Impure ADT Reader.fs

open System
open FSharpx.Reader

[<AutoOpen>]
module LoggerModule =

    /// A fake logger type.
    type Logger =
        private { __ : unit }

    [<RequireQualifiedAccess; CompilationRepresentation (CompilationRepresentationFlags.ModuleSuffix)>]
    module Logger =
        let info (_ : string) (_ : Logger) = ()
        let warn (_ : string) (_ : Logger) = ()
        let error (_ : string) (_ : Logger) = ()
        let make () = { __ = () }

[<AutoOpen>]
module DatabaseConnectionModule =

    /// A fake database connection type.
    type DatabaseConnection =
        private { __ : unit }

    [<RequireQualifiedAccess; CompilationRepresentation (CompilationRepresentationFlags.ModuleSuffix)>]
    module DatabaseConnection =
        let getName (_ : DatabaseConnection) = "MyDatabase"
        let getValue (_ : DatabaseConnection) = 10
        let make (_ : string) = { __ = () }

[<AutoOpen>]
module FrobServiceModule =

    /// A service that can frobnicate at the given sanitation capacity and finalization rate.
    type FrobService =
        private
            { SanitationCapacity : int
              FinalizationRate : int
              Logger : Logger
              Database : DatabaseConnection }
    
    [<RequireQualifiedAccess; CompilationRepresentation (CompilationRepresentationFlags.ModuleSuffix)>]
    module FrobService =
    
        let private queryFrobData frobService =
            DatabaseConnection.getValue frobService.Database

        let private trySanitizeFrobData frobData frobservice =
            Some <| max frobData frobservice.SanitationCapacity
    
        let private finalizeFrobData frobData frobService =
            frobData * frobService.FinalizationRate
    
        /// Attempt to frobnicate.
        let tryFrobnicate frobService =
    
            // query frob data
            Logger.info ("Querying frob data from source " + DatabaseConnection.getName frobService.Database) frobService.Logger
            let frobData = queryFrobData frobService

            // attempt to sanitize frob data
            match trySanitizeFrobData frobData frobService with
            | Some frobData ->

                // finalize frob data
                let frobData = finalizeFrobData frobData frobService
                Some frobData

            // warn of failure!
            | None ->
                Logger.warn ("Failed to sanitize frob data " + string frobData) frobService.Logger
                None

        /// Make a frob service.
        let make sanitationCapacity finalizationRate logger database =
            { SanitationCapacity = sanitationCapacity; FinalizationRate = finalizationRate; Logger = logger; Database = database }

/// Program entry point.
let [<EntryPoint>] main _ =

    // build frob service
    let logger = Logger.make ()
    let database = DatabaseConnection.make "DB=MyDatabase;User=Me;Password=12345"
    let frobService = FrobService.make 20 60 logger database
    
    // attempt to frobnicate using the Reader monad
    let frobber = reader { return! FrobService.tryFrobnicate }
    match Reader.ask frobber frobService with
    | Some frobData -> printfn "Frobnication result: %i frobs." frobData
    | None -> Logger.error "Frobnication failure!" logger
    
    // return an integer exit code
    0