CoCoA » CoCoA-5

Initially I was unconvinced, but then realised that an error message is necessary, and naturally the error message should be as helpful as possible. Cannot find ... in scope is surely less clear than Function unavailable because CoCoA was not built with Normaliz (exact wording still to be decided)

We do not make arity checks for unavailable functions: it is surely more useful to the user (and simpler for us) to report unavailable rather than wrong arity.

Here is a reasonable proposal for how we could implement the ideas above.
We define a CPP macro called DECLARE_MISSING_EXTLIB which registers the name as a function with any arity, and which always produces a Missing external library error.

Here is how it might look in the file BuiltinFunctions-Normaliz.C

#ifndef CoCoA_WITH_NORMALIZ

  DECLARE_MISSING_EXTLIB(NormalizHilbertBasis,    "NORMALIZ");
  DECLARE_MISSING_EXTLIB(NormalizNormalToricRing, "NORMALIZ");
  DECLARE_MISSING_EXTLIB(NormalizHilbertBasis,    "NORMALIZ");
  DECLARE_MISSING_EXTLIB(NormalizHilbertBasis,    "NORMALIZ");

#else

DECLARE_STD_BUILTIN_FUNCTION(NormalizHilbertBasis, 1) {...}

The main defect is that the name of each function has to be written twice: once when using the DECLARE_MISSING_EXTLIB macro, and once when giving the proper definition. However, I think the block of calls to DECLARE_MISSING_EXTLIB could also function as a handy index of the functions defined in the file.