How does Lmod convert TCL modulefile into Lua

Lmod uses a TCL program called 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:

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 problems. In particular, suppose you have two TCL modulefiles:


setenv SYSTEM_NAME Centos

And B:

module load Centos

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

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

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):

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

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.