Question

我正在尝试使用ghci将自定义模块导入到main中，但出现了我不太了解的错误。

Main.hs

module Main where

import Newton (my_sqrt)

main = my_sqrt 25

Newton.hs

module Newton where

deriv f x = (f(x + dx) - f(x))/dx
    where dx = 0.0001


newton f = until satis improve
    where   satis y = abs(f y) < eps
            eps = 0.0001
            improve y = y - (f y/deriv f y)


my_sqrt x = newton f x
    where f y = y^2 - x

my_cubrt x = newton f x
    where f y = y**3 - x

我尝试使用将它们加载到ghci中

:l Main.hs

我收到此错误

Main.hs:9:8: error:
    • No instance for (Fractional (IO t0))
        arising from a use of ‘my_sqrt’
    • In the expression: my_sqrt 25
      In an equation for ‘main’: main = my_sqrt 25

Main.hs:9:16: error:
    • No instance for (Num (IO t0)) arising from the literal ‘25’
    • In the first argument of ‘my_sqrt’, namely ‘25’
      In the expression: my_sqrt 25
      In an equation for ‘main’: main = my_sqrt 25
Failed, modules loaded: Newton.

我该如何解决这个问题？

Answer 1

main的类型必须为IO ()。

sqrt 25的类型显然是Fractional t => t（在程序中始终建议包含顶级实体的类型签名是很明智的；您会错过这些签名）。

要使两者协调一致，您可以定义例如

main :: IO ()
main = print (sqrt 25)

因为print的类型为print :: Show a => a -> IO ()。

在ghci中导入自定义模块时出现“无实例”错误

1 个答案: