How to check if a module exists in Lua? - lua

Question

I'm using xdg-menu-to-awesome-wm to generate a Lua file containing the GNOME menu, for inclusion in Awesome WM. Since the generator script may not be installed, I need some way for Lua to only require the menu module if it exists.
I don't want to go looking through file names, since it could be anywhere in package.path. One option would be to ignore the exception created when the module does not exist, but I'd rather not ignore any other exceptions - I do want to know if the module contains any syntax or other errors. The reference unfortunately doesn't specify which exceptions can be generated, so I'm not sure how to do it.

Answer 1

If you need to distinguish between a missing module and a syntax error, you can directly access the searcher functions in package.searchers.
These functions will:
Return a loader function if successful
Return a string if the module is not found
Throw an error if there is a syntax error
So what you can do is mimic the way require searches for a module, calling each searcher in turn until one of them returns a function. Unlike require, we need not throw an error if the module is not found, i.e. if every searcher function returns a string.
function isModuleAvailable(name)
if package.loaded[name] then
return true
else
for _, searcher in ipairs(package.searchers or package.loaders) do
local loader = searcher(name)
if type(loader) == 'function' then
package.preload[name] = loader
return true
end
end
return false
end
end

Answer 2

Look, I had the same problem with 'luafilesystem' module, I worked it out like this,
local haslfs,lfs = pcall(require,"lfs")
if haslfs then
configLines["PROJECT_HOME"] = lfs.currentdir()
else
configLines["PROJECT_HOME"] = prompt("Project path > ")
end
'lfs' here is the module handle . And pcall is used to know if the module is really loaded without propagating the error.

Answer 3

What I do is wrap the require in a pcall so that the module is loaded and a fail to load can be caught. There is a fully worked function which I use to download and install the missing modules from our servers here:
http://www.fhug.org.uk/wiki/doku.php?id=plugins:code_snippets:module_require_with_load
function loadrequire(module)
local function requiref(module)
require(module)
end
res = pcall(requiref,module)
if not(res) then
-- Do Stuff when no module
end
end
loadrequire('menu')