[Rd] RFC: adding an 'exact' argument to [[

Seth Falcon sfalcon at fhcrc.org
Thu May 17 18:10:33 CEST 2007

Hi all,

One of the things I find most problematic in R is the partial matching
of names in lists.  Robert and I have discussed this and we believe
that having a mechanism that does not do partial matching would be of
significant benefit to R programmers.  To that end, I have written a
patch that modifies the behavior of "[[" as follows:

   1. [[ gains an 'exact' argument with default value NA

   2. Behavior of 'exact' argument:

          partial matching is performed as usual, however, a warning
          will be issued when a partial match occurs.  This is the

          no partial matching is performed.

          partial matching is allowed and no warning issued if it

This change has been discussed among R-core members and there appeared
to be a general consensus that this approach was a good way to
proceed.  However, we are interested in other suggestions from the
broader R developer community.

Some additional rationale for our approach:

Lists are used as the underlying data structures in many R programs
and in these cases the named elements are not a fixed set of things
with a fixed set of names.  For these programs, [[ will be used with
an argument that gets evaluated at runtime and partial matching here
is almost always a disaster.  Furthermore, dealing with data that has
common prefixes happens often and is not an exceptional circumstance
(a precondition for partial matching issues).

We have tested a similar patch that simply eliminated partial matching
for [[ on all CRAN and Bioconductor packages and did not see any
obvious failures.

A downside of this approach is that S4 methods on [[ will need to be
modified to accommodate the new signature.  However, by adding an
argument, we are able to move more slowly towards a non-partially
matching [[ (eventually, the default could be exact=TRUE, but that is
a discussion for another day).

+ seth

Seth Falcon | Computational Biology | Fred Hutchinson Cancer Research Center

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