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

From: Seth Falcon <sfalcon_at_fhcrc.org>
Date: Thu, 17 May 2007 09:10:33 -0700

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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