大家好。由于一定的需要，要在短时间内对Haskell进行一个扫盲。今天晚上在网上搜索了一下，发现Yet Another Haskell Tutorial是一个很好的开始。在看到第三章的时候，有个习题想不出来，特此请教一下；

问题是

Exercise 3.10 Write a program that will repeatedly ask the user for numbers until she
types in zero, at which point it will tell her the sum of all the numbers, the product of
all the numbers, and, for each number, its factorial. For instance, a session might look
like:

即：

Give me a number (or 0 to stop):
5
Give me a number (or 0 to stop):
8
Give me a number (or 0 to stop):
2
Give me a number (or 0 to stop):
0
The sum is 15
The product is 80
5 factorial is 120
8 factorial is 40320
2 factorial is 2

我是这样做的：

module ReadNum
     where 

import IO 

fibonacci 1 = 1
fibonacci 2 = 1 
fibonacci n = fibonacci (n-2) + fibonacci (n-1)

readnum  = do 
      hSetBuffering stdin LineBuffering
      putStrLn "Give me a number (or 0 to stop) :"
      num <- getLine 
      let value = read num
      if value == 0
          then do
                   return []
          else do
                   rest <- readnum 
                   return ( value : rest )

result = do 
        list <- readnum 
        let fib = map fibonacci list
        putStrLn ("The sum is " ++ show(foldr (+) 0 list))
        putStrLn ("The product is " ++ show(foldr (*) 1 list))
        putStrLn (show(head list) ++ " factorial is " ++ show(head fib))

结果是

Give me a number (or 0 to stop) :
5
Give me a number (or 0 to stop) :
8
Give me a number (or 0 to stop) :
2
Give me a number (or 0 to stop) :
0
The sum is 15
The product is 80
5 factorial is 5

这一行 5 factorial is 5 也是错的

后面不知道怎么办是好。不知道如何遍历一个List还能“递归”地打印出来

首先，你的 fibonacci 是干嘛的，不是要求 factorial 的嘛。

其次，最后一行只取了 head，当然也只有一个结果啦，可以用 mapM_ 。

Here comes my toy..

module Main where

main :: IO ()
main = do
    xs <- sgetline
    mapM_ (\s -> putStrLn s) ([showSum xs , showProduct xs] ++ (map showFactorial xs))
  where 
    showSum x = "The sum is: " ++ (show . sum) x
    showProduct x = "The product is: " ++ (show . product) x
    showFactorial x = (show x) ++ " factorial is: " ++ (show . factorial) x
    
sgetline :: IO [Int]
sgetline = do
    putStrLn "Give me a number (or 0 to stop) :"  
    x <- getLine
    if x `elem` ["", "0"] then -- isStringEmpty??
      do return []
      else
      do xs <- sgetline
         return ((read x):xs)

factorial n = product [1..n]