Week 2 Lecture 1

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import Distribution.Fields.Field (getName)
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type Name = String
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type Age = Int
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data Country
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    = Denmark
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    | DPRK
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    | Latvia
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personAge :: Name -> Maybe Age
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personAge "Troels" = Just 38
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personAge "Toxo" = Just 8
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personAge "Sebastian" = Just 29
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personAge _ = Nothing
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mayVote0 :: Name -> Maybe Bool
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mayVote0 name = fmap (>= 18) (personAge name)
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mayVote1 :: (Functor f) => (Name -> f Age) -> Name -> f Bool
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mayVote1 getAge name = fmap (>= 18) (getAge name)
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anotherPersonAge :: Name -> Either String Age
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anotherPersonAge "Troels" = Right 38
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anotherPersonAge "Doctor Doom" = Left "Dr. Doom is ageless"
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anotherPersonAge name = Left $ "Unknown person: " ++ name
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votingAge :: Country -> Maybe Age
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votingAge Denmark = Just 18
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votingAge DPRK = Just 17
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votingAge Latvia = Nothing
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mayVote2 :: Applicative f => (Name -> f Age) -> (Country -> f Age) -> Name -> Country -> f Bool
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mayVote2 getAge getVotingAge name country = 
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    -- (>=) <$> (getAge name) <*> (getVotingAge country) -- Name check first
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    (<=) <$> (getVotingAge country) <*> (getAge name) -- Age check first
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    -- pure (>=) <*> (getAge name) <*> (getVotingAge country)
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-- "Talking to the outside world is dangerous and uncomfortable for computer scientists"
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findVoters :: Monad f => (Name -> f Age) -> (Country -> f Age) -> [Name] -> Country -> f [Name]
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findVoters getAge getMinAge [] country = pure []
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findVoters getAge getMinAge (name: names) country = canVote name >>= filtering
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    where
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        remaining = findVoters getAge getMinAge names country
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        filtering b = if b then (name:) <$> remaining else remaining
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        canVote name = (>=) <$> getAge name <*> getMinAge country
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myVoters :: Maybe [Name]
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myVoters = findVoters personAge votingAge ["Troels", "asd"] Latvia
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getAgeFromUser :: Name -> IO Age
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getAgeFromUser name = do
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    putStrLn $ "How old is " ++ name ++ "?"
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    a <- getLine
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    pure (read a)
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