Haskell中的实例声明 [英] Instance declaration in Haskell
问题描述
我有这两个函数:
primes = sieve [2 ..]
其中
筛(p:xs)= p:筛[x | x < - xs,x`mod'p> 0]
isPrime number = number / = 1&& null [x | x< - takeWhile(\ x - > x<(ceiling。sqrt)number)primes,mod number x == 0]
问题是,当我试图加载包含这些函数的模块时,我看到以下错误消息:
[2 of 2]编译Main(euler37.hs,解释)
euler37.hs:6:70:
(RealFrac Int)$ b没有实例$ b使用`ceiling'
可能的修正:为(RealFrac Int)添加一个实例声明
在`(。)'的第一个参数中,即`ceiling'
In表达:天花板。 sqrt
在'(<)'的第二个参数中,即'(ceiling。sqrt)number'
euler37.hs:6:80:
没有Floating Int)使用`sqrt'引起的
可能的修正:为(Floating Int)添加一个实例声明
在'(。)'的第二个参数中,即`sqrt'
在表达式中:天花板。 sqrt
在'(<)'的第二个参数中,即'(ceiling。sqrt)数字'
我真的不明白是什么问题,因为当我试图从一段代码中创建一个小函数时,据我所知,这些代码会导致这些错误,就像gb中的$ b一样$ b let f number x = x < (ceiling。sqrt)number
我看不到任何错误消息。
问题在于 primes
是整数列表(由于您使用 mod
),但 x < (天花板上的sqrt.quatrt)数字
,那么它会正常工作。 fromIntegral
只是将整数转换为任何其他数字类型:
fromIntegral ::(积分a,数字b)=> a - > b
在这种情况下,由于您没有指定任何特定的浮点类型来转换,它将默认使用Double值来计算平方根。您可以通过将 fromIntegral
更改为类似于(fromIntegral :: Integer - > Float)
的方式指定其他类型。 p>
你在GHCi中看不到这个错误的原因是因为你的条件很好;它只适用于你在这里使用的不同类型。只是验证一段代码是孤立的,是不够的;因为它通过类型检查器,它也必须在上下文中有意义。
您可能需要考虑使用整数平方根算法以获得准确性。
I have these two functions:
primes = sieve [2..]
where
sieve (p:xs) = p : sieve [x|x <- xs, x `mod` p > 0]
isPrime number = number /= 1 && null [x | x <- takeWhile (\x -> x < (ceiling . sqrt) number) primes, mod number x == 0]
The thing is that when I'm trying to load module which contains those functions, I see following error message:
[2 of 2] Compiling Main ( euler37.hs, interpreted )
euler37.hs:6:70:
No instance for (RealFrac Int)
arising from a use of `ceiling'
Possible fix: add an instance declaration for (RealFrac Int)
In the first argument of `(.)', namely `ceiling'
In the expression: ceiling . sqrt
In the second argument of `(<)', namely `(ceiling . sqrt) number'
euler37.hs:6:80:
No instance for (Floating Int)
arising from a use of `sqrt'
Possible fix: add an instance declaration for (Floating Int)
In the second argument of `(.)', namely `sqrt'
In the expression: ceiling . sqrt
In the second argument of `(<)', namely `(ceiling . sqrt) number'
I really can't understand what's the problem, because when I'm trying to make a small function from piece of code, which, as far as I understand, cause these errors, right in ghci, like let f number x = x < (ceiling . sqrt) number I don't see any error messages.
The problem is that primes
is a list of Integers (due to your use of mod
), but sqrt
operates on floating-point numbers. If you do x < (ceiling . sqrt . fromIntegral) number
, then it'll work fine. fromIntegral
just converts an integral number into any other numeric type:
fromIntegral :: (Integral a, Num b) => a -> b
In this case, since you don't specify any specific floating-point type to convert to, it'll default to using Double values to compute the square root. You could specify another type by changing fromIntegral
to something like (fromIntegral :: Integer -> Float)
.
The reason you don't see this error in GHCi is because your conditional is fine; it just works on different types to the ones you're using here. Just verifying that a piece of code is correct in isolation isn't enough; for it to pass the type checker, it has to make sense in context too.
You might want to consider using an integer square root algorithm for accuracy.
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