macros.lua2p

--[[============================================================
--=
--=  LuaPreprocess example: Macros.
--=
--=  Here we define a better assert function using a macro.
--=
--============================================================]]

!(
local DEBUG = true

function assert(conditionCode, messageCode)
	if not DEBUG then
		-- Make assert() calls do absolutely nothing if we're not in debug mode.
		return ""
	end
	if not messageCode then
		messageCode = string.format("%q", "Assertion failed: "..conditionCode)
	end
	return "if not ("..conditionCode..") then  error("..messageCode..")  end"
end
)

local i = 4

--
-- A call to Lua's normal assert function might look something like this. A
-- problem is that the message expression is evaluated even if the condition
-- is true, which is completely unnecessary. (This example is very simple of
-- course, but imagine more costly operations happening.)
--
assert(i > 1 and i < 7, "Invalid index. It must be 1<=i<=7 but is "..i)

--
-- By prepending @insert we actually call the assert function in the
-- metaprogram which, as we defined above, separates the condition and the
-- message arguments so that the message expression never evaluates as long as
-- the condition is true.
--
@insert assert(i > 1 and i < 7, "Invalid index. It must be 1<=i<=7 but is "..i)

-- We also made the default message better by including the condition code
-- itself in the message.
@insert assert(i > 1 and i < 7)

-- Note that @insert can also be written as @@.
@@assert(i > 1 and i < 7)

print("'i' is all good!")