Warn for Lua 5.3 and below

Lua 5.4 introduces the warn function, which allows the user to output warning messages to stderr when warning output has been turned on.

This library adds this functionality to older versionf of Lua.

It can safely be required in Lua 5.4, in which case it simply does nothing.

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Usage:

require 'warn' -- On Lua 5.4+ this does absolutely nothing

warn("@on")
warn("Hello", "World")

Alternatively, libraries can optionally use this feature locally with

local warn = warn or require 'warn.compatible'

Or use this wrapper to use warn only if it's available as a global

local warn = require 'warn.maybe'

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To run Lua with warnings enabled, require the w module:

lua -lw file_with_warnings.lua

Unlike the -W flag, this also works on older versions and requires warn if there's no global warn available.

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Differences from vanilla warn (only applies below version 5.4)

• File name and line number are part of the output message
• The global function WARN_PREFIX can be used to customize this prefix

Also please note that this obviously doesn't add the -W flag to the Lua interpreter.