Re: [Haskell-cafe] Problems with function recursion in Haskell

9 May 2016

      On Mon, May 09, 2016 at 10:30:44AM -0300, Henson wrote:
...
Hi,
I'm new in Haskell and I need help in recursion.
That function below is returning "*** Exception: Prelude.head: empty list"
and
I need resolve that:
execNTimes 0 [] = return()
execNTimes n xs = if n<=0  || null xs
    then return()
    else do
        si <- getLine
        let s = words si
            l = read (s !! 1) :: Int
            r = read (s !! 2) :: Int
        if head s=="Q"
            then do
                let z = slice l r xs
                    m = foldl lcm 1 z
                print (m `mod` (toInteger 1000000007))
            else do
                let s1 = update l r xs
                execNTimes (n-1) s1
        execNTimes (n-1) xs
Anybody can me help?
Hello Josenildo,

    the error indeed tells us you are passing an empty list to the function
`head`.

    si <- getLine
        let s = words si
    -- other lines
    if head s == "Q"

so I suppose you are giving an empty string as input to getLine (I cannot
be sure as the code doesn't compile, missing `slice` and `update`).

Re: [Haskell-cafe] Problems with function recursion in Haskell

Francesco Ariis