Adding a new exercise and solution to Section 5.1

[Liable5607]

Hi! I was playing around with the server example in Section 5.1 and I think it would make a nice exercise. I refactored handleRequest to use Either. I found it a nice challenge that helped make me more comfortable with parametric types. This pull request:

1. adds a prompt in the 5.1 exercises
2. puts the solution in the solutions section.

I did have to append some values with an apostrophe (e.g. handleRequest', UnknownUser') to get it to compile in the same file as the solution to the previous exercise.

Thank you for the tutorial. I'm really enjoying it. For me at least the exercises are the most helpful part.

[stefan-hoeck]

Thanks for adding this. To avoid having to use primed function names, you could put everything in its own namespace:

namespace Ex2
  data Failure : Type where
    UnknownUser : Email -> Failure
    InvalidPassword : Failure
    AccessDenied : Email -> Album -> Failure

  handleRequest : DB -> Request -> Either Failure Album
  handleRequest db req = case find (\u1 => u1.email == req.credentials.email) db of
    Nothing => Left (UnknownUser req.credentials.email)
    Just u2 => case (u2.email == req.credentials.email && u2.password == req.credentials.password) of
      False => Left InvalidPassword
      True => case elem req.album u2.albums of
        False => Left (AccessDenied req.credentials.email req.album)
        True => Right req.album

[Liable5607]

Thank you, I didn't think of that. And in hindsight find ((==) req.credentials.email . email) db is probably more "functional" than making a lambda

[Liable5607]

I wasn't sure how to handle the fact that this branch was created before but submitted after the other pull request. So I manually changed the initMaybe to reflect the current version. that's why that file has changed too

[stefan-hoeck]

I wasn't sure how to handle the fact that this branch was created before but submitted after the other pull request. So I manually changed the initMaybe to reflect the current version. that's why that file has changed too

Please don't do this. Git is able to deal with this kind of thing automatically. Otherwise, the preferred way to do this would be a rebase. Now, in order not to further complicate things, you can leave things as they currently are. So, once the linting issues are fixed (there seems to be some trailing whitespace), this should be ready for merging.