[Rd] Source references from the parser

Duncan Murdoch murdoch at stats.uwo.ca
Sat Nov 25 22:37:16 CET 2006

On 11/25/2006 3:12 PM, Deepayan Sarkar wrote:
> On 11/25/06, Duncan Murdoch <murdoch at stats.uwo.ca> wrote:
>> I have just committed some changes to R-devel (which will become R 2.5.0
>> next spring) to add source references to parsed R code.  Here's a
>> description of the scheme:
>> The design is done through 2 old-style classes.
>> "srcfile" corresponds to a source file: it contains a filename, the
>> working directory in which that filename is to be interpreted, the last
>> modified timestamp of the file at the time the object is created, plus
>> some internal components.  It is implemented as an environment so that
>> there can be multiple references to it.
>> "srcref" is a reference to a particular range of characters (as the
>> parser sees them; I think that really means bytes, but I haven't tested
>> with MBCSs) in a source file.  It is implemented as a vector of 4
>> integers (first line, first column, last line, last column), with the
>> srcfile as an attribute.
>> The parser attaches a srcref attribute to each complete statement as it
>> gets parsed, if option("useSource") is TRUE.  (I've left the old source
>> attribute in place as well for functions; I think it won't be needed in
>> the long run, but it is needed now.)
>> When printing an object with a srcref attribute, print.default tries to
>> read the srcfile to obtain the text.  If it fails, it falls back to an
>> ugly display of the reference.  Using a new argument useSource=FALSE in
>> printing will stop this attempt:  when printing language, it will
>> deparse; when printing a srcref, it will print the ugly fallback.
>> source(echo=T) will echo all the lines of the file including comments
>> and formatting.  demo() does the same, and I would guess Sweave will do
>> this too, but I haven't tested that yet.  I think this will improve
>> Sweave output, but will need changes to the input file:  people may have
>> comments there that they don't want shown.  Some sort of
>> "useSource=FALSE" option will need to be added.
>> The browser used with debug() etc. will display statements as they were
>> formatted in the original source.  It will not display leading or
>> following comments, but will display embedded comments.
>> Parsing errors display the name of the source file that was parsed, and
>> display verbose error messages describing what's wrong.  This display
>> could still be improved, e.g. by displaying the whole source line with a
>> pointer to the error, instead of just the text up to the location of the
>> error.
>> I plan to add some sort of equivalent of C "#line" directives, so that
>> preprocessed source files (e.g. the concatenated source that is
>> installed) can include references back to the original source files, for
>> syntax error reporting, and/or debugging.  This will require
>> modification of the INSTALL process, but I haven't started on this yet.
>> It would probably be a good idea to have some utility functions to play
>> with the srcref records for debugging and other purposes, but I haven't
>> written those yet.  For example, the current source record on a function
>> could be replaced with a srcref, but only by expanding the srcref to
>> include some of the surrounding comments.
>> Comments and problem reports are welcome.
> I haven't tested this, but the idea seems useful. Will this have any
> effect on code parsed using parse(text = "...")? Can it be extended to
> have some such effect? I ask because this is relevant in the context
> of Sweave, where I have always wanted the ability to retain the
> original formatting. I'm currently testing a patch that allows me to
> do this specifically for Sweave, but a more general solution is
> obviously preferable.

It won't currently, but that's on the todo list.

When it will arrive depends on how many other things land on my desk in 
the next few weeks:  if I don't get it done before January, it probably 
won't make 2.5.0.

Duncan Murdoch

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