[Rd] withAutoprint({ .... }) ?

[Duncan Murdoch]

On 02/09/2016 7:56 AM, Martin Maechler wrote:
> On R-help, with subject
>    '[R] source() does not include added code'
>
>>>>>> Joshua Ulrich <josh.m.ulrich at gmail.com>
>>>>>>     on Wed, 31 Aug 2016 10:35:01 -0500 writes:
>
>     > I have quantstrat installed and it works fine for me.  If you're
>     > asking why the output of t(tradeStats('macross')) isn't being printed,
>     > that's because of what's described in the first paragraph in the
>     > *Details* section of help("source"):
>
>     > Note that running code via ‘source’ differs in a few respects from
>     > entering it at the R command line.  Since expressions are not
>     > executed at the top level, auto-printing is not done.  So you will
>     > need to include explicit ‘print’ calls for things you want to be
>     > printed (and remember that this includes plotting by ‘lattice’,
>     > FAQ Q7.22).
>
>
>
>     > So you need:
>
>     > print(t(tradeStats('macross')))
>
>     > if you want the output printed to the console.
>
> indeed, and "of course"" ;-)
>
> As my subject indicates, this is another case, where it would be
> very convenient to have a function
>
>    withAutoprint()
>
> so the OP could have (hopefully) have used
>    withAutoprint(source(..))
> though that would have been equivalent to the already nicely existing
>
>    source(.., print.eval = TRUE)
>
> which works via the  withVisible(.) utility that returns for each
> 'expression' if it would auto print or not, and then does print (or
> not) accordingly.
>
> My own use cases for such a withAutoprint({...})
> are demos and examples, sometimes even package tests which I want to print:
>
> Assume I have a nice demo / example on a help page/ ...
>
> foo(..)
> (z <- bar(..))
> summary(z)
> ....
>
> where I carefully do print parts (and don't others),
> and suddenly I find I want to run that part of the demo /
> example / test only in some circumstances, e.g., only when
> interactive, but not in BATCH, or only if it is me, the package maintainer,
>
> if( identical(Sys.getenv("USER"), "maechler") ) {
>   foo(..)
>   (z <- bar(..))
>   summary(z)
>   ....
> }
>
> Now all the auto-printing is gone, and
>
> 1) I have to find out which of these function calls do autoprint and wrap
>    a print(..) around these, and
>
> 2) the result is quite ugly (for an example on a help page etc.)
>
> What I would like in a future R, is to be able to simply wrap the "{
> .. } above with an 'withAutoprint(.) :
>
> if( identical(Sys.getenv("USER"), "maechler") ) withAutoprint({
>   foo(..)
>   (z <- bar(..))
>   summary(z)
>   ....
> })
>
> Conceptually such a function could be written similar to source() with an R
> level for loop, treating each expression separately, calling eval(.) etc.
> That may cost too much performnace, ... still to have it would be better than
> not having the possibility.
>
> ----
>
> If you read so far, you'd probably agree that such a function
> could be a nice asset in R,
> notably if it was possible to do this on the fast C level of R's main
> REPL.
>
> Have any of you looked into how this could be provided in R ?
> If you know the source a little, you will remember that there's
> the global variable  R_Visible  which is crucial here.
> The problem with that is that it *is* global, and only available
> as that; that the auto-printing "concept" is so linked to "toplevel context"
> and that is not easy, and AFAIK not so much centralized in one place in the
> source. Consequently, all kind of (very) low level functions manipulate R_Visible
> temporarily.... and so a C level implementation of  withAutoprint() may
> need considerable more changes than just setting R_Visible to TRUE in one
> place.
>
> Have any efforts / experiments already happened towards providing such
> functionality ?

I don't think the performance cost would matter.  If you're printing 
something, you're already slow.  So doing this at the R level would make 
most sense to me --- that's how Sweave and source and knitr do it, so it 
can't be that bad.

Duncan Murdoch