本文介绍了Haskell中的实例声明的处理方法,对大家解决问题具有一定的参考价值,需要的朋友们下面随着小编来一起学习吧! 问题描述 29岁程序员,3月因学历无情被辞! 我有这两个函数: primes = sieve [2 ..] 其中筛(p:xs)= p:筛[x | x 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 我看不到任何错误消息。解决方案问题在于 primes 是整数列表(由于您使用 mod ),但 sqrt 操作浮点数。如果你做 x ,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 . sqrtIn 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 . sqrtIn 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) numberI don't see any error messages. 解决方案 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 -> bIn 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. 这篇关于Haskell中的实例声明的文章就介绍到这了,希望我们推荐的答案对大家有所帮助,也希望大家多多支持! 上岸,阿里云!