Module:CallAssert

This module is rated as ready for general use. It has reached a mature form and is thought to be relatively bug-free and ready for use wherever appropriate. It is ready to mention on help pages and other Wikipedia resources as an option for new users to learn. To reduce server load and bad output, it should be improved by sandbox testing rather than repeated trial-and-error editing.

This Lua module is used in system messages, and on approximately 245,000 pages.
Changes to it can cause immediate changes to the Wikipedia user interface.
To avoid major disruption and server load, any changes should be tested in the module's /sandbox or /testcases subpages, or in your own module sandbox. The tested changes can be added to this page in a single edit. Please discuss changes on the talk page before implementing them.

This metamodule provides function callAssert, which simplifies error checking by throwing an error if a given function returns nil or false. This is similar to the built-in function assert, but unlike it, callAssert itself calls the function, whose return value is to be checked. This allows it to include the function name and argument values in the error message.

Usage

local callAssert = require('Module:CallAssert')
local result1, result2, etc = callAssert(func, 'funcName', ...)

callAssert calls func(...) and checks if the first returned value evaluates to true. If it does, then it returns all the returned values. If not, it throws an error with a message in the form 'funcName(argument_values) failed'.

local function pack(...)
	return {...}, select('#', ...)
end

local function mapArray(func, array, count)
	local result = {}
	for i = 1, count or #array do
		result[i] = func(array[i])
	end
	return result
end

local function quote(value)
	if type(value) == 'string' then
		return (string.gsub(string.format('%q', value), '\\\n', '\\n'))  -- Outer parentheses remove second value returned by gsub
	end
	local str = tostring(value)
	if type(value) == 'table' and str ~= 'table' then
		return '{' .. str .. '}'
	end
	return str
end

local function callAssert(func, funcName, ...)
	local result, resultCount = pack(func(...))
	if not result[1] then
		local args, argsCount = pack(...)
		args = mapArray(quote, args, argsCount)
		local message = mw.ustring.format(
			'%s(%s) failed',
			funcName,
			table.concat(args, ', ')
		)
		error(message, 2)
	end
	return unpack(result, 1, resultCount)
end

return callAssert