Re: [Libguestfs] [PATCH 2/3] v2v: add Var_expander
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.
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)
> +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
--
Pino Toscano
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