A tiny, little logger <3


Set up

The default log level is Level.Warning and the default printer is prerr_endline.

To change these, use the set_log_level and set_printer functions.


Logging functions come in two flavors: those that accept a message (e.g., info, error), and those that accept strings directly (e.g., sinfo, serror).

A message is simply a unit -> string thunk. Because message is a thunk, its evaluation won't happen unless the message actually needs to be printed. The type message is just a name given to the thunk to clarify the function signatures.

The functions that take a string directly are prefixed with an s (for string).


A printer is a function with the following signature string -> unit. The name printer is simply used for clarity.

Common choices for a printer would be prerr_endline to print to stderr, or Async.prerr_endline to asynchronously print to stderr using Async.Writer (e.g., if you are using Jane Street's Async library.

Message Levels

Message levels are hierarchical: given the log level threshold (get_log_level), all messages of equal or higher priority will be logged.

In other words, if a message level is greater than or equal to the logging threshold, it will be printed.

For example, Level.Trace would print all messages, Level.Silent would print no messages, Level.Error would print Level.Error, Level.Fatal, and Level.Unknown messages.

The default log level is Level.Warning. All messages of equal or higher priority than this will be logged.


Note: the examples assume you have open Little_logger at the top of your file.

Logging messages

let () = Logger.error (fun () -> "This is an error")
let () = Logger.serror "This is an error, but using `serror` instead"

let () = (fun () ->
      sprintf "I can use %s strings like %s" "format" "this")

Using the thunk accepting functions let you avoid potentially expensive calls in cases where the log level would prevent a message from being printed.

let () = Logger.set_log_level Logger.Level.Error

(* This won't run and the expensive thing won't take up extra time. *) let
() = Logger.debug (fun () -> (* ...Call some expensive log message
generating function here... *) )

Changing the logging level

The default level is Warning. Here we lower it to Debug.

let () = Logger.set_log_level Logger.Level.Debug 

Changing the printer

The default printer is prerr_endline, which prints to stderr. We can change to printing to stdout like this.

let () = Logger.set_printer print_endline 

If you're using the Async library, just change to an async printer.

let () = Logger.set_printer Async.prerr_endline 

Writing directly to a file

If you want to get crazy you could do something like this to log directly to a file, but I don't know if it is the best idea :)

let log_fname = "silly_file.txt"
let log_chan = Out_channel.create "silly_file.txt"

let printer msg = Out_channel.output_string log_chan (msg ^ "\n")
let () = Logger.set_printer printer

let () = (fun () -> "Hi file!")

let () = Out_channel.close log_chan



module Level : sig ... end

Logging levels

Type aliases

type printer = string -> unit

Type alias for printer functions

type message = unit -> string

Type alias for message thunks

Getters and setters

val get_log_level : unit -> Level.t
val set_log_level : Level.t -> unit
val set_printer : printer -> unit

Message accepting log functions

val unknown : message -> unit

unknown msg logs an unknown message. Unknown messages are printed when log level is Level.Unknown or below.

val fatal : message -> unit

fatal msg logs an fatal message. Fatal messages are printed when log level is Level.Fatal or below.

val error : message -> unit

error msg logs an error message. Error messages are printed when log level is Level.Error or below.

val warning : message -> unit

warning msg logs an warning message. Warning messages are printed when log level is Level.Warning or below.

val info : message -> unit

info msg logs an info message. Info messages are printed when log level is Level.Info or below.

val debug : message -> unit

debug msg logs an debug message. Debug messages are printed when log level is Level.Debug or below.

val trace : message -> unit

trace msg logs an trace message. Trace messages are printed when log level is Level.Trace or below.

String accepting log functions

These are analogous to the message accepting log functions.

val sunknown : string -> unit
val sfatal : string -> unit
val serror : string -> unit
val swarning : string -> unit
val sinfo : string -> unit
val sdebug : string -> unit
val strace : string -> unit