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[Anders Logg <logg@xxxxxxxxx>]

User-defined functions are currently defined by overloading eval().
The eval() function is available in two different versions, for vectors
(tensors) and scalars:

  void eval(double* values, const double* x) const;

  double eval(const double* x) const;

This might be ok, but functions may also depend on the current cell,
the current facet, and perhaps on the current time and other data.
These are accessible by this->cell, this->facet.

So, a function depends on x and other things and it's not obvious what
additional data a function does depend on. Are there any suggestions
for what the interface should look like for user-defined functions?

One option could be to let functions be defined by expressions
(function pointers) given to the constructor of Function. That way,
one could define a function by

  void source(const double* x) { sin(x[0]); }

  f = Function(mesh, source);

This way, there could be many different types of function pointers,
all taking a different number of arguments, and we could differentiate
between the different types.

-- 
Anders

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