Re: [5.1.4/newbie] "attempt to index ... 'file' (a nil value)"?

[Steve Litt <slitt@...>]

On Friday 18 February 2011 08:53:25 Axel Kittenberger wrote:
> > BTW, is there a tool that can parse a Lua script to spot syntax
> > errors?
> 
> Best is to add a metatable to globals _G that errors on index. Put
> this on top of your program.
> 
> do
>   local mt = getmetatable(_G) or {}
>   mt.__index = function(k) error("Global "..k.." does not exist", 2) end
>   setmetatable(_G, mt)
> end

Axel,

This is wonderful. It's like "use strict" for Lua. However, I had a problem 
with your code because k was always a table, regardless of the global I read 
or wrote (I also added a __newindex() to catch writes to globals). So I 
changed your code to add a v to the __index() and __newindex() args, and it 
seems now to do what you intended:

==============================
#!/usr/bin/lua 

-- LEGAL GLOBALS BECAUSE THEY COME BEFORE METATABLE TWEAK
boisenberry = waterbury

do
	local mt = getmetatable(_G) or {}
	mt.__index = function(k,v)
		error("Global ".. tostring(v) .." does not exist", 2)
	end
	mt.__newindex = function(k,v)
		error("Cannot set global vars: ".. tostring(v), 2)
	end
	setmetatable(_G, mt)
end

-- COMMENT OUT ONE OF THE PRECEDING TO TEST READ OR WRITE OF A GLOBAL
strawberry = 4
strawberry = blueberry
==============================

Just to make it more memorable, I put it in a function called use_strict():
==============================
local function use_strict()
	local mt = getmetatable(_G) or {}
	mt.__index = function(k,v)
		error("Global ".. tostring(v) .." does not exist", 2)
	end
	mt.__newindex = function(k,v)
		error("Cannot set global vars: ".. tostring(v), 2)
	end
	setmetatable(_G, mt)
end
==============================

Axel, you have no idea how much work you've saved me with this suggestion. I'm 
very careless with typing, so it will save me hours and hours of debugging.

Just yesterday I was reminiscing about the "use strict" of my Perl youth, and 
wishing Lua had it. Today you showed me Lua DOES have it.

Thanks!

SteveT

Steve Litt
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