Module:Arguments

    From Nonbinary Wiki
    Revision as of 10:30, 18 April 2014 by m>Mr. Stradivarius (memoize nils using a separate nilArgs table; this fixes a bug where nil values were being iterated over with pairs())

    Documentation for this module may be created at Module:Arguments/doc

    -- This module provides easy processing of arguments passed to Scribunto from
    -- #invoke. It is intended for use by other Lua modules, and should not be
    -- called from #invoke directly.
    
    local libraryUtil = require('libraryUtil')
    local checkType = libraryUtil.checkType
    
    local arguments = {}
    
    -- Generate four different tidyVal functions, so that we don't have to check the
    -- options every time we call it.
    
    local function tidyValDefault(key, val)
    	if type(val) == 'string' then
    		val = val:match('^%s*(.-)%s*$')
    		if val == '' then
    			return nil
    		else
    			return val
    		end
    	else
    		return val
    	end
    end
    
    local function tidyValTrimOnly(key, val)
    	if type(val) == 'string' then
    		return val:match('^%s*(.-)%s*$')
    	else
    		return val
    	end
    end
    
    local function tidyValRemoveBlanksOnly(key, val)
    	if type(val) == 'string' then
    		if val:find('%S') then
    			return val
    		else
    			return nil
    		end
    	else
    		return val
    	end
    end
    
    local function tidyValNoChange(key, val)
    	return val
    end
    
    function arguments.getArgs(frame, options)
    	checkType('getArgs', 1, frame, 'table', true)
    	checkType('getArgs', 2, options, 'table', true)
    	frame = frame or {}
    	options = options or {}
    
    	--[[
    	-- Get the arguments from the frame object if available. If the frame object
    	-- is not available, we are being called from another Lua module or from the
    	-- debug console, so assign the args to a new variable so we can
    	-- differentiate them.
    	--]]
    	local fargs, pargs, luaArgs
    	if type(frame.args) == 'table' and type(frame.getParent) == 'function' then
    		if not options.parentOnly then
    			fargs = frame.args
    		end
    		if not options.frameOnly then
    			pargs = frame:getParent().args
    		end
    		if options.parentFirst then
    			fargs, pargs = pargs, fargs
    		end
    	else
    		luaArgs = frame
    	end
    	
    	-- Set the order of precedence of the argument tables. If the variables are
    	-- nil, nothing will be added to the table, which is how we avoid clashes
    	-- between the frame/parent args and the Lua args.	
    	local argTables = {fargs}
    	argTables[#argTables + 1] = pargs
    	argTables[#argTables + 1] = luaArgs
    
    	--[[
    	-- Generate the tidyVal function. If it has been specified by the user, we
    	-- use that; if not, we choose one of four functions depending on the
    	-- options chosen. This is so that we don't have to call the options table
    	-- every time the function is called.
    	--]]
    	local tidyVal = options.valueFunc
    	if tidyVal then
    		if type(tidyVal) ~= 'function' then
    			error(
    				"bad value assigned to option 'valueFunc'"
    					.. '(function expected, got '
    					.. type(tidyVal)
    					.. ')',
    				2
    			)
    		end
    	elseif options.trim ~= false then
    		if options.removeBlanks ~= false then
    			tidyVal = tidyValDefault
    		else
    			tidyVal = tidyValTrimOnly
    		end
    	else
    		if options.removeBlanks ~= false then
    			tidyVal = tidyValRemoveBlanksOnly
    		else
    			tidyVal = tidyValNoChange
    		end
    	end
    
