package vlt

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A variant of Bolt logging tool






VLT - A Variant of Bolt OCaml Logging Tool


This package provides yet another variant of Bolt OCaml logging tool called VLT derived from Volt logging tool. Unlike Volt, VLT relies on ppx instead of camlp4 to preprocess source code. In addition to the original Bolt (except preprocessor), VLT offers the following features:

  • Pass filter concept Each logger has an associated pass filter, which ensures that events will never be propagated to the ancestor loggers when the events do not satisfy the filter.

  • Original logging syntax modified for ppx
    log_expr ::= [%LOG level (string|ident) {arg}* {attr}* ]

    level ::=
    | [%FATAL]
    | [%ERROR]
    | [%WARN]
    | [%INFO]
    | [%DEBUG]
    | [%TRACE]

    attr ::=
    | [%NAME string ]
    | [%(PROPERTIES | WITH) expr ]
    | [%(EXCEPTION | EXN) expr ]

  • Extended logging syntax
    log_expr ::= ...
    | [%fatal_log (string|ident) {arg}* {attr}* ]
    | [%error_log (string|ident) {arg}* {attr}* ]
    | [%warn_log (string|ident) {arg}* {attr}* ]
    | [%info_log (string|ident) {arg}* {attr}* ]
    | [%debug_log (string|ident) {arg}* {attr}* ]
    | [%trace_log (string|ident) {arg}* {attr}* ]

    block_expr ::=
    | begin%fatal_block expr_seq end
    | begin%error_block expr_seq end
    | begin%warn_block expr_seq end
    | begin%info_block expr_seq end
    | begin%debug_block expr_seq end
    | begin%trace_block expr_seq end

  • More informative default logger name In addition to a capitalized source file name without suffix, surrounding module names, class names, method names, or function names are used to compute a logger name, unless [%NAME ... ] attribute is provided. Note that, unlike Volt, you have to enclose each toplevel structure_item in [%%capture_path ... ].

  • Suppression of unwanted evaluation of arguments Arguments in the [%LOG ... ] and *_log expressions are not evaluated when the defined logger does not record events.

  • Additional keys for use by the pattern and csv layouts.
    monthname - name of month e.g. January, February, ...
    monthnm - abbreviated name of month e.g. Jan, Feb, ...


The development sources are available from GitHub.


As in the original version, this tool is free software released under the LGPLv3.


$ dune build
$ dune install


The original Bolt is Copyright (C) 2009-2012 Xavier Clerc and released under the LGPLv3. The official website of Bolt is here.

Copyright © 2023 Codinuum Software Lab