tcl changing a global variable while in a child interpreter (interp) - tcl

Question

I am trying to use a TCL program to read TCL modulefiles and translate them into another language. This has worked quite well until now. For reasons too complicated explain I have to treat "puts stderr" differently in different parts of the modulefile. I am asking for help in trying to figure out a way to do this.
Below is an extremely abbreviated modulefile called "modfile". This "modfile" is translated or "sourced" by a second tcl program.
proc ModulesHelp { } {
 puts stderr "(1) This is a help message"
}
puts stderr "(2) Here in modfile"
The puts statement inside ModulesHelp has to be treated differently from the second puts statement. Note that any solution CAN NOT CHANGE "modfile". That file is not under my control.
Here is my attempt at a solution:
#!/usr/bin/env tclsh
proc myPuts { stream msg } {
global putMode
puts stdout "putMode: $putMode" # <====== HERE 1
puts stdout $msg
}
proc report { message } {
puts stderr "$message"
}
proc execute-modulefile { m } {
global MODFILE putMode
set putMode "normal"
set slave "__mod"
interp create $slave
interp alias $slave puts {} myPuts
interp alias $slave report {} report
interp eval $slave {global putMode }
interp eval $slave [list "set" "putMode" $putMode]
interp eval $slave [list "set" "m" $m]
set errorVal [interp eval $slave {
set sourceFailed [catch {source $m } errorMsg]
if {[info procs "ModulesHelp"] == "ModulesHelp" } {
set putMode "InHelp" # <======= HERE 2
ModulesHelp
}
if {$sourceFailed} {
report $errorMsg
return 1
}
}]
interp delete $slave
return $errorVal
}
eval execute-modulefile $argv
To run this I do: $ ./try.tcl modfile where obviously the above script is "try.tcl" and the modulefile is "modfile". I am running this on a linux system with tcl 8.4.
What I would like to have happen is that at the line labelled "HERE 2" I like to somehow change the global variable of "putMode" from "normal" to "InHelp" so that I can change the behavior at the line labelled "HERE 1". No matter what I have tried to do I can not change the value of putMode at "HERE 1" by doing something at "HERE 2". The puts statement at "HERE1 always says "normal".
Using a global variable seems like the easiest solution but if someone could show me how to use namespaces or some other technique, I'll be happy with that as well.
Thanks for any insight.
I greatly appreciate the time that others have looked at my question. I am trying to use the proposed solution and I'm not quite seeing it. Here is my new attempt at a solution (This doesn't work at all). Can someone suggest how I modify this code to change "putMode" to inHelp where "HERE 2" is? Also is there something special that needs to go where "HERE 1" is?
#!/usr/bin/env tclsh
proc myPuts { stream msg } {
global putMode
puts stdout "putMode: $putMode" # <=== HERE 1
puts stdout $msg
}
proc report { message } {
puts stderr "$message"
}
proc PutModeTrace {childInterp operation realPutMode} {
# Alias the main array element for the purposes of this procedure
upvar \#0 PutMode($childInterp) realPutMode
if {$operation eq "read"} {
interp eval $childInterp [list set putMode $realPutMode]
} elseif {$operation eq "write"} {
set realPutMode [interp eval $childInterp {set putMode}]
}
}
proc execute-modulefile { m } {
global MODFILE putMode
set putMode "normal"
set slave [interp create]
interp alias $slave puts {} myPuts
interp alias $slave report {} report
interp eval $slave {global putMode }
interp eval $slave [list "set" "putMode" $putMode]
interp eval $slave [list "set" "m" $m]
interp eval $slave [list "set" "slave" $slave]
interp eval $slave {trace add variable putMode {read write} PutModeTrace}
interp alias $slave PutModeTrace {} PutModeTrace $slave
set errorVal [interp eval $slave {
set sourceFailed [catch {source $m } errorMsg]
if {[info procs "ModulesHelp"] == "ModulesHelp" } {
set start "help(\[\["
set end "\]\])"
PutModeTrace $slave "write" "inHelp" # <=== HERE 2
puts stdout $start
ModulesHelp
puts stdout $end
}
if {$sourceFailed} {
report $errorMsg
return 1
}
}]
interp delete $slave
return $errorVal
}
eval execute-modulefile $argv

Answer 1

The problem is that the slave and the master are different interpreters. This means that every interpreter has it's own
commands
variables
namespaces
channels
You can't simply change a variable in the master from the slave, so the easiest solution would be:
interp alias $slave InHelp {} set ::putMode InHelp
and calling this alias instead.
Some other notes:
An other option would be to change the puts alias when InHelp is called. Example
proc InHelp {slave} {
interp alias $slave puts {} HelpPuts
}
and using it with interp alias $slave {} InHelp $slave
You don't have to assign a name for the slave. Just do
set slave [interp create]
Single words don't have to be quoted, so
list "a" "b" "c"
is equal to
list a b c
If you need argument expansion (and use at least Tcl 8.5) use {*}$argv instead eval.
But because execute-modfile only accept one argument, execute-modfile [lindex $argv 0] should do the job.

Answer 2

As Johannes writes, variables are entirely separate in different interpreters; they're not shared at all.
However, you can use trace and some aliases to couple things together. I'll show how to do it for a simple scalar variable (with the parent having an array of them, presumably one for each child interpreter), under the assumption that you never want to have the setting of the variable in the master interpreter trigger a trace in the child.
interp eval $child {trace add variable putMode {read write} PutModeTrace}
interp alias $child PutModeTrace {} PutModeTrace $child
proc PutModeTrace {childInterp varName elementName operation} {
# Ignore the elementName argument
# Alias the main array element for the purposes of this procedure
upvar \#0 PutMode($childInterp) realPutMode
if {$operation eq "read"} {
interp eval $childInterp [list set putMode $realPutMode]
} elseif {$operation eq "write"} {
set realPutMode [interp eval $childInterp {set putMode}]
}
}
This makes it so that whenever the child interpreter reads or writes the putMode variable, the read/write gets reflected into the master.
It's easier to map a command (via an alias) though, and if you were using Tcl 8.6 I'd suggest stacking and unstacking custom transformations on stderr instead. (But that's a massively more sophisticated technique.)

Answer 3

Thanks for all the help. It has taken me a while to understand what was being proposed. Here is the code that does what I want:
#!/usr/bin/env tclsh
proc myPuts { stream msg } {
global putMode
if {$putMode != "inHelp"} {
puts stderr $msg
} else {
puts stdout $msg
}
}
proc report { message } {
puts stderr "$message"
}
proc setPutMode { value } {
global putMode
set putMode $value
}
proc execute-modulefile { m } {
global MODFILE putMode
set putMode "normal"
set slave [interp create]
interp alias $slave puts {} myPuts
interp alias $slave setPutMode {} setPutMode
interp alias $slave report {} report
interp eval $slave {global putMode }
interp eval $slave [list "set" "putMode" $putMode]
interp eval $slave [list "set" "m" $m]
interp eval $slave [list "set" "slave" $slave]
interp eval $slave {trace add variable putMode {read write} PutModeTrace}
interp alias $slave PutModeTrace {} PutModeTrace $slave
set errorVal [interp eval $slave {
set sourceFailed [catch {source $m } errorMsg]
if {[info procs "ModulesHelp"] == "ModulesHelp" } {
set start "help(\[\["
set end "\]\])"
setPutMode "inHelp"
puts stdout $start
ModulesHelp
puts stdout $end
setPutMode "normal"
}
if {$sourceFailed} {
report $errorMsg
return 1
}
}]
interp delete $slave
return $errorVal
}
eval execute-modulefile $argv
Thanks again.