initialization of S4 classes/methods

I'm seeing weird issues in methods initialization, i.e. loading
marrayClasses loads Biobase, and when explicitly done, as in

   library(Biobase)
   library(marrayClasses)

is fine, but when Biobase is loaded via a require statement in
marrayClasses' .First.Lib, I end up with:

  Warning message: In the method signature for function "coerce", class
  "exprSet" has no current definition in: matchSignature(signature,
  fdef, where)

Yes.  See below.  Does this actually cause problems later on?  When
methods are dispatched, it's likely that "exprSet" will be found.

and based on the "band-aid" working above, suggests an initialization
or evaluation timing problem.

Not exactly.  The problem is one of the behavior of require() combined
with rules for parents of environments; it's bitten us before, but so
far hasn't been fixed.

1.  The effect of require() is to put Biobase on the search path
_before_ marrayClasses, whereas in the case that library(Biobase) has
been done previously, the opposite is true.

2.  In looking for the definiton of class exprSet, the call to exists
from setMethod will start with the environment of package marrayClasses
and search through the chain of parent environments; i.e., all the
packages _after_ marrayClasses in the search list.  And so it doesn't
find class "exprSet".

It's possible to hack our way around point 2 (by having a special case
for the environment not being a namespace), but point 1 seems not quite
correct.  For example, if the owner of marrayClasses wanted to
deliberately override an object in Biobase, require() would prevent it. 
(Not that overriding is necessarily a good practice, but it should
either work or be an error.)
 
John

(and exprSet is defined in the Biobase package).

best,
-tony

--
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Biomedical and Health Informatics   University of Washington
Biostatistics, SCHARP/HVTN          Fred Hutchinson Cancer Research Center
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