Wire-Cell News

Jsonnet is now a required external dependency.

It is just far too useful to keep optional and it's a very light-weight package so easy to build. However, it is not yet added to wire-cell-spack (see inaugural issue). Building it yourself is simple but non-standard. Follow this guide:

1. Get the source and do make all
2. Copy the two header under include/ at some installation path.
3. Similarly, copy the two shared libraries to lib/ at some installation path.
4. Finally, copy the executable binary jsonnet to some bin/.

One annoyance with the elaborate configuration which Jsonnet makes easy is that, for some things, you do not want to constantly edit a file just to make some small change. In particular, initial input and final output files are often best given directly on the command line rather than in some configuration file. Jsonnet also comes to the rescue here by allowing external parameters to be "injected" into the configuration using its std.extVar("name") function.

For this to work, the author of some configuration calls this function where they would otherwise type in the value. Then the user must supply that value on the wire-cell command line or if compiling the Jsonnet to JSON via the jsonnet CLI.

An example is in the configuration supporting the new "multi-ductor" feature (stay tuned for details). One spot it is used can be found in depos.jsonnet where the input file holding depositions is set. The user of wire-cell or jsonnet sets a value for this variable in the same way: by simply adding a -V flag. Here is a full example with some comments to explain:

$ wire-cell \
   -V detector=uboone \                         # (1)
   -V depofile=g4tuple-qsn-v1-fixed.json.bz2 \  # (2)
   -V framefile=uboone.root \                   # (3)
   -c multi/init.jsonnet \                      # (4) 
   -c multi/multiductor.jsonnet                 # (5)

Notes:

1. A variable detector is used in various places of the configuration to switch between some global parameters specific to that detector.
2. The input depofile is set. Remember that WCT will look for JSON/Jonnet files in directories given in the WIRECELL_PATH environment variable.
3. The output framefile is set. This will hold all the frames of traces (aka the "event") that get simulated.
4. The first of two configuration files that being the list of configurables.
5. The "meat" of the configuration.