尝试在Haskell中“打印”列表时出错

Question

I have the following problem: given a max(max) apacity, and given a list of values(listOfValues) i need to return a list with values from the listOfValues. The sum of the elements must be <= max and i need to prioritize the higher values.

Example: typing solvingProblem 103 [15, 20, 5, 45, 34] i must get: [45, 45, 5, 5]

To solve the problem i create the following code:

solvingProblem max [] = 0
solvingProblem max listOfValues | max == 0 = 0
                                | otherwise = createList max listOfValues []


createList max [] result = -1
createList max listOfValues result | smaller listOfValues > max = -1
                               | higher listOfValues > max = createList max (remove (higher listOfValues) listOfValues) result
                               | otherwise = createList (max - higher listOfValues) listOfValues (insert (higher listOfValues) result)

higher [a] = a
higher (a:b:x) | a > b = higher (a:x)
              | otherwise = higher (b:x)

smaller [a] = a
smaller (a:b:x) | a < b = smaller (a:x)
              | otherwise = smaller (b:x)

remove x [] = []
remove x (h:t) | x == h = remove x t
                | otherwise = h : remove x t

insert x (h:t) = x : h : t

In the two lines where i'll returning "-1" should be the parameter "result", but if i change "-1" to "result" the code don't load on ghci.

Can someone help me?

Thank you and sorry for my bad english.

Answer 1

If I may begin with a bit of a side note, some of your functions already exist in Haskell (now that I come to think of it you might have written them for an exercise, but just in case it wouldn't be the case, let's discuss that): your higher is maximum , your smaller is minimum and your insert is just (:) , beacause like you write it yourself insert x list = x:list . Note that your version will fail if you give it the empty list because the pattern matching is non-exhaustive. Also you could write remove in terms of filter: remove x list = filter (== x) list .

Now why doesn't your code load properly? ghci tells you:

• Non type-variable argument in the constraint: Num [a]
  (Use FlexibleContexts to permit this)
• When checking the inferred type
    solvingProblem :: forall a.
                      (Ord a, Num [a], Num a) =>
                      a -> [a] -> [a]

Which I agree is pretty cryptic, but what it's saying is that the return type of solvingProblem is a list of a and for some reason it is also an instance of the Num type class. The reason why it says it's an instance of Num is because one of the return value of solvingProblem is 0 which is a number, which is a bit odd because it is also a list. Changing the 0 with [] makes the code compile and work (if you change insert with (:) otherwise you get the non-exhaustive pattern matching I was talking about earlier).

λ> solvingProblem 103 [15,20, 5, 45, 34]
[5,5,45,45]
it :: (Ord t, Num t) => [t]

Answer 2

The problem is with the last guard clause in createList .

The type you intended for createList seems to be:

createList :: Int -> [Int] -> Int -> Int

but if you look at the last guard clause you have:

| otherwise = createList (max - ...) listOfValues (insert ...)
                         ^^^^^^^^^^^ ^^^^^^^^^^^^ ^^^^^^^^^^^^
                          Int        [Int]        [Int]

Even though GHC is very good at inferring types, always adding type signatures to your code is a good way of catching these kinds of errors early.