无法将[Char]与实际类型绑定（也许是a）

Question

So I have this:

lstDelete :: [Char] -> Lst a -> Lst a
lstDelete k inp@(lstNode key)
    | k == key && lstIsEmpty = Map.delete key (Map.fromList inp)

The problem is, k is of type [Char], and key is of type (Maybe a):

data Lst a = lstNode (Maybe a)

So I'm getting the error saying:

Couldn't match expected type ‘[Char]’ with actual type ‘Maybe a’

How can I fix this without changing lstDelete :: [Char] -> Lst a -> Lst a ? I also want to keep the Maybe a since I want it to be able to be empty too.

Answer 1

There is several problem with your code:

1. Constructors names should start with capital letter.

    data Lst a = LstNode (Maybe a)

2. Your list can contain only one value

3. lstDelete doesn't make any sense. It accept [Char] as key and Lst a as list. There is no way to compare them (there is no nontrivial function that can accept those parameters). You should either change type to

   lstDelete :: [Char] -> Lst [Char] -> Lst [Char]

or write more generic

   lstDelete :: Eq a => a -> Lst a -> Lst a

4. Map.fromList accepts [(k, v)] , you can't pass Lst a to it.
5. lstIsEmpty is not defined.