====== Introduction to Haskell ======

Different ways of defining the ''sum'' function in Haskell:

<code haskell>
sum1 :: [Int] -> Int -> Int -> Int 
sum1 v crt_sum crt_index = if crt_index == (length v)
                           then crt_sum
                           else sum1 v (crt_sum + (v !! crt_index)) (crt_index + 1)

sum2 v crt_sum crt_index 
    | crt_index == (length v) = crt_sum
    | otherwise = sum2 v (crt_sum + (v !! crt_index)) (crt_index + 1)
    
sum3 v crt_sum 
    | v == [] = crt_sum
    | otherwise = sum3 (tail v) (crt_sum + (head v))
    
sum4 [] crt_sum = crt_sum
sum4 (x:xs) crt_sum = sum4 xs (crt_sum + x)

sum5 :: [Int] -> Int
sum5 v = local v 0
    where local [] acc = acc
          local (x:xs) acc = local xs (acc + x)           
</code>

Different ways of writing ''atLeastTwo'':
<code haskell>
atLeastTwo1 :: [Int] -> Bool
atLeastTwo1 l
  | l == [] = False
  | otherwise = atLeastOne (tail l)
            where atLeastOne ls 
                        | ls == [] = False
                        | otherwise = True

atLeastTwo2 [] = False
atLeastTwo2 (x:xs) = atLeastOne xs
                    where atLeastOne [] = False
                          atLeastOne _ = True

atLeastTwo3 [] = False
atLeastTwo3 [x] = False
atLeastTwo3 (x:y:xs) = True

atLeastTwo4 (_:_:_) = True
atLeastTwo4 _ = False                          

</code>