[ragel-users] combining ragel and lemon (update)

[Mark Olesen]

I figured that I should provide a followup on this topic.

OpenFOAM-v1912 (Dec 2019 release) now includes expression evaluation 
using a combination of ragel and lemon - my thanks to Adrian for a few 
pointers regarding simplifying my use of ragel.

You could say that the OpenFOAM expressions are essentially a glorified 
calculator, except that we handle various vector-space fields 
(scalar/vector/tensor/symmTenor/sphericalTensor) for internal meshes and 
patch fields etc.

The scanner is ragel, the parser is a version of lemon that has been 
minimally modified to allow for C++ namespaces - to allow for good 
encapsulation without forcing any particular interface on the user or 
affecting the use of lemon for C-code. To managing the large number of 
similar definitions, the lemon grammar is modularized using m4 macros.

Here are some code references as of Jan-2020:

The m4-augmented lemon make rules:
https://develop.openfoam.com/Development/openfoam/blob/master/wmake/rules/General/lemon

The lemon wrapper with m4 filtering:
https://develop.openfoam.com/Development/openfoam/blob/master/wmake/scripts/wrap-lemon

The m4 macros used for lemon:
https://develop.openfoam.com/Development/openfoam/tree/master/src/OpenFOAM/include/m4/lemon

An example of m4-augmented lemon grammar:
https://develop.openfoam.com/Development/openfoam/blob/master/src/OpenFOAM/expressions/fields/fieldExprLemonParser.lyy-m4

The corresponding ragel scanner:
https://develop.openfoam.com/Development/openfoam/blob/master/src/OpenFOAM/expressions/fields/fieldExprScanner.rl

The expected tokens for the lemon parser are pre-built, which allows the 
rest of the code to build in parallel without issue.

Cheers,
/mark