The conflict with expected and actual argument

Question

belongs :: Eq a => a -> [a] -> [a]

belongs e (h:t) = if ( h == e) then [] else belongs e t
belongs e [] = [e]




nub :: Eq a => [a] -> [a]
nub l = nub' l [] 
    where 
        nub' [] new_list = new_list
        nub' (x:xs) new_list = (nub' xs ((\ e new_list ->  
            case (belongs e new_list) of
            []    ->  new_list
            (h:t) ->  (h:new_list))(x new_list)))

Couldn't match expected type [a] with actual type [a0] -> [a0] In the return type of a call of nub' Probable cause: nub' is applied to too few arguments In the expression: nub' l [] In an equation for nub:
 nub l
  = nub' l []
  where
      nub' [] new_list = new_list
      nub' (x : xs) new_list
        = (nub'
             xs
             ((\ e new_list
                 -> case (belongs e new_list) of {
                      [] -> ...
                      (h : t) -> ... })
                (x new_list)))
Couldn't match expected type [a0] -> [a0] with actual type [a1] In the second argument of nub', namely [] In the expression: nub' l [] In an equation for nub:
nub l
  = nub' l []
  where
      nub' [] new_list = new_list
      nub' (x : xs) new_list
        = (nub'
             xs
             ((\ e new_list
                 -> case (belongs e new_list) of {
                      [] -> ...
                      (h : t) -> ... })
                (x new_list)))

Why it doesn't work? I cannot understand. Please give me a hand. The errors are understandable but I don't know their causes.

MicroVirus MicroVirus · Accepted Answer · 2016-03-07T12:19:06

Are you trying to force a function call with the lambda and the extra parenthesis ((\e new_list -> ...)(x new_list))? If so, that doesn't work for one, and even if that syntax would work then your parenthesis would be mismatched.

Forego the 'attempted function call' and the lambda and just use x and new_list directly in nub', or if you really want to rename use a let e = x in to bind e. That should fix the type errors.

If you really want the lambda, then you should know that function application is done using spaces and not parenthesis, so like (\x -> x + 1) 3.

The conflict with expected and actual argument

1 Answers