Haskell parse error, for averaging program

Question

I need to get the average, so input is stopped until user puts in a negative number and then the output is the average

  {
    getFloat :: IO Float
    getFloat = do line <- getLine
                    return (read line:: Float) 


    average :: IO Float
    average =  helper summ n
                    where
                    helper :: Float->Float->IO Float
                    helper summ n = do val<-getFloat
                                       if (val<0)
                                          then (return average)
                                          else ( do summ = summ + val
                                                    n = n+1
                                                    average= summ/n
                                                    average)
                                                         }

Please paste the error. Also, any question usually should contain at least one ? — Yuras
– Yuras, Commented Feb 21, 2013 at 19:17
how do i fix this parse error, and alogotihm to get the correct program — user1988365
– user1988365, Commented Feb 21, 2013 at 19:26
Your indentation in getFloat is broken. Is it the same in your file? — Daniel Fischer
– Daniel Fischer, Commented Feb 21, 2013 at 21:35

ДМИТРИЙ МАЛИКОВ · Accepted Answer · 2013-02-21 19:47:59Z

2

It's better to separate pure and impure computations. I.e., average could be just [a] -> b without IO effects.

main = print =<< average `fmap` go []
  where
    go :: [Int] -> IO [Int]
    go xs = do
      x <- (read :: String -> Int) `fmap` getLine
      if x < 0 then return (x : xs) else go (x : xs)

    average :: [Int] -> Float
    average xs = (fromIntegral $ sum xs) / (fromIntegral $ length xs)

This is not the best solution, by the way:

readLn :: Read a => IO a
average value could be get easier without traversing a List twice

edited Feb 21, 2013 at 19:47

answered Feb 21, 2013 at 19:28

ДМИТРИЙ МАЛИКОВ

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Comments

ErikR · Accepted Answer · 2013-02-21 19:28:22Z

0

Try something like this:

main = do (s,n) <- getnums
          putStrLn $ show $ s / (fromIntegral n)

getnums :: IO (Float, Int)
getnums = go 0.0 0
  where go s n = do line <- getLine
                    let f = read line :: Float
                    if f < 0 then return (s,n)
                             else go (s+f) (n+1)

answered Feb 21, 2013 at 19:28

ErikR

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2 Comments

Cubic Over a year ago

getLine >>= \line -> return $ read line is the same as read <$> getLine which is the same as readLn. And you don't need an explicit type signature for read if you do something with the result that fixes its type.

ErikR Over a year ago

True. I think the OP's real problem was how to perform looping in a monadic computation, so I tried to make my code as similar to the original.

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