Env not modify when loading module in modulefile

Question

I would like to load a module into a modulefile (to resolve dependencies).

MyModule:

#%Module########################################
##
##  Modulefile
#
proc ModulesHelp { } {
    puts stderr "Env for MyProg"
}
proc addPath {var val} {
    prepend-path $var $val
}
module load MyOtherModule 
addPath   PATH   /opt/MyModule/bin

MyOtherModule:

#%Module########################################
##
##  Modulefile
#
proc ModulesHelp { } {
    puts stderr "Env for MyOtherProg"
}
proc addPath {var val} {
    prepend-path $var $val
}
addPath   PATH   /opt/MyOtherModule/bin

When I run module load MyModule, both modules seem to be loaded but environment is not right :

$module list
Currently Loaded Modulefiles:
  1) MyModule   2) MyOtherModule
$echo $PATH
/opt/MyModule/bin:/usr/bin:/bin

If I add the line foreach p [array names env] { set tmp $env($p) } or at least set tmp $env(PATH) in the MyModule after the module load MyOtherModule line, the environment is correctly modified. It also work fine if I don't use my function addPath but I use the prepend-path command directly, which is a bit annoying because I would like to do more things in the addPath function of course.

Anyone as an idea on what is going on and what I am missing ?

Answer 1

The prepend-path is probably doing some “clever” stuff to manage a variable; what exactly it is is something I don't know and don't need to know, because we can solve it all using generic Tcl. To make your wrapping of it work, use uplevel to evaluate the code in the proper scope, though you need to consider whether to use the global scope (name #0) or the caller's scope (1, which is the default); they're the same when your procedure addPath is called from the global level, but otherwise can be quite different, and I don't know what other oddness is going on with the modules system processing.

To demonstrate, try this addPath:

proc addPath {var val} {
    puts stderr "BEFORE..."
    uplevel 1 [list prepend-path $var $val]
    puts stderr "AFTER..."
}

We use list to construct the thing to evaluate in the caller's scope, as it is guaranteed to generate substitution-free single-command scripts. (And valid lists too.) This is the whole secret to doing code generation in Tcl: keep it simple, use list to do any quoting required, call a helper procedure (with suitable arguments) when things get complicated, and use uplevel to control evaluation scope.

(NB: upvar can also be useful — it binds local variables to variables in another scope — but isn't what you're recommended to use here. I mention it because it's likely to be useful if you do anything more complex…)