Haskell Data.Sequence试图同时更改2个值

Question

Let's say I have a = [[1],[2,3,4],[5,6,7]] and I would like to change 2 values in this list, let's call every list in a "level" and it starts from "0". I know that the 2 values are in neighbouring levels and I know their positions. Let's say I want to change the numbers "4" and "5" both with "9". Here is my code :

import qualified Data.Sequence as Seq

updateList :: Seq.Seq a -> Int -> a -> a -> Int -> Int -> Seq.Seq a
updateList list levelAt firstNew secondNew pos1 pos2
 | 0 == (levelAt - 1) = (Seq.update pos1 firstNew (Seq.take 1 list)) Seq.>< (updateList (Seq.drop 1 list) (levelAt - 1 ) firstNew secondNew pos1 pos2 )
 | 0 == levelAt = (Seq.update pos2 secondNew (Seq.take 1 list)) Seq.>< (Seq.drop 1 list)
 | otherwise = (Seq.take 1 list) Seq.>< (updateList (Seq.drop 1 list) (levelAt - 1) firstNew secondNew pos1 pos2 )

For my example I have to call this function with :

updateList (Seq.fromList [[1],[2,3,4],[5,6,7]]) 2 9 9 2 0
Btw when I call this function it would always be with [[a]] type.

But It gives me an error No instance for (Num t0) arising from the literal '1' The type variable 't0' is ambiguous
Can you explain me why this is happening and how to fix it ?
Thank you.

Answer 1

Your Seq.update functions operate on sequences, not on the sequences within your sequence, because Seq.update :: Int -> a -> Seq.Seq a -> Seq.Seq a ; it doesn't even know you have sequences inside the sequence.

The type error you got results from it trying to make a list of integers out of 1 , so complaining that it didn't have a Num instance for list.

ghci> updateList (Seq.fromList [[1],[2,3,4],[5,6,7]]) 0 [9] [9] 1 2
fromList [[1],[2,3,4],[5,6,7]]

Unfortunately this means your function isn't doing what you intended.

Answer 2

EDIT

As a commenter pointed out, the type of your sequence is [a] . Thus the third and fourth arguments of updateList need to have the type [a] , but they have type a .

From the signature of updateList , you can tell that updateList is polymorphic because of the letter a . When you call updateList , the types of the polymorphic arguments must be inferred from the context, or given explicitly. In your case, the type of the sequence that you pass in is unclear. Is it a sequence of Int s, Integer s, or something else? You can fix this problem by explicitly giving the type of the sequence:

updateList (Seq.fromList [[1 :: Int],[2,3,4],[5,6,7]]) 2 [9] [9] 3 1

or

updateList (Seq.fromList [[1],[2,3,4],[5,6,7]] :: Seq.Seq [Int]) 2 [9] [9] 3 1

Alternatively, since the polymorphic type a appears in several places in the signature, you only need to specify the type of one argument. Thus you could give an explicit type to either of the 9s:

updateList (Seq.fromList [[1],[2,3,4],[5,6,7]]) 2 [9::Int] [9] 3 1