module Env

type Field =
    | Arg of int
    | Local of int

type Env = private Env of int * Map<string, Field>

let ofArgs args =
    let map =
        args
        |> List.mapi (fun i local -> (local, Arg i))
        |> Map.ofList
    Env(0, map)

let addLocal name env =
    match env with
    | Env(next, map) ->
        let map' = Map.add name (Local next) map
        (Env(next + 1, map'), next)

let tryGetField name env =
    match env with
    | Env(_, map) -> Map.tryFind name map