Ok thanks, got it working: import System.IO import Control.Monad.Fix nHead :: Int -> String -> String nHead n str = let rec = \x n str -> if n == 0 then str else x (n - 1) (tail str) in if (length str) < n then str else fix rec n str main = interact (nHead 5) In the end I will be using: import System.IO nHead :: Int -> String -> String nHead n str = let rec n str = if n == 0 then str else x (n - 1) (tail str) in if (length str) < n then str else rec n str main = interact (nHead 5) But it's good to know that it was possible. On Mon, Apr 09, 2012 at 12:28:42PM -0500, Antoine Latter wrote:

On Mon, Apr 9, 2012 at 10:57 AM, sarfraz wrote:

...

Hi,

Can't seem to be able to debug the following:

import System.IO

nHead :: Int -> String -> String nHead n str = if (length str) < n              then str              else if n ==0                   then str                   else (\x n str -> if n == 0 -- this lambda does not work                                     then str                                     else x (n - 1) (tail str))

main = interact (nHead 5)

I see where the problem is. But surely it is possible to make a recursive lambda. (I want to use a lambda, i know how to do otherwise)

You can't make a recursive lambda directly - you need a helper function:

http://hackage.haskell.org/packages/archive/base/latest/doc/html/Data-Functi...

Example usage:

...

let factNonResursive = \fact n -> if n < 2 then 1 else n * fact (n - 1) fix factNonRecursive 5

Antoine

_______________________________________________ Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners

-- Sarfraz K.