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

Robert Gentleman rgentlem at fhcrc.org
Sat May 7 11:00:23 CEST 2005

  In Bioconductor, we have something called copySubstitute, which does  
what you want, I believe,

  x="select @var1@ from @tab1@"
  copySubstitute(textConnection(x), symbolValues= list(var1="Race",  
tab1="ReallyBigTable"), dest=stdout())

select Race from ReallyBigTable

you can read in from any connection and write out to any connection,  
change the delimiter, etc.
We use it to autogenerate manual pages and other documentation for  
packages that have lots of similar structure, as well as for things  
like what you want to do.

Best wishes,

On May 7, 2005, at 1:36 AM, charles loboz 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.
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| Robert Gentleman              phone: (206) 667-7700                    
| Head, Program in Computational Biology   fax:  (206) 667-1319   |
| Division of Public Health Sciences       office: M2-B865               
| Fred Hutchinson Cancer Research Center                                 
| email: rgentlem at fhcrc.org                                              

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