Question

If I have:

data Data = T | F
    deriving (Eq,  Show, Ord)

type PhoneBook = [(String, Data)] 

testBook :: Book
testBook = [("aT", T), ("bF", F)]

I’m trying to make a look up function, but because I’m using a user defined type it’s throwing me off. Any suggestions? I’m trying:

lookup1 :: PhoneBook->String->Data
lookup1 [] key = Nothing
lookup1 ((k,v):xs) key = if key == k
                            then Just v
                            else lookup1 xs key

Answer 1

When you have a statement like type Book = [(String, Answer)] then Book is just a synonym for [(String, Answer)]. This means that you can replace Book with [(String, Answer)] anywhere in your types without changing them. This means that looking up a value in a Book is exactly the same as looking up a value in a [(String, Answer)].

Thus you can just use the normal lookup function to get the answer corresponding to a particular string.