open Microsoft.Extensions.Configuration
open Microsoft.Extensions.DependencyInjection
open Argu

type HostedConfigurationReader(config: IConfiguration) =
    interface IConfigurationReader with
        member this.GetValue(k) = config.[k]
        member this.Name = ".NET Core hosted configuration"

type IServiceCollection with
    member this.AddArgu<'T when 'T : not struct and 'T :> IArgParserTemplate>(configure: IConfigurationReader -> ParseResults<'T>) =
        this.AddSingleton<IConfigurationReader, HostedConfigurationReader>()
        this.AddSingleton<ParseResults<'T>>(fun services ->
            services.GetRequiredService<IConfiguration>()
            |> HostedConfigurationReader
            |> configure)

    member this.AddArgu<'T when 'T : not struct and 'T :> IArgParserTemplate>() =
        this.AddArgu<'T>(fun config -> ArgumentParser.Create<'T>().Parse(configurationReader = config))

//// Example use:

type Args =
    // Passed as --file XXX on the CLI
    // or { "file": XXX } in appSettings.json
    // or FILE environment variable
    | File of string
    // Passed as --url on the CLI
    // or { "myService": { "url": XXX } } in appSettings.json
    // or MYSERVICE__URL envirenment variable
    | [<CustomAppSettings "myService:url">] Url of string

    interface IArgParserTemplate with
        member this.Usage =
            match this with
            | File _ -> "a file."
            | Url _ -> "the url of my service."

// Service using it:
type MyService(args: ParseResults<Args>) =
    let file = args.GetResult Args.File
    let url = args.GetResult Args.Url

// App startup config:
type Startup() =
    member this.ConfigureServices(services: IServiceCollection) =
        services.AddArgu<Args>()
        |> ignore