Tcl: How to do a callback accross namespace borders? - tcl

Question

Consider a library structured like this:
package provide ::mylib 1.0
namespace eval ::mylib {
namespace export f
proc f { action } {
# pass action around
g $action
}
proc g { action } {
eval $action
}
}
If I try to use it like this, it won't work:
....
namespace eval ::user {
set x 10
::mylib::f { puts $x }
}
The reason is that $x is not known in mylib. I can fix it like this:
namespace eval ::user {
set x 10
::mylib::f { puts $::user::x }
}
This works, but qualifying each variable in the argument to ::mylib::f is awkward. Another possibility is to wrap the code inside another namespace eval:
namespace eval ::user {
set x 10
::mylib::f { namespace eval { puts $x } }
}
Better, but still ugly. If it were Ruby or Perl, I would simply pass a closure to ::mylib::f. What is the best practice in Tcl?
BTW, I'm currently on Tcl 8.3, but hope to have the possibility to upgrade to a newer version soon, so solution in terms of 8.3 AND more recent versions are wellcome.

Answer 1

tl;dr version: namespace code
The namespace code command is intended for this situation. You pass it the script that you want to be encapsulated, and it hands you back a version that has magic in it so that it handles being called from anywhere. Here's how you might use it.
namespace eval ::user {
variable x 10
::mylib::f [namespace code { puts $x }]
}
Internally, it uses namespace inscope to do this. You're recommended to not use that directly; namespace code is the convenient way of doing this.
Note that it even works if the callback site passes arguments to it, so long as that site is doing:
eval $callback [list "the argument is this"]
(In fact, in 8.3 the eval isn't necessary because of a gross hack in unknown, but please do it the way I recommend above because we took that hack out in later versions. It was pretty awful.)