Question

我已经在这一段很长一段时间了，我无法弄清楚什么是错的哈斯克尔只是让我感到愚蠢

data Operation
    = Nth Integer


fib :: (Integral i, Integral j) => i -> j

fib   n | n == 0         = 1
        | n == 1         = 1
        | n == 2         = 1
        | n == 3         = 1        
        | otherwise = (fib(n-1)+fib(n-2))* fib(n-3) `div` fib(n-4)
main = do
command <- getLine
case command of
    Nth op -> show $ fib op
    Nothing -> "Invalid operation"

因此，当用户输入Nth 9时，需要使用n = 9调用fib函数并将输出提供给用户。我觉得我的案件控制结构是合适的，但我不能让它完全工作!!!

Answer 1

你差不多完成了。使用deriving (Read)将String作为Operation阅读。 http://en.wikibooks.org/wiki/Haskell/Classes_and_types#Deriving

如果您想处理读取错误，请参阅How to catch a no parse exception from the read function in Haskell?

data Operation = Nth Integer deriving (Read)

fib :: (Integral i, Integral j) => i -> j

fib   n | n == 0         = 1
        | n == 1         = 1
        | n == 2         = 1
        | n == 3         = 1
        | otherwise = (fib(n-1)+fib(n-2))* fib(n-3) `div` fib(n-4)
main = do
  command <- getLine
  print $ case read command of
    Nth op -> fib op

调用函数的haskell案例

1 个答案: