Szathmáry László honlapja @ DEIK | Nim2 / execute a proc before quitting

Example

import std/exitprocs

proc one() = echo "one"

proc two() = echo "two"

proc main() =
echo "hello"

when isMainModule:
addExitProc(one)
addExitProc(two)
#
main()

The registered procs go into a stack (LIFO), i.e. the procs will be called in a reverse order.

Output:

hello
two
one

Unhandled exception

What happens if we have an unhandled exception and the program terminates with an ugly error message?

import std/exitprocs

proc one() = echo "one"

proc two() = echo "two"

proc main() =
echo "hello"
echo readFile("doesnt_exists.txt")

when isMainModule:
addExitProc(one)
addExitProc(two)
#
main()

Output:

hello
Error: unhandled exception: cannot open: doesnt_exists.txt [IOError]

The registered procs are not executed :( Unhandled exceptions bypass exit procs.

Handled exception / quit

import std/exitprocs

proc one() = echo "one"

proc two() = echo "two"

proc main() =
echo "hello"
echo readFile("doesnt_exists.txt")

when isMainModule:
addExitProc(one)
addExitProc(two)
#
try:
main()
except Exception as e:
stderr.writeLine "Error: ", e.msg
quit(1)

Output:

hello
Error: cannot open: doesnt_exists.txt
two
one

To overcome the previous problem, we need to catch the exception. A simple way is to put the whole main() call inside a try block.

quit() triggers the exit procs.

Alternatively, we could also put just readFile() inside a try block:

# ...
try:
echo readFile("doesnt_exists.txt")
except IOError as e:
stderr.writeLine "File error: ", e.msg
quit(1)
# ...