"Proposal" for global/local parameter scope in CASA

Version umpteenth+1 (2008-04-23)

S.T. Myers 

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See discussion on the wiki for more on A,B,C:
https://wikio.nrao.edu/bin/view/Software/CasaVariableScope

Previous proposals:
http://www.aoc.nrao.edu/~smyers/naug/notes/2008-04-21-myers-globals.txt
and
http://www.aoc.nrao.edu/~smyers/naug/notes/2008-04-22-myers-globals.txt
(Part A only)

NOTE: the discussion is about tasks, toolkit useage is
unaffected, and the "meta-tasks" like go, inp, tget (which
use the "taskname" global) also have to work independent of
scope etc.

This is essentially David Whysong's proposal.

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Remove casaglobals param.

Tasks invoked with "go" ALWAYS use the current global parameter
values except for those specified as args.  Tasks invoked as

> go task(var1=val1) 

will use the globals except for the vars listed as args, AND
the global values for input parameters are NOT changed by tasks.

NOTE: output variables as parameters will need to be always set in the
task and are thus "global" and changed in the task.

Tasks invoked as calls with arguments NEVER use the current 
global parameter values, but use defaults except for those 
given as explicit args:

> task (var1=val1, etc.)  

Note that upon successful completion the actual parameters specified
in the call will be in the .last file and thus accessible via tget.
But the current value of the globals will NOT be updated to these
just by running the task.

An exception is a task called without args, e.g.

> task
> task()

These would fail if defaults were used because no task will actually
run on the defaults (they at least need some dataset to work on).  A
task call like these with NO specified arguments will therefore be
treated as

> go task 

and uses the current globals.  Note that this will alleviate
most of the breakage of current examples and regressions.

Pros: reasonably simple and clean, no toggles, easy to explain

      makes scripts using call style straightforward and 
      deterministic

      fulfills ALMA req OL-2.4-R4 and EVLA R2.4-R2.2

      is expected behavior for most users used to python
      or function calls (miriad?)

      most current users used to doing "go" or just "task()"
      so will not see change

      gets rid of the "casaglobals" parameter

      straightforward to put into effect in tasks

      only a few changes needed to docs

Cons: two different behaviors depending on how called

      a few changes needed in cookbook and other docs

      is NOT expected behavior for difmap users

      breaks previous behavior to mix setting params and 
      then doing task(var1=val1) to change that one only

      subtle distinction between "go task(var1=val1)" and
      "task(var1=val1)"

      will need to make the distinction clear to users in 
      documentation