F＃地图和不同的对象

Question

I have some nondescript but distinct objects (specifically, unnamed variables in logic expressions) that I want to put in a map that associates them with their values. As I understand it, map needs to distinguish objects by some ordered field, so I can't just have

type Term =
...
| Var

as this would not allow different variables distinguishable from each other. Instead I could presumably have

type Term =
...
| Var of int64

and then have a new_var function that increments a global int64 counter and returns a new variable with the incremented value. This seems slightly inelegant, but should work.

Is the global counter the recommended way to handle this, or is there a more idiomatic method?

Answer 1

It's not really a "map having to distinguish objects" thing - when you declare a type like this:

type Term = 
    | Var

you have a type with a single valid value - Var . If you're saying you want to have objects that are distinct - this is not what you want. You can still use that type as a key in a map - not a particularly useful one though, since it will have at most a single element.

Using a counter is a good enough way to handle it. If you don't want a "global" one, you can roll it into a function using a ref cell to hold it:

type Term = 
    | Var of int

let make = 
    let counter = ref 0
    fun () ->
        counter := !counter + 1
        Term.Var (!counter)

Or use GUIDs if you don't care about the values and want the counter out of the picture:

 type Term = 
     | Var of System.Guid

 let make () = 
     Term.Var (System.Guid.NewGuid())