[R] string syntactic sugar in R? - long post

Gabor Grothendieck ggrothendieck at gmail.com
Sat May 7 14:22:40 CEST 2005

On 5/7/05, charles loboz <charles_loboz at yahoo.com> wrote:
> Currently in R, constructing a string containing
> values of variables is done using 'paste' and can be
> an error-prone and traumatic experience. For example,
> when constructing a db query we have to write,
>          paste("SELECT " value " FROM table  where
> date ='",cdate,"'")
> we are getting null result from it, because without
> (forgotten...) sep=""  we get
>         SELECT value FROM table where date='
> 2005-05-05 '
> instead of
>        SELECT value FROM table where date='2005-05-05'
> Adding sep="" as a habit results in other errors, like
> column names joined with keywords - because of
> forgotten spaces. Not to mention mixing up or
> unbalancing quote marks etc. The approach used by
> paste is similar to that of many other languages (like
> early Java, VB etc) and is inherently error-prone
> because of poor visualization. There is a way to
> improve it.
> In the Java world gstrings were introduced
> specifically for this purpose. A gstring is a string
> with variable names embedded and replaced by values
> (converted to strings, lazy eval) before use. An
> example in R-syntax would be:
> >alpha <- 8; beta="xyz"
> >gstr <- "the result is ${alpha} with the comment
> ${beta}"
> >cat(gstr)
>      the result is 8 with the comment xyz
> This syntactic sugar reduces significantly the number
> of mistakes made with normal string concatenations.
> Gstrings are used in ant and groovy - (for details see
> http://groovy.codehaus.org/Strings, jump to GStrings).
> They are particularly useful for creating readable and
> error-free SQL statements, but obviously the simplify
> 'normal' string+value handling in all situations. [ps:
> gstrings are not nestable]
> I was wondering how difficult it would be to add such
> syntactic sugar to R and would that create some
> language problems? May be it is possible that it could
> be done as some gpaste function, parsing the argument
> for ${var}, extracting variables from the environment,
> evaluating them and producing the final string?
> I admit my bias - using ant for years and groovy for
> months and having to do a lot of SQL queries does not
> put me in the mainstream of R users - so it may be
> that this idea is not usable to a wider group of
> users.

Here is one attempt.  It eliminates the necessity to quote the
elements altogether but in exchange requires that the
argument be a valid R expression.  It is based on the 
R bquote function.  

gpaste <- function(expr, where = parent.frame()) {
  dequote <- function(e) as.name(noquote(as.character(e)))
  unquote <- function(e) {
      if (length(e) <= 1) 
      else if (e[[1]] == as.name(".")) 
          dequote(eval(e[[2]], where))
      else as.call(lapply(e, unquote))
  rval <- paste(unquote(substitute(expr)), collapse = " ")
  rval <- gsub("+ ", "", rval, fix = TRUE)
  gsub("`", "", rval)

# test
var <- "myvar"
gpaste( select + .(var) + from + table + where + 
	date +" =" + .(sQuote(Sys.Date())) )

When you run it you get this:

> gpaste( select + .(var) + from + table + where + 
+                   date +" =" + .(sQuote(Sys.Date())) )
[1] "select myvar from table where date  = '2005-05-07'"

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