Haskell filtering a nested list with data constructors

Question

Let\'s say I have the following data type

data Number = Positive Integer | Negative Integer
     deriving (Eq, Show)

I have a function definiti

Answer 1

One way is pattern matching on the nested list to peek at the first element of the first list to figure out what you need to filter:

-- Note you could give this the more general type Eq a => [[a]] -> [[a]]
-- (as well as a more appropriate name)
removePos :: [[Number]] -> [[Number]]
removePos [] = [] -- Empty list case.
removePos xss@[[]:_] = xss -- If the first inner list is empty,
                           -- return the whole thing unchanged
removePos ((x:_):xss) = filter (notElem x) xss