Haskell转换类型错误

Question

我正在Haskell中编写一个简单的程序,它允许用户输入2的幂的指数

power a = 2^a
main = do
   number <- readLn
   let x = (read number :: Int)
   power x

我想出我需要转换String为Int但仍然会出现以下错误:No instance for (Num (IO t0)) arising from a use of 'power'
我如何使其工作？

Answer 1

当你写出类型时它会有所帮助.

power :: (Num a, Integral b) => b -> a
power a = 2^a

main :: IO ()
main = do
  number <- getLine
  let x = read number :: Int
  power x

但让我们来看看这些东西真的会回归.power看起来不错,但是main试图回归Num a,而不是回归IO ().main并不意味着返回值 - 你应该对它们做些什么.也许你想要print结果？print :: Show a => a -> IO ()

main :: IO ()
main = do
  number <- getLine
  let x = read number :: Int
  print $ power x