[ET Trac] [Einstein Toolkit] #798: allow calculations in parameter assignments in parameter files

[Einstein Toolkit]

#798: allow calculations in parameter assignments in parameter files
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  Reporter:  rhaas        |       Owner:          
      Type:  enhancement  |      Status:  reopened
  Priority:  optional     |   Milestone:          
 Component:  Cactus       |     Version:          
Resolution:               |    Keywords:          
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Comment (by knarf):

 Right. The usage with (x+y) is so common that I didn't think of other
 possibilities. However, the only other usage I found is ((x>y)*x+!(x>y)*y)
 - essentially max(x,y) - which is also commutative. I don't think we
 should really support non-commutative expressions here, this was probably
 never intended. But you are right, this is beyond the scope of this
 ticket.

 What would be important here is: should entries within parameter files be
 order-independent? I would argue that this would be a good idea.

 Concerning an implementation: Yes, the parameter file would need to be
 parsed multiple times: first for the active thorns and at least once more
 for the actual parameters (#705). And yes, Cactus would need to track
 dependencies.

 > Writing to a parameter that has already been read would be flagged as
 error.

 Consider:

 {{{
 A::a = "=B::b"
 B::b = 5
 }}}

 When parsing this line-by-line, A::a would get the default value of B::b
 (let us assume 4), would be marked as being read, and the second line
 would throw an error. But what we actually want is that A::a = 5, not 4,
 and no error.

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Ticket URL: <https://trac.einsteintoolkit.org/ticket/798#comment:25>
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