F# typed environment config with validation

open System

type Config = { Host: string; Port: int; Debug: bool }

let loadConfig () =
    let get key fallback =
        match Environment.GetEnvironmentVariable key with
        | null | "" -> fallback
        | value -> value
    let port =
        match Int32.TryParse(get "APP_PORT" "8080") with
        | true, p when p > 0 && p < 65536 -> p
        | _ -> failwith "APP_PORT must be an integer between 1 and 65535"
    { Host = get "APP_HOST" "127.0.0.1"
      Port = port
      Debug = (get "APP_DEBUG" "false").ToLowerInvariant() = "true" }