firstEvenSquares n = [i*i | i<-[1..n], even i]
firstEvenSquares' n = [i*i | i<-unsquares]
  where unsquares = [2,4..n]

firstEvenSquares'' n = let unsquares = [2,4..n] in
  [i*i | i<-unsquares]

mySum xs = foldr (+) 0 xs

myLength xs = foldl oneMore 0 xs
  where oneMore n _ = n+1

reverse' xs = foldl prefix [] xs
  where prefix xs x = x:xs

join _ [] = "" 
join _ (w:[]) = w
join sep (w:ws) = w ++ sep ++ join sep ws

joincomma = join ", "