Re: [RFC] Logically composable variant of errexit

[Andreas Grünbacher]

2014-10-10 3:29 GMT+02:00 Chet Ramey chet.ra...@case.edu:
 What does logically composable mean in this context?

I would like the hypothetical errfail option to:

 * Behave like errexit at the top level (outside of functions).

 * Be inherited by functions,  subshells, and command substitution.

 * Inside functions, it should trigger like errexit would outside of
   functions, no matter how the function is called.
   It should return instead of exiting, though.

With errexit, you get vastly different results from functions depending
on how the functions are called, for example,

   foo() {
  echo foo: top
  false
  echo foo: bottom
   }

   set -o errexit

   # bottom of foo reached:
if foo; then
  echo success  # reached
   fi

   # bottom of foo not reached:
   foo

With errfail, foo:bottom and success would not be reached.

Command substitutions would continue to behave as basic commands
do with respect to control flow: inside functions, a failure would cause
the function to return; outside of functions, the script would exit.

Thanks,
Andreas

Re: [RFC] Logically composable variant of errexit

[Andreas Grünbacher]

2014-10-10 8:38 GMT+02:00 Dan Douglas orm...@gmail.com:
 I would still propose that a simple and powerful way to extend Bash with
 exception handling would be to extend the ERR trap by passing it some metadata
 about the type and location of the exception incurred so that it can be 
 handled
 by user code. This proposal allows us to largely avoid having to answer the
 question of when and where does it make sense for this to get triggered? for
 the user.

 It might require a new type of trap that just fires on anything that returns
 false or fails so that the user handler can choose what to do about it in
 various contexts.

Well, currently, the ERR trap only triggers when errexit would
trigger, so another
type of trap would certainly be needed. I don't think it's reasonable
to expect from
users to use trigger tricks to get basic error handling working in a sane way,
though; this should be built in and trivial to activate.

Thanks,
Andreas

[RFC] Logically composable variant of errexit

[Andreas Grünbacher]

Hi all,

the errexit option can be very useful in simple scripts. This option
is being ignored in many contexts like lists and conditionals though.
I understand that this is by design and that errexit cannot be
fixed to behave more reasonably. Still, this makes bash a lot less
useful than it could be; working around this limitation is painful,
ugly, and leads to fragile code.

So, since we cannot fix errexit, can we maybe introduce another
option like errfail that behaves like errexit for simple commands,
but is also logically composable? Let me show what I mean with the
following pseudo-code:

   set -o errfail
   fail() {
  false
  echo 'oops!' 2  # not reached
   }
   ! fail
   fail || :
   if fail; then
  :
   fi
   set -- `fail`  # script fails here
   echo 'oops!' 2  # not reached

Having such an option would certainly make bash a lot more useful to me.
What do you guys think, could such an option be implemented with
reasonable effort?

Thanks,
Andreas