[Haskell-cafe] Problems with function recursion in Haskell

[Chris Smith]

On Mon, May 9, 2016 at 6:30 AM, Henson <jfsihenson at gmail.com> wrote:

> 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
>

Your code is making the assumption that (words si)  is non-empty.  When
that's not true, such as when the input from the console is empty, your
program will crash at the use of `head`, since it throws an error when
applied to an empty list.  That's what you are seeing.  Incidentally, by
using `l` and `r` later on, you're also assuming that the input contains at
least 2 more words after that first one.  Violating that will also crash
your program.  Finally, your use of `read` means that the program will
crash if the second two inputs are not numbers.  All three of these
functions should be used with great caution, or not at all, since they
threaten to crash your program!

Here's a very concise way to write this, using pattern guards:

    case words si of
        [ a, ls, rs ]
            , Right l <- readEither ls :: Either String Int
            , Right r <- readEither rs :: Either String Int
            -> if a == "Q"
                    then do
                        let x = slice l r xs
                        let m = foldl lcm 1 z
                        print (m `mod` (toInteger 1000000007))
                    else do
                        let s1 = update l r xs
                        execNTimes (n-1) s1
        _ -> putStrLn "Invalid input."
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