haskell - 调用函数的haskell案例

Question

我已经在这里待了很长时间了，我不知道出了什么问题
Haskell只是让我感到如此愚蠢

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。我觉得我的案例控制结构合适，但我根本无法使它正常工作!!!

Answer 1

你快完成了。
使用deriving (Read)读取String作为Operation。
http://en.wikibooks.org/wiki/Haskell/Classes_and_types#Deriving

如果要处理读取错误，请参见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