Re: [Caml-list] Creating a Map from custom type

[Mark Shinwell <mshinwell@janestreet.com>]

On 14 November 2013 10:39, Ollie Frolovs <of12343@my.bristol.ac.uk> wrote:
> My current implementation uses associative lists, in subroute.ml:
>
> open Core.Std
> type t = ((int list * int) * (int * int list)) list
> let empty = []
> let to_list x = x
> let add t k v = List.Assoc.add t k v
> let find t k = List.Assoc.find t k
>
> I would like to use a Map instead. The key for my Map must have the type (int list * int) and the values have the type (int * int list). If i understood it correctly, because the key is a tuple and not one of the built-in simple data types (string, int, etc), because of this i have to provide a custom comparator. This is first thing that i am not sure of.

I think you need something like:

module Key = struct
  type t = int list * int with sexp, compare
end

module My_map = Map.Make (Key)

Then, for example:

  let map =
    My_map.add My_map.empty
      ~key:([42], 0)
      ~data:(1, [2])

The "with compare" auto-generates a "compare" function for
you.  Using explicit comparison functions, as Core strongly
encourages, does unfortunately mean a bit more code at times
but is arguably vastly more sustainable as your codebase
grows.  In such scenarios it becomes increasingly unlikely
that the correct notion of comparison on one of your many
abstract types coincides with the structural comparison
on whatever implementation those types happen to have today.

Mark