On 26 May 2011 10:45, Jacek Generowicz
What is the Haskell approach to efficient comparison and lookup of objects by their identity?
Often you just provide your own and implement Eq.
I should be able to run the program on data that becomes available at run time.
Typically you define an id generator and zip anything coming from the input stream up with that generator. So: persons <- fmap (zipWith addId [1..] . map parsePerson) (hGetContents handle) where addId id person = person { personId = id }
Whatever algorithm I choose to use for the optimization, will have to do lots of comparisons of Groups and Persons where their *identity* is all that matters: you don't need to look inside the objects.
To achieve this abstraction the usual way is just implementing Eq: instance Eq Person where Person{personId=id1} == Person{personId=id2} = id1 == id2 You can also implement ordering if required: instance Ord Person where Person{personId=id1} `compare` Person{personId=id2} = id1 `compare` id2 Then you can write person1 == person2 and person1 > person2, etc. without "looking inside" the object, and all the library functions you have available like the list functions that work on Eq instances and Ord instances will work for your data type automatically. There're also some packages on Hackage for generating unique ids.