Find all the ways to add up to a number

sumTo.hs

sumTo target n pool = loop n 0 [] 
  where
    loop 0 runningTotal accumulator = do
      guard (runningTotal == target)
      return accumulator

    loop counter runningTotal accumulator = do 
      current <- pool
      case accumulator of
        []            -> guard True
        (previous:_)  -> guard (current < previous)
      guard (current + runningTotal <= target)
      loop (counter-1) (runningTotal+current) (current:accumulator)