Catching exception of a shell command in Perl 6

Question

I had to run a shell program exiting with an error from Perl 6, so I've decided to test how it works. I've made a bash script producing an error to run it from a Perl 6 program:

$ cat prog.sh 
echo "error" >&2
exit 1

Here is how I call it from Perl 6:

put "start";
  try {
    shell "./prog.sh";
  }
put "end";

The output shows that the program exited after running the shell command.

start
error
The spawned command './prog.sh' exited unsuccessfully (exit code: 1)
  in block <unit> at b.p6 line 2

If I add a CATCH block

put "start";
  try {
    shell "./prog.sh";
    CATCH { default {} }
  }
put "end";

everything is OK, and the program works to the last line:

start
error
end

So my question: why is it necessary to add the CATCH block, while try by itself cannot tackle the error?

Answer 1

shell doesn't throw the Exception until the sink.

The try block with only the shell in it fully executes without an exception being thrown, returning the last value in the block, which then gets sunk outside the context of the try, which then throws the Exception.

You can see this with:

put "start";
  try {
    shell "./prog.sh";
    'something';
  }
put "end";

Now the shell gets sunk inside the try, which gets caught by the implicit CATCH of the try. The try block returns the last value in the block, the 'something', which then gets safely sunk outside the try.

You can also force the sink to happen inside the try:

put "start";
try {
   sink shell "./prog.sh"
}
put "end";

Your added CATCH block is just preventing the try block from returning the return value from the shell.

You can re-arrange them and see that this still blows up:

put "start";
try {
    CATCH { default {} }
    shell "./prog.sh";
}
put "end";

The best, most clear way to handle this IMHO would be to check the return from the shell yourself rather than letting it sink and throw the Exception:

put "start";
if shell "./prog.sh" {
    say 'ok'
}
else {
    say 'failed'
}
put "end";