R help - Aug 2010 - Modifying glm.fit() / execution path

Hello all,
I'm sure I'm missing something simple here, but I can't figure out
how to
modify the glm.fit() function and then get R to use it (sort of). I'm doing
something along the lines of:

glm.fit<-edit(glm.fit) # add something trivial to the top of the glm.fit
function like: print("Hello world!")

#now have a modified glm.fit in position 1/.GlobalEnv

x<-rnorm(20)
y<-rnomr(20)

glm(y~x)  # I don't get the trivial bit of glm.fit back (i.e. "Hello
world!"
doesn't print)

#but

glm(y~x,method=glm.fit) # works! I get "Hello world!"

My understanding is that glm() should call glm.fit() by default but it seems
to be calling the glm.fit() in "package:stats" and not
".GlobalEnv". Why
isn't the function in ".GlobalEnv" superseding the version in
"package:stats"?

Thanks,
Ben


version:

platform       i386-pc-mingw32
arch           i386
os             mingw32
system         i386, mingw32
status
major          2
minor          11.1
year           2010
month          05
day            31
svn rev        52157
language       R
version.string R version 2.11.1 (2010-05-31)

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On Sun, 1 Aug 2010, Benjamin Ridenhour wrote:
> Hello all,
> I'm sure I'm missing something simple here, but I can't figure
out how to
> modify the glm.fit() function and then get R to use it (sort of). I'm
doing
> something along the lines of:
>
> glm.fit<-edit(glm.fit) # add something trivial to the top of the glm.fit
> function like: print("Hello world!")
>
> #now have a modified glm.fit in position 1/.GlobalEnv
>
> x<-rnorm(20)
> y<-rnomr(20)
>
> glm(y~x)  # I don't get the trivial bit of glm.fit back (i.e.
"Hello world!"
> doesn't print)
>
> #but
>
> glm(y~x,method=glm.fit) # works! I get "Hello world!"
>
> My understanding is that glm() should call glm.fit() by default but it
seems
> to be calling the glm.fit() in "package:stats" and not
".GlobalEnv". Why
> isn't the function in ".GlobalEnv" superseding the version in
> "package:stats"?
Because that is the way scoping works in R -- you need to understad 
the concept of name spaces.

This is not the way to do what you seem to want: rename your fitter 
function and use

   method: the method to be used in fitting the model.  The default
           method ?"glm.fit"? uses iteratively reweighted least
squares
           (IWLS), whereas ?"model.frame"? which returns the model
frame
           and does no fitting.  User-supplied fitting functions can be
           supplied either as a function or a character string naming a
           function, with a function which takes the same arguments as
           ?glm.fit?.

to specify it.
>
> Thanks,
> Ben
-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595