Tcl Script: Invalid command name return when executing proc body

Question

In a Tcl script, I want to catch the return of a Tcl proc in order to execute some finalization actions. For example my code could be as follows:

proc X10 { a } { return [expr $a * 10] }
proc procF {} {
    set a 13
    catch {[info body X10]} __result
    return $__result
}
procF

The previous code gives me an error: invalid command name " return [expr $a * 10] "

Although that replacing info body X10 with return [expr $a * 10] works as expected. What I initially thought is that both of them are exchangeable and should give the same output. So, why the first gives an error and what is the difference between both of them ?

Answer 1

Your code is failing because you are getting the body of X10 and treating that as a command name. Tcl doesn't split things up automatically for you — you have to ask for it — and this is a critical language safety factor. You'd have to do something like this:

proc procF {} {
    set a 13
    catch {eval [info body X10]} __result
    return __result
}

or this (because the first argument to catch is a script):

proc procF {} {
    set a 13
    catch [info body X10] __result
    return __result
}

But I'd actually be inclined in your case (as presented exactly, and trying to interpret what you've said) to do:

proc procF {} {
    set a 13
    catch {X10 $a} __result
    return __result
}

Note also that if you did this:

proc procF {} {
    set a 13
    catch {info body X10} __result
    return __result
}

then the result would be the definition of X10 without it being evaluated.