    	--[[
    	-- Set up the args, metaArgs and nilArgs tables. args will be the one
    	-- accessed from functions, and metaArgs will hold the actual arguments. Nil
    	-- arguments are memoized in nilArgs, and the metatable connects all of them
    	-- together.
    	--]]
    	local args, metaArgs, nilArgs, metatable = {}, {}, {}, {}
    	setmetatable(args, metatable)
    
    	local function mergeArgs(iterator, tables)
    		--[[
    		-- Accepts multiple tables as input and merges their keys and values
    		-- into one table using the specified iterator. If a value is already
    		-- present it is not overwritten; tables listed earlier have precedence.
    		-- We are also memoizing nil values, but those values can be
    		-- overwritten.
    		--]]
    		for _, t in ipairs(tables) do
    			for key, val in iterator(t) do
    				if metaArgs[key] == nil then
    					local tidiedVal = tidyVal(key, val)
    					if tidiedVal == nil then
    						nilArgs[key] = true
    					else
    						metaArgs[key] = tidiedVal
    					end
    				end
    			end
    		end
    	end
    
    	--[[
    	-- Define metatable behaviour. Arguments are memoized in the metaArgs table,
    	-- and are only fetched from the argument tables once. Fetching arguments
    	-- from the argument tables is the most resource-intensive step in this
    	-- module, so we try and avoid it where possible. For this reason, nil
    	-- arguments are also memoized, in the nilArgs table. Also, we keep a record
    	-- in the metatable of when pairs and ipairs have been called, so we do not
    	-- run pairs and ipairs on the argument tables more than once. We also do
    	-- not run ipairs on fargs and pargs if pairs has already been run, as all
    	-- the arguments will already have been copied over.
    	--]]
    
    	metatable.__index = function (t, key)
    		--[[
    		-- Fetches an argument when the args table is indexed. First we check
    		-- to see if the value is memoized, and if not we try and fetch it from
    		-- the argument tables. When we check memoization, we need to check
    		-- metaArgs before nilArgs, as both can be non-nil at the same time.
    		-- If the argument is not present in metaArgs, we also check whether
    		-- pairs has been run yet. If pairs has already been run, we return nil.
    		-- This is because all the arguments will have already been copied into
    		-- metaArgs by the mergeArgs function, meaning that any other arguments
    		-- must be nil.
    		--]]
    		local val = metaArgs[key]
    		if val ~= nil then
    			return val
    		elseif metatable.donePairs or nilArgs[key] then
    			return nil
    		end
    		for _, argTable in ipairs(argTables) do
    			local argTableVal = tidyVal(key, argTable[key])
    			if argTableVal == nil then
    				nilArgs[key] = true
    			else
    				metaArgs[key] = argTableVal
    				return argTableVal
    			end
    		end
    		return nil
    	end
    
    	metatable.__newindex = function (t, key, val)
    		-- This function is called when a module tries to add a new value to the
    		-- args table, or tries to change an existing value.
    		if options.readOnly then
    			error(
    				'could not write to argument table key "'
    					.. tostring(key)
    					.. '"; the table is read-only',
    				2
    			)
    		elseif options.noOverwrite and args[key] ~= nil then
    			error(
    				'could not write to argument table key "'
    					.. tostring(key)
    					.. '"; overwriting existing arguments is not permitted',
    				2
    			)
    		elseif val == nil then
    			--[[
    			-- If the argument is to be overwritten with nil, we need to erase
    			-- the value in metaArgs, so that __index, __pairs and __ipairs do
    			-- not use a previous existing value, if present; and we also need
    			-- to memoize the nil in nilArgs, so that the value isn't looked
    			-- up in the argument tables if it is accessed again.
    			--]]
    			metaArgs[key] = nil
    			nilArgs[key] = true -- Memoize nils.
    		else
    			metaArgs[key] = val
    		end
    	end
    
    	metatable.__pairs = function ()
    		-- Called when pairs is run on the args table.
    		if not metatable.donePairs then
    			mergeArgs(pairs, argTables)
    			metatable.donePairs = true
    			metatable.doneIpairs = true
    		end
    		return pairs(metaArgs)
    	end
    
    	metatable.__ipairs = function ()
    		-- Called when ipairs is run on the args table.
    		if not metatable.doneIpairs then
    			mergeArgs(ipairs, argTables)
    			metatable.doneIpairs = true
    		end
    		return ipairs(metaArgs)
    	end
    
    	return args
    end
    
    return arguments