R devel - Aug 2013 - Inconsistency between eval and withVisible (with patch)

R-team,

The $value element of the return value of *withVisible* does not agree with
the return value of *eval* when *withVisible* is passed a variable (symbol)
containing an expression object or anonymous code/expressions which
generates an expression object when evaluated (such as calls to *parse* or *
expression*).

I have attached a patch against the svn trunk which addresses this.

Example (under devel r63577):
> withVisible(parse(text="5+pi"))
$value
*expression(5+pi)*

$visible
[1] TRUE
> eval(parse(text="5+pi"))
*[1] 8.141593*



With the attached patch this is no longer the case:

> withVisible(parse(text="5+pi"))
$value
*[1] 8.141593*

$visible
[1] TRUE

The patch changes only the withVisible function in eval.c. I'm happy to
work with / at the direction of an R-core member to get the patch into an
different form/coding style/fix strategy/etc if its current form is not
acceptable.

Thanks,
~G
> sessionInfo()
R Under development (unstable) (2013-08-14 r63577)
Platform: x86_64-unknown-linux-gnu (64-bit)

locale:
 [1] LC_CTYPE=en_US.UTF-8       LC_NUMERIC=C
 [3] LC_TIME=en_US.UTF-8        LC_COLLATE=en_US.UTF-8
 [5] LC_MONETARY=en_US.UTF-8    LC_MESSAGES=en_US.UTF-8
 [7] LC_PAPER=en_US.UTF-8       LC_NAME=C
 [9] LC_ADDRESS=C               LC_TELEPHONE=C
[11] LC_MEASUREMENT=en_US.UTF-8 LC_IDENTIFICATION=C

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base



-- 
Gabriel Becker
Graduate Student
Statistics Department
University of California, