MO_FAIL_ON_UNSET should ignore unrendered variables

[eherot]

Prior to 3.0.0, MO_FAIL_ON_UNSET/-u used to ignore unset variables as long as they weren't being rendered:

# 2.4.1
$ unset TESTVAR ; echo 'TESTVAR={{#TESTVAR}}{{TESTVAR}}{{/TESTVAR}}' | mo -u
TESTVAR=

but now that doesn't work anymore:

# 3.0.4
unset TESTVAR ; echo 'TESTVAR={{#TESTVAR}}{{TESTVAR}}{{/TESTVAR}}' | mo -u
ERROR: Environment variable not set: TESTVAR

This was a very handy feature and it would be nice to have it back (or perhaps there is another way of doing this that I'm not seeing?)...

[fidian]

The earlier behavior was buggy. The option is intended to indicate an error whenever a variable is accessed and helps guard against typos. Previously, {{#TESTVAR}}...{{/TESTVAR}} would not use the same mechanism to get the variable. So, if you mistakenly used {{#TEST_VAR}}...{{/TEST_VAR}} it would not realize that the variable was not defined.

The erroring is supposed to mirror set -u in Bash, where any usage of an undefined variable would result in an error. It would be extremely handy in user-provided scripts or other untrustworthy sources. In those environments, one can define a set of allowed variables as empty values and the template will work. Perhaps this solution would work for you?

To get similar functionality, you may be able to write a function, but then your template would look quite a bit more ugly. The syntax would likely appear like {{#IF 'TESTVAR'}}...{{/IF 'TESTVAR'}}.

Another option is to modify mo and add a special command to ignore the MO_FAIL_ON_UNSET variable, or another flag could be added to ignore unset on just conditions, though I don't know of any similar approach with Bash scripts.

Maybe you could share more about why you want to use -u to flag undefined variables but you also don't want to care about them in conditions?

[fidian]

Closing due to inactivity. Feel free to reopen with an example of where you would want to error when the variable is used as a variable but not as a condition.