Looks up an entry depending on the phone book entry
Add type signature
Solve small exercise.
Write a function which tests whether a name appears in a PhoneBook, returning a Boolean value. Hint: Use psci to find the type of the Data.List.null function, which test whether a list is empty or not.
Add a showBook function
Add removeDuplicates function
Write a function removeDuplicates which removes duplicate phone book entries
with the same first and last names.
Hint: Use psci to find the type of the Data.List.nubBy function,
which removes duplicate elements from a list based on an equality predicate.