noop_printf.fs

module Logger =
  open System
  open System.IO
  open System.Text

  type LogLevel = TRACE | DEBUG | INFO

  type Logger0(writer: TextWriter) =
    member x.Log (fmt: PrintfFormat<'Printer, _, _, _>): 'Printer =
      Printf.fprintfn writer fmt

  type LevelLogger0(writer: TextWriter, level: LogLevel) =
    inherit Logger0(writer)
    member x.Info (fmt: PrintfFormat<'Printer, _, _, _>): 'Printer =
      if level <= LogLevel.INFO then
        x.Log fmt

    member x.Debug (fmt: PrintfFormat<'Printer, _, _, _>): 'Printer =
      if level <= LogLevel.DEBUG then
        x.Log fmt


  type Logger1(writer: TextWriter) =
    member x.Log tw (fmt: PrintfFormat<'Printer, _, _, _>): 'Printer =
      Printf.fprintfn tw fmt

  type LevelLogger1(writer: TextWriter, level: LogLevel) =
    inherit Logger1(writer)
    
    let getTw(threshold: LogLevel) =
      if level <= threshold then writer else TextWriter.Null
    
    member x.Info (fmt: PrintfFormat<'Printer, _, _, _>): 'Printer =
        x.Log (getTw LogLevel.INFO) fmt

    member x.Debug (fmt: PrintfFormat<'Printer, _, _, _>): 'Printer =
        x.Log (getTw LogLevel.DEBUG) fmt


  type Logger2() =
    member val StringBuilder = StringBuilder()
    member x.Log k (fmt: PrintfFormat<'Printer, _, _, _>): 'Printer =
      Printf.kbprintf k (x.StringBuilder.Clear()) fmt

  type LevelLogger2(writer: TextWriter, level: LogLevel) as x =
    inherit Logger2()
    
    let action(threshold: LogLevel) () =
      if level <= threshold then writer.WriteLine(x.StringBuilder)
    
    member x.Info (fmt: PrintfFormat<'Printer, _, _, _>): 'Printer =
        x.Log (action LogLevel.INFO) fmt

    member x.Debug (fmt: PrintfFormat<'Printer, _, _, _>): 'Printer =
        x.Log (action LogLevel.DEBUG) fmt


module App =
  open System

  (*
  let main0 args =
    let logger = Logger.LevelLogger0(Console.Out, Logger.INFO)
    logger.Debug "launch at %A" DateTime.UtcNow
    logger.Info "hello with args %A" args
    logger.Debug "shutdown at %A" DateTime.UtcNow
    0 *)

  let main1 args =
    let logger = Logger.LevelLogger1(Console.Out, Logger.INFO)
    logger.Debug "launch at %A" DateTime.UtcNow
    logger.Info "hello with args %A" args
    logger.Debug "shutdown at %A" DateTime.UtcNow
    0

  let main2 args =
    let logger = Logger.LevelLogger2(Console.Out, Logger.INFO)
    logger.Debug "launch at %A" DateTime.UtcNow
    logger.Info "hello with args %A" args
    logger.Debug "shutdown at %A" DateTime.UtcNow
    0

[<EntryPoint>]
let main args = App.main1 args
    

​noop_printf.output.txt

hello with args [||]

​​.sharplab.json

{
    "version": 1,
    "target": "Run",
    "mode": "Release",
    "branch": "core-x64"
}