{- This uses lazy evaluation to define Pascals triangle -}
 
 module Main where
 
 pascal :: [[Int]]
 pascal = [1] : [[x+y | (x,y) <- zip ([0]++r) (r++[0])] | r <- pascal]
 
 tab :: Int -> ShowS
 tab 0     = id
 tab (n+1) = showChar ' ' . tab n
 
 showRow :: [Int] -> ShowS 
 showRow []     = showChar '\n'
 showRow (n:ns) = shows n . showChar ' ' . showRow ns
 
 showTriangle 1     (t:_)  = showRow t
 showTriangle (n+1) (t:ts) = tab n . showRow t . showTriangle n ts
 
 main = appendChan stdout "Number of rows: " abort $
        readChan stdin abort $ \input ->
        appendChan stdout
          (showTriangle (read (head (lines input))) pascal "")
          abort done