[Haskell-beginners] Re: Empty list Exception

[Sean Bartell]

When you use filter in this case, it's checking for users that are the same
as ("Joao Claudino", "1240", 2). There aren't any in the list, so filter
returns []. You can't use head on [], because there's no first element.

Other notes about your code:

   - if x then True else False is the same as just x.
   - The otherwise case for userRegistration can never be reached.


I would write it more like this:

checkUser user userList = elem user userList

userRegistration user list
    | checkUser user list = list
    | otherwise = user:list
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