OCaml: How to traverse a list parameter in a record type

Question

If I have a data structure such as

type cost = int

type 'a map = {
  cities : 'a list
  routes : ('a * 'a * cost) list
}

And say I have a function with a header such as

let nearest_cities (m : 'a map) = ...

and I want to recursively traverse the routes list, what would be the best and cleanest way of going about this? I tried to match on the record, however I'm not sure how to recursively update the parameters of the record

Thanks

Answer 1

You say you want to update the record, but the record type as you have defined it is immutable. You can't change the values of the fields of such a record. The usual way to handle this in a functional language is to create a new record with the desired new contents.

You say you want to "traverse" the list, but this isn't specific enough to offer a detailed suggestion. There are functions in the List module for various kinds of traversal: List.iter , List.map , List.fold_left , and so on.

As an example, here is a function that increments all the costs of a value of type 'a map :

 let cost_incr map =
      { map with routes =
            List.map (fun (a, b, c) -> (a, b, c + 1)) map.routes
      }

(Also note you need ; after the definition of the cities field in the record type.)