The proposed behavior of `-config-var s` is as follows:
- if `s` is an existing configuration variable, print its value as
a string and exit with a success return value (0)
- if `s` is not an existing configuration variable, print nothing
and exit with a failure return value (non-0)
Note that we do not print a newline after the value of the
configuration variable. In particular, if the value is an empty
string, the output is undistinguishable from the output for
non-existing variables, the return value has to be considered instead.
The following alternative behaviors were considered:
- We could print a newline after the configuration value, which
would let users distinguish empty values from non-existing variables
by counting the lines of output, and would also be more pleasant for
users invoking the option from the command-line. However, the way
bash works on Windows means that $(ocamlc -config-var foo) would keep
a trailing \r in its output, and portable scripts would have to use
$(ocamlc -config-var foo | tr -d '\r') instead, which is a pain.
(This issue was pointed out by David Allsopp)
- We could print a message on the error output if the configuration
variable does not exist. This is clearer to a human user, but it is
annoying for scripts if they forget to silence the error output and
get their output mixed with our error messages. The main use of this
new feature is for scripting purposes.
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