Updating record when referenced by multiple data structures

Question

Suppose I have a record, e.g. Person, and I want to be able to look this person up through multiple data structures. Maybe there\'s an index by name, another in

Answer 1

Since I answered this, a few people in #haskell on Freenode recommended alternative, premade solutions:

• mm_freak_ recommended Data.IxSet.
• donri recommended Data.Store, which was said to provide lenses.

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You can make a data structure that contains your lookup tables, as well as a Vector of actual Persons. The lookup tables will give you an Int or a list of Ints (rather than a Person or a list of Persons) which is the index into the Vector Person. For example:

data PersonStuff = PersonStuff {
                                 persons              :: Vector Person,
                                 firstNameLookupTable :: LookupTable Name,
                                 ...
                               }

data LookupTable a = LookupTable {
                                   table  :: Map a Int,
                                   update :: Person -> Person -> Map a Int -> Map a Int
                                 }

The update function is given the old Person, the updated Person, and will update the table only if the relevant details have changed. When a Person is modified through the convenient PersonStuff functions you'll write, those functions will update all the lookup tables for you, returning a new PersonStuff with all associated data. This makes for a pure data structure with quick lookups.

You can make functions like updatePeopleWithFirstName :: Name -> (Person -> Person) -> PersonStuff -> PersonStuff that will get all the people with a first name, apply a Person -> Person to each of them, modify their entries in the Vector, and use the update functions to update all of the lookupTables.