -- Defined in ‘GHC.Float’
      instance RealFrac Float -- Defined in ‘GHC.Float’
      instance Integral a => RealFrac (GHC.Real.Ratio a)
        -- Defined in ‘GHC.Real’
    In the expression: (floor (sqrt x))
    In the second argument of ‘filter’, namely
      ‘[2 .. (floor (sqrt x))]’
    In the first argument of ‘length’, namely
      ‘(filter (\ y -> x `mod` y == 0) [2 .. (floor (sqrt x))])’

Euler.hs:34:66:
    No instance for (Floating a0) arising from a use of ‘sqrt’
    The type variable ‘a0’ is ambiguous
    Relevant bindings include
      x :: a0 (bound at Euler.hs:34:12)
      isPrime :: a0 -> Bool (bound at Euler.hs:34:1)
    Note: there are several potential instances:
      instance Floating Double -- Defined in ‘GHC.Float’
      instance Floating Float -- Defined in ‘GHC.Float’
    In the first argument of ‘floor’, namely ‘(sqrt x)’
    In the expression: (floor (sqrt x))
    In the second argument of ‘filter’, namely
      ‘[2 .. (floor (sqrt x))]’
Failed, modules loaded: none.

似乎mod功能和楼层功能存在一些问题？

Answer 1

你的第二个函数是一个没有签名推断的奇怪类型：

isPrime :: (RealFrac a, Integral a, Floating a) => a -> Bool

我不确定任何类型是所有这些类的实例（Int和Double绝对不是）。所以你的功能很好，你无法提供它正在运行的值。

为什么我的isPrime功能无法正常工作？

1 个答案: