Using local name with Merl

It looks like you're trying to write a generic generator, i.e. one that can be used for any metamodel. They are actually rather hard, because you need reflection capabilities to read the metamodel information. A normal generator only needs to read the model information, and the generator itself contains direct references to particular types, local names etc.

If you really want your model transformed in a watertight way into XML, I'd suggest you look instead at MetaEdit+'s XML export. That already takes care of the many XML challenges such as object references, circularity, illegal characters etc. that you will face after overcoming this hurdle. It should be easy enough to massage the XML export format into the format you want, e.g. with XSLT.

On the other hand, if this was just a quick example to play around with MERL, I'd suggest you look at a different task. For a generator to add value, it has to understand the different semantics of each piece of information in the models, rather than just treating them all the same. For instance, normally one property will go to make up the name of a file, another to become a function name, still another will become part of a condition. Each property will thus be named explicitly in the generator, rather than iterating over all of them with "do :()".

If you really want the local names, MERL doesn't have direct access to them [Edit 2018: now it does!]: it's designed to operate on models, not metamodels. You can however get them by saving the metamodel into text with:
   internal 'document:into: "' type '" metamodels' execute
and reading that into a variable with:
   variable 'metamodel' write
      filename 'metamodels' sep type sep 'metamodel.txt' read
   close
You can then iterate over the lines of that variable with "do $metamodel" - you'll have to do a little processing, but you can look at the "Reverse engineer Java" generator in UML's Class Diagram for hints.

Of course, if you're only going to do this for one metamodel, there's no need to write a metaprogram that could do it for all metamodels. Just stick direct references to the local names into the generator: it's almost certainly faster in the long run, and also gives you a measure of indirection to cope with name changes in the future (e.g. if you want to keep the same XML schema but make the user-visible names in the modeling tool more user-friendly).

do .State {
'<state name="' :Name%xml '" description="' :Description%xml '/>' newline
}

Note how that is already simpler than what you have above, and easier to read. I'm a big fan of meta solutions, but even I have to admit that in some cases a simple solution, even one with some apparent duplication, may be best!

Edited by stevek - 29.May.2018 at 09:29

It actually wouldn't be trivial to add local name: inside do :(), the elements we are iterating over are properties. A property doesn't know its local name, because the same property can be shared between several objects, maybe of different types or in different slots, each with its own local name.

Great! That's a nice combination of a meta-solution with pragmatism.

Hi,

How about iterating the collection with:

'   void On'
do :In protocols; { id ' '} newline

Or (if you don't want to have extra space after the last reported item)

'   void On'
dowhile :In protocols; { id ' '} newline

While in your sample you have metatype testing for the property field, I assume you are iterating over all properties (have do :() {...}). If that is the case, then you need to index the inner do loop with 1, following sample should work:

do :() {
  if type ='In protocols' then
    ' void On'
    dowhile :In protocols;1 {id ' ' } newline
  endif
}

More samples and full MERL documentation can be found from: http://www.metacase.com/support/45/manuals/mwb/Mw.html


Edited by janne - 16.Feb.2012 at 11:10