Re: comparative functions and "same metamethod" gotcha

[Andrew Starks <andrew.starks@...>]

On Wed, Jun 4, 2014 at 11:31 PM, Roberto Ierusalimschy <roberto@inf.puc-rio.br> wrote:

> I THINK (and I'm not certain) that differing line numbers in the debug
> info is enough to mark them as "different". I think "similar enough"
> essentially refers to two functions being instantiated by different
> iterations of the same line of code.

Right.

Okay. This is (pretty much) the whole constructor:

local function url(url_str)

local self

if type(url_str) == "string" then

self = url_pat:match(url_str)

elseif type(url_str) == "table" and url_str.url then

return url(url_str.url)

end

if self then

self.socket = self.host .. 

(self.port and (":"..("%d"):format(self.port)) or "")

self[0] = self.socket

self.xpath = xpath:match(url_str)

self.url = "" .. "://" .. self.xpath

self.route = route

return setmetatable({}, {

--for brevity, I'm only including the function in question.

__eq = function(a, b) return a.url == b.url end ,

})

end

end

This would seem to support "similar enough". 

If I run tests in the same file as this code, it never fails to see the functions as the same. 

--...

--complete module definition is above this comment...

for i = 1, 10001 do

local a = url("mcp://host/thread/name/"..tostring(i))

collectgarbage()

local b =  url("mcp://host/thread/name/"..tostring(i) .."/")

assert(a == b)

end

print("passed!")

-->  passed!

If I run them in a different file (require the module), it _often_ does. That is, it's not only within Lua Busted.

If I run the garbage collector between assignments, it will fail every time. 

local url = "">

local a = url("mcp://host/thread/name")

collectgarbage()

local b = url("mcp://host/thread/name/")

print(a == b)

--> false

print(getmetatable(a).__eq, getmetatable(b).__eq)

--> function: 00000000004337D0 function: 00000000003E9550

Is this expected? Can you provide some insight as to how I can understand this better?

-Andrew