package easy_logging

  1. Overview
  2. Docs

Easy logging

Logging infrastructure inspired by the Python logging module. The aim of this module is to provide a quick and easy to use logging infrastructure.

It has the following features :

  • one line logger creation
  • log messages printf style, or provide string or string lazy_t message
  • tree architecture for easy configuration
  • handlers associated to each logger will format, filter and treat the message independantly
  • use the infrastructure with your own handlers with the MakeLogging functor.
  • use tags to add contextual information to log messages
Basic example
open Easy_logging
logger = Logging.make_logger "my_logger" (Some Debug) [Cli Debug];;
logger#info "log_message";; 

will output to the stdout a message of the form

1.306  my_logger    Info    log_message


  1. Overall description (see also Easy_logging)
  2. Defaults

    1. Handlers (see also Easy_logging__Handlers)
    2. Loggers (see also Easy_logging.Logging.logger )
    3. The Logging module (see also Easy_logging.Logging)
  3. The MakeLogging functor (see also Easy_logging.MakeLogging )
  4. More examples

Overall description

The logging infrastructure is based on four concepts:

loggers, handlers, log items and logging tree.

A call to logger will create a log item, which it will pass to its handlers. Each handler will treats the item (e.g. transform it to a string, and then outputs to stdout or to a file).

                                    | handler 1 |
               _______________      |-----------|
              |     logger    | ==> |   ( * )   |
              |---------------|     |___________|
(message) ==> | -> log item   |      ___________
              [_______________| ==> | handler 2 |
                                    |   ...     |

Logging tree

Loggers are stored in a tree structure, with the following properties :

  • messages handled by a logger (after level filtering) are passed to its ancestor's handlers (this can be overriden with the set_propagate method).
  • if not set, the level of a logger is that of its closest ancestor whose level is set (or NoLevel if there is no such ancestor)
                       A (Info, Cli Debug)
                     /    \
                  A.B      A.C
                 /            \
            A.B.D (Warning)    A.C.E (Debug, (File "f", Debug),

For example, with the above logging tree

  • logs made to A, A.B or A.C of level Info or more are written to the stdout.
  • logs made to A.B.D of level Warning or above are written the stdout.
  • logs made to A.C.E are only written to the file "f"


To each logger and log message are associated a level, which will be used to filter the messages going through the logging infrastructure.

The predefined levels are, in increasing order of precedence :

  1. Debug : used for debugging.
  2. Trace : used between info and debug
  3. Info : used to trace program execution.
  4. Warning : used for warnings.
  5. Error : used for errors.
  6. Flash : used for one-shot debugging: displays an easy to spot message.
  7. NoLevel : used to filter out all messages.

Log items

A log item has type

type log_item = {
    level : level;
    logger_name : string;
    msg : string;
    tags : tag list

where the tag type is defined by the Handlers module.



Two handler creation helper function are provided. They instantiate handlers with a level of their own to filter messages :

  • Cli handler: outputs colored messages to stdout.

    let h = Default_handlers.make (Cli Debug) 
  • File handler : outputs messages to a given file.

    let h = Default_handlers.make (File ("filename", Debug)) 

Note that file handlers will write to files in the logs folder in the current path, creating it if it doesn't exist. See more about default handlers at Easy_logging__Default_handlers.

See more at Easy_logging.Handlers


See complete class documentation at Easy_logging.Logging.logger


A logger object can be created directly (in which case it will not be part of the logging tree)

let logger1 = new Logging.logger "my_logger1" 

or through helper functions of the The Logging module module.


A logger object has three methods for each of the log levels:

  • one that takes a formatting string and parameters (printf like)

    logger1#debug "Myvar : %s" (to_string myvar); 
  • one that takes a string lazy_t

    logger1#ldebug (lazy (heavy_calculation ())); 
  • one that takes a string

    logger1#sdebug (to_string myvar);

The Logging module

The Easy_logging.Logging module is that application of MakeLogging over DefaultHandlers. It provides two functions :

  • val make_logger :
    ?propagate:bool -> string -> log_level -> Default_handlers.desc list 

    Instantiates a logger with some paramters, and adds it to the logging tree.

  • val get_logger : string -> logger 

    Returns a registered logger, or creates a new one if it doesn't exist.

The MakeLogging functor

The MakeLogging functor takes a Easy_logging__.Easy_logging_types.HandlersT typed module, and creates a Logging module.


When declaring your Handlers module, do not coerce it the type HandlersT, because then its internal types t and desc won't be accessible.


See the examples page for more examples.


Innovation. Community. Security.