Assertion function with formatting support.

assert_fmt.lua

-- Release as Public Domain.
local function fmt_error(fmt, ...)
	local f_type = type(fmt)
	if f_type == 'string' then
		if select('#', ...) > 0 then
			return string.format(fmt, ...)
		else
			-- only a single string, no formating needed.
			return fmt
		end
	elseif f_type == 'function' then
		-- fmt should be a callable function which returns the message to log
		return fmt(...)
	end
	-- fmt is not a string and not a function, just call tostring() on it.
	return tostring(fmt)
end

function assert_fmt(v, fmt, ...)
	if v then return v, fmt, ... end
	local good, msg = pcall(fmt_error, fmt, ...)
	if not good then
		msg = "Failed to format error message: " .. msg
	end
	error(msg, 2)
end