Opening files after return statement in a proc inside a namespace? - file-io

Question

I am not able to open a file and write to it from inside a procedure which is inside a namespace. Just wanted to know if TCL allows doing this.
namespace eval ::ms {}
proc ::ms::add {a b} {
return [expr {$a + $b}]
set fp [open "dummy.txt" w+]
puts $fp "test\ntest1\ntest2"
close $fp
}
puts [::ms::add 10 20]
I expected a file named dummy.txt to be present in my current directory after execution of this piece of code.

Answer 1

When a procedure executes a return (without an option that you're not supplying), it stops the procedure executing immediately. Code that comes afterwards in the procedure is simply not executed. To make the file, move the commands that create it and write content into it before the return.
proc ::ms::add {a b} {
set fp [open "dummy.txt" w+]
puts $fp "test\ntest1\ntest2"
close $fp
# You should also brace your expressions always
return [expr {$a + $b}]
}
There is however a way to have the file writing code after the return. Returning is done internally by throwing a kind of exception (typically caught by the hull of the procedure, which is the same bit of code that prepares and cleans up the stack frame), which can be intercepted. Tcl has two commands that can intercept return exceptions: catch and try. The latter makes putting code after the return easy:
proc ::ms::add {a b} {
try {
return [expr {$a + $b}]
} finally {
set fp [open "dummy.txt" w+]
puts $fp "test\ntest1\ntest2"
close $fp
}
}
You can do the same with catch (which is a bit more of a primitive operation) but it's fiddly and not really recommended.