Question

I have:

type Person     = String
type Book       = String
type Database   = [(Person,[Book])]

I’m trying to define a function:

books :: Database -> Person -> [Book]

that takes in
1) a list of tuples (which contain a string and a list of strings
2) a String name

and returns a list of strings (namely the books from the database)

I want to use list comprehension, but I don’t know how to access elements within a list that is in the tuple inside of the database list.

Thoughts?

Example database would look like:

db = [("Bob", ["Red Riding Hood", "Alice in Wonderland"]), ("Carol", ["Game of Thrones"])]

And if I ask for say “Carol”, it should return [“Game of Thrones”].

Answer 1

Since you have an association list you can use the lookup function. It pretty much works exactly like you want:

getVal :: Database -> String -> Maybe [String]
getVal  = lookup

The only difference is that it returns a Maybe but IMHO that is the right behavior, consider what would happen if you didn’t have a value in your database when you looked it up?

Since you want to use pattern matching here is the source of lookup

lookup                  :: (Eq a) => a -> [(a,b)] -> Maybe b
lookup _key []          =  Nothing
lookup  key ((x,y):xys)
    | key == x          =  Just y
    | otherwise         =  lookup key xys

Or in your case

getVal :: Database -> String -> [String] --Avoids a Maybe if you want
getVal _key []          =  []
getVal  key ((x,y):xys)
    | key == x          =  y
    | otherwise         =  getVal key xys