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] /boston/2008/01/11.1 | [5] http://luacheck.readthedocs.org/ | [6] /boston/2014/03/05.1 | [7] http://www.inf.puc-/

=> Gemini Mention this post | Contact the author