The R language has two common methods of assignment. The = and <- binary
operators are semantically equivalent in most cases, so which to use largely
comes down to convention for the particular project. At times it may be
desirable to convert between the two styles.
Converting all instances of <- to use = assignment is fairly straightforward.
<- only performs assignment and is unique, so one can simply do a regular
expression replacement for all occurrences of <-. The regex uses a negative
lookbehind to
avoid corner cases such as user defined %<-% operator or the <<- operator
and should work in nearly every case.
Converting all instances of = to use <- is more challenging. = can also
occur in the equality operators ==, >=, <= as well as named arguments in
function calls fun(x = 1). It is not possible to use a regular expression to
disambiguate all cases for this transformation.
Fortunately we can use information provided by R’s parser to handle this issue
for us. The
assignment_linter
in the lintr uses this method, so
we can discriminate the assignment = from the argument =. So we can lint
the file using the assignment_linter to find the locations of all =
assignments, and convert them to <- assignments using a little Rcpp.
This gives us a robust method of performing this replacement without error for any number of files. A similar linter and replacement function could be written to replace the regular expression approach for modifying arrow assignment.
