On Fri, Mar 29, 2019 at 01:38:55PM +0100, Pino Toscano wrote:
On Monday, 25 March 2019 15:55:46 CET Richard W.M. Jones wrote:
On Mon, Feb 25, 2019 at 05:22:51PM +0100, Pino Toscano wrote: [...]
After being burned a few times with custom parsing (hello, guestfish) I'm not a big fan.
I can perfectly understand that, however ...
Is there not an existing C or OCaml library/facility we could use here? It's a shame we can't use Perl Template Toolkit because it would be ideal here.
... sadly I did not find anything simple convering the use case that this Var_expander module covers. The closest thing I found was the usage of ${..}/$(..) variables in dune (the OCaml build system), with the following differences: - manually tokenizes the string - allows anything as variable name, splitting it in two if a ':' is found - ${..}/$(..) instead of %{..}
There are all kinds of questions that aren't answered such as: Should variables be replaced recursively? [...] Should we allow loops or similar constructs?
I don't think they are needed.
How do you escape %{..} if you don't want it to be replaced?
Good question... dune does not seem to allow that; maybe we can allow a trailing '$'/'\' as escape character.
RPM uses double %%
Existing template systems solve these kinds of problems already.
Yes, however they are much more complex, and with a number of features like conditionals, filters, loops, etc. Considering this is needed so far for the file paths of disks in -o json, IMHO plain variables are enough.
+let var_re = PCRE.compile "%{([^}]+)}"
Are we planning to allow a completely free choice for variable names, or could we limit this regexp to only matching ASCII alphanumeric + underscore?
This regex allows anything as variables for two reasons:
1) we do not miss any variable-like pattern (so we do not silently skip any now, while handling it in the future in case we accept more characters)
So I think you're saying that if we see (for example) this input: { foo: "%{" } we want %{ to be flagged as an error instead of being ignored? I guess could argue this one both ways, but yours is a reasonable explanation.
+let check_variable var = + String.iter ( + function + | '0'..'9' + | 'a'..'z' + | 'A'..'Z' + | '_' + | '-' -> () + | _ -> raise (Invalid_variable var) + ) var
... and then this function would presumably go away.
2) we can check that a variable has only allowed characters, and report that to the user
Anyway that's for investigating. If there's no alternative I guess we'll need to stick to hand-parsing. Rich. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Read my programming and virtualization blog: http://rwmj.wordpress.com virt-df lists disk usage of guests without needing to install any software inside the virtual machine. Supports Linux and Windows. http://people.redhat.com/~rjones/virt-df/