util.warn does not warn against unexisting table members

[jucor]

Hi Jeff, Andreas @akfidjeland

I just noticed (the hard way) that util.warn only warns against reading unexisting variables, not unexisting table members:

require 'util.warn'

-- generates an error
-- local x = y

local t = {}
-- does not generate an error
local x = t.y

This is very problematic when using object-style, since self is always a table: we can thus read from inexisting fields. Would it be possible to warn against this, the same way you check for unexisting members in _G?

[fidlej]

I'm posting the code for the simplest checking here.
If you want to check torch classes and nested tables,
some extra logic would need to be integrated.
You can experiment with the metatable methods __index and __newindex.

local function guardReads(table)
    setmetatable(table, {
        __index = function (table, key)
            error("missing key: "..tostring(key), 2)
        end,
    })
end

local t = {}
guardReads(t)
-- will generate an error
local x = t.y

[jucor]

Thanks Ivo. As discussed by email, is it possible to package this guardReads to util.warn? I can then call it on self in the __init() of my class.

[fidlej]

I worry that calling guardReads(self) would not work correctly.
The torch class has already an existing metatable. The metatable provides a fallback to read attributes from the parent.

You can start using the above guardReads() in your code and discover the problems.
In other words: pull-requests are welcome.