scoping problem?

Peter Dalgaard BSA <p.dalgaard at biostat.ku.dk> writes:

In general, you need to know that scoping and evaluation rules can be
broken in the R system. It is possible for a function to get a hold of
the expression that is passed for an argument and evaluate it in an
environment of its own choice, or even use the symbolic expression as
such (that is how you get away with saying "library(tree)" without
getting a "there is no object called `tree'" type error.)

tree() is doing some such trickery at the start of the function.
Basically, it gets hold of the entire call, and then replaces the
function being called with model.frame.default and reevaluates in the
parent frame. Or rather, it tries to do that. The code is using
eval(model, sys.parent()) and somehow that fails. 

Replacing sys.parent() with parent.frame() inside tree does make it
work, which points to a possible bug in eval() when used with an
integer envir= argument. Will have a look.

Hmm. Digging a little deeper got me to the following code in do_eval
(eval.c) 

    case INTSXP:
    case REALSXP:
        nback = asInteger(env);
        if (nback==NA_INTEGER)
            errorcall(call,"invalid environment");
        if (nback > 0 )
            nback -= framedepth(R_GlobalContext);
        nback = -nback;
        PROTECT(env = R_sysframe(nback,R_GlobalContext));
        break;

This indicates that the interpretation of an integer argument to eval
is the number of frames to go back if it is *positive*. This is the
opposite of what sys.call/sys.frame does, and besides, R_sysframe does
the positive/negative conversion itself. Apparently, this code has
been sitting there "forever"... 

I think it ought to be just

    case INTSXP:
    case REALSXP:
        nback = asInteger(env);
        if (nback==NA_INTEGER)
            errorcall(call,"invalid environment");
        PROTECT(env = R_sysframe(nback,R_GlobalContext));
        break;

(possibly with a change of variable name for "nback").

O__  ---- Peter Dalgaard             Blegdamsvej 3  
  c/ /'_ --- Dept. of Biostatistics     2200 Cph. N   
 (*) \(*) -- University of Copenhagen   Denmark      Ph: (+45) 35327918
~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk)             FAX: (+45) 35327907
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