* * * * *
                                        
                         Notes about a Lua lint checker
                                        
> Luacheck is a static analyzer and a linter for Lua [1]. Luacheck detects
> various issues such as usage of undefined global variables, unused
> variables and values, accessing uninitialized variables, unreachable code
> and more.
> 

“Luacheck [2]”

The one real issue I have with Lua [3] is its dynamic typing [4]. Of all the
bugs I fix in my own Lua code, I would say that the majority are due to typos
(wrong variable name) or an unexpected type. So I was quite happy to come
across and try out Luacheck. And fortunately, it's pretty straightforward to
run [5].

I ran it over “Project: Sippy- Cup [6]” and … wow. The extensive regression
test I have has already flushed out the typos and the unexpected type errors
I tend to make. But Luacheck found quite a few unused variables (which is
nice—it also found a bunch of unsused LPeg [7] expressions) and a ton of
unintentional global variables (because I forgot to declare them with local).

The output is easy to read (here's a representative sample from some non-
work related code I have):

> Checking ptest-cr-select.lua                      **Failure**
> 
>     ptest-cr-select.lua:53:9: variable **amount** was previously defined as an argument on line 52
>     ptest-cr-select.lua:128:9: variable **okay** is never accessed
>     ptest-cr-select.lua:193:40: unused argument **event**
>     ptest-cr-select.lua:197:43: shadowing upvalue **conn** on line 194
>     ptest-cr-select.lua:213:21: shadowing upvalue **argument** event on line 193
>     ptest-cr-select.lua:215:15: unused variable **rem**
>     ptest-cr-select.lua:215:15: shadowing upvalue **rem** on line 194
> 
> Total: **7** warnings / **0** errors in 1 file
> 

About the only false positive it finds is this idiom:

> function foo(param1,param2)
>   local param1 = param1 or "default value"
>   local param2 = param2 or 3
>   local a = ...
>   -- ...
> end
> 

where it will flag param1 and param2 as shadowing an upvalue. This idiom
though, is used to provide a default value if a parameter isn't given to a
function. It's easy enough to fix, either:

> function foo(param1,param2)
>   param1 = param1 or "default value"
>   param2 = param2 or 3
>   local a = ...
>   -- ...
> end
> 

or

> function foo(param1,param2)
>   local param1 = param1 or "default value" -- luacheck: ignore
>   local param2 = param2 or 3 -- luacheck: ignore
>   local a = ...
>   -- ...
> end
> 

Overall, I'm glad I found this tool. It's been a real eye opener.

[1] http://www.lua.org/
[2] https://github.com/mpeterv/luacheck
[3] http://www.lua.org/
[4] gopher://gopher.conman.org/0Phlog:2008/01/11.1
[5] http://luacheck.readthedocs.org/
[6] gopher://gopher.conman.org/0Phlog:2014/03/05.1
[7] http://www.inf.puc-/

Email Sean Conner at sean@conman.org

.