当返回的对象不是字符串时,在Haskell中抛出异常.

Question

所以我在Haskell中编写一个程序,它接收一个数字n告诉它返回从2开始的第n个素数作为第一个素数.程序的那部分工作然而我不理解的是如何让程序在数字为0或更少时抛出异常.

pr :: Int -> Int
pr n = (filter(\x -> (getter) == []) [2..]) !! (n-1)

getter引用了我编写的另一个解决主要问题的方法.它工作正常.

Answer 1

默认情况下,如果函数的方程式与给定的参数不匹配,则会出现运行时错误:

fromJust :: Maybe a -> a
fromJust (Just a) = a
-- No case for Nothing
-- fromJust Nothing throws error at runtime

但是,这对数字不起作用.相反,警卫会做类似的事情:

assertOver0 :: Int -> ()
assertOver0 n | n > 0 = ()
-- No case for n <= 0
-- assertOver0 (-1) throws error at runtime

虽然这是默认行为,但是不完整的模式/警卫是不好的风格.相反,使用error或显式导致错误undefined:

pr n | n >= 0    = filter (null . getter) [2..] !! n
     | otherwise = error "Table flip"
-- undefined is just like error, except that error lets you give an error message
-- and undefined doesn't (undefined is more useful when you know it will never
-- be evaluated, and you don't need to give an error message)
-- undefined :: a; error :: String -> a
-- That is, they can take on any type you want them to have, because whatever code
-- is after them will never be executed anyway
-- I took liberties with your definition of pr. Your filtering function didn't use
-- x, so I wrote what I think you meant. I also made it 0-indexed.
-- Prelude.null checks for [], but doesn't incur an Eq constraint, so I replaced
-- (== []) with it.
-- Parens are not needed around the filter, because function application has
-- the highest precedence.

Haskell也有一个更复杂的异常机制Control.Exception,但你可能不需要这里.一般来说,异常和部分函数被忽略,(因为你只能处理它们IO),你应该努力寻找monad Maybe或者monad Either.

import Control.Monad
pr n = do guard $ n >= 0 -- guard True = Just (); guard False = Nothing (in this case)
          return $ filter (null . getter) [2..] !! n
pr 2 = Just 5
pr (-1) = Nothing

但这一切都是不必要的.(!!)已经出现负面指数的错误

ghci> "abc" !! -1
*** Exception: Prelude.!!: negative index

所以我们回到了我们开始的地方:

pr n = filter (null . getter) [2..] !! n

还有一个库重新定义列表操作(包括(!!))是monadic而不是partial.