Question

I’m trying to figure out a solution for Problem 27 of 99 Haskell questions.
Here’s how I want it to be:

  group :: (Eq a) => [Int] -> [[a]] -> [[[[a]]]]
  group []     _  = []
  group (i:is) xs 
    | sum (i:is) /= length xs = error "invalid arguments"
    | otherwise               = ...

An example from the link:

group [2,2,5] [“aldo”,”beat”,”carla”,”david”,”evi”,”flip”,”gary”,”hugo”,”ida”]
[[[“aldo”,”beat”],[“carla”,”david”],[“evi”,”flip”,”gary”,”hugo”,”ida”]],…]
(altogether 756 solutions)

Thus, I want to firstly check whether the sum of Int list equals to the length String list as above. What I run into is that no matter whether the two values equal or not it always print “invalid arguments”. I also tried this:

group (i:is) xs 
     | (sum (i:is) == length xs) = ...
     | otherwise                 = error "invalid arguments"

still doesn’t work
Any ideas?

UPDATES: thanks guys, my carelessness. Here’s the recursive part of the function:

 group (i:is) xs 
     | (sum (i:is) == length xs) = filter (/= []) $ concatGroups (combinations i xs) (group is xs)
     | otherwise                 = error ("invalid arguments: " ++ show (sum(i:is)) ++ "/=" ++ show(length xs))

As you can tell, group is xs reduces the sum but not the length so it will always complain when going recursive. I think I will just remove that guard and wish the user would never do it wrong.

Answer 1

To expand on hammar’s point, if you change your code to

group :: (Eq a) => [Int] -> [[a]] -> [[[[a]]]]
group []     _  = []
group (i:is) xs 
  | sum (i:is) /= length xs = error ("invalid arguments to group: sum "
                                     ++ show (i:is) ++ " /= " ++ show (length xs))
  | otherwise               = ...

this will help you track down what is going wrong in the ... part.