Dependency injection, revisited

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// The `isCurrentMonth` depends on an implementation of a
// timer that tells the current time or a mock time for tests

type ITime =
  abstract Now : System.DateTime

let mutable time:ITime = 
  { new ITime with 
    member x.Now = failwith "Not initialized" }

let isCurrentMonth (t:System.DateTime) = 
  t.Month = time.Now.Month

// Implementation for production

time <- 
  { new ITime with 
    member x.Now = System.DateTime.Now }

isCurrentMonth (System.DateTime(2018, 7, 5))

// Implementation for unit testing

time <- 
  { new ITime with 
    member x.Now = System.DateTime(2018, 1, 1) }

isCurrentMonth (System.DateTime(2018, 7, 5))

abstract member ITime.Now : System.DateTime

Full name: Script.ITime.Now

namespace System

Multiple items
type DateTime =
  struct
    new : ticks:int64 -> DateTime + 10 overloads
    member Add : value:TimeSpan -> DateTime
    member AddDays : value:float -> DateTime
    member AddHours : value:float -> DateTime
    member AddMilliseconds : value:float -> DateTime
    member AddMinutes : value:float -> DateTime
    member AddMonths : months:int -> DateTime
    member AddSeconds : value:float -> DateTime
    member AddTicks : value:int64 -> DateTime
    member AddYears : value:int -> DateTime
    ...
  end

Full name: System.DateTime

--------------------
System.DateTime()
   (+0 other overloads)
System.DateTime(ticks: int64) : unit
   (+0 other overloads)
System.DateTime(ticks: int64, kind: System.DateTimeKind) : unit
   (+0 other overloads)
System.DateTime(year: int, month: int, day: int) : unit
   (+0 other overloads)
System.DateTime(year: int, month: int, day: int, calendar: System.Globalization.Calendar) : unit
   (+0 other overloads)
System.DateTime(year: int, month: int, day: int, hour: int, minute: int, second: int) : unit
   (+0 other overloads)
System.DateTime(year: int, month: int, day: int, hour: int, minute: int, second: int, kind: System.DateTimeKind) : unit
   (+0 other overloads)
System.DateTime(year: int, month: int, day: int, hour: int, minute: int, second: int, calendar: System.Globalization.Calendar) : unit
   (+0 other overloads)
System.DateTime(year: int, month: int, day: int, hour: int, minute: int, second: int, millisecond: int) : unit
   (+0 other overloads)
System.DateTime(year: int, month: int, day: int, hour: int, minute: int, second: int, millisecond: int, kind: System.DateTimeKind) : unit
   (+0 other overloads)

val mutable time : ITime

Full name: Script.time

type ITime =
  interface
    abstract member Now : DateTime
  end

Full name: Script.ITime

val x : ITime

property ITime.Now: System.DateTime

val failwith : message:string -> 'T

Full name: Microsoft.FSharp.Core.Operators.failwith

val isCurrentMonth : t:System.DateTime -> bool

Full name: Script.isCurrentMonth

val t : System.DateTime

property System.DateTime.Month: int

property System.DateTime.Now: System.DateTime