How does Lmod convert TCL modulefile into Lua
=============================================

Lmod uses a TCL program call **tcl2lua.tcl** to read TCL modulefiles
and convert them to lua.  The whole TCL modulefile is run through.
However instead of executing the "module functions" they are converted
to Lua.  For example, suppose you have the following simple TCL
modulefile for git::

    #%Module
    set appDir          $env(APP_DIR)
    set version         2.0.3

    prepend-path        PATH "$appDir/git/$version/bin"

Assuming that the environment variable APP_DIR is */apps* then the output of the
**tcl2lua.tcl** program would be::

   prepend_path("PATH", "/apps/git/2.0.3/bin")

Note that all the normal TCL code has been evaluated and the TCL
**prepend-path** command  has been converted to a lua **prepend_path**
function call.

Normally this works fine.  However, because Lmod does evaluate the
actions of a TCL module file as a two-step process, it can cause
problem.  In particular, suppose you have two TCL modulefiles:

Centos::

    #%Module
    setenv SYSTEM_NAME Centos

And B::

    #%Module
    module load Centos

    if { $env(SYSTEM_NAME) == "Centos" } {
       # do something
    }

When Lmod tries to translate the B modulefile into lua it fails::

   load("Centos")
   LmodError([[/opt/mfiles/B: (???): can't read "env(SYSTEM_NAME)": no such variable]])

This is because unlike the TCL/C Module system, the **module load
Centos** command is converted to a function call, but it won't load the module
in time for the test to be evaluated properly.

The only solution is convert the B modulefile into a Lua modulefile (B.lua)::

   load("Centos")
   if (os.getenv("SYSTEM_NAME") == "Centos") then
     -- Do something
   end

The Centos modulefile does not have to be translated in order for this to
work, just the B modulefile.


As a side note, you are free to put Lua modules in the same tree that the
TCL/C Module system uses.  It will ignore files that the first line is
not #%Module and Lmod will pick B.lua over B.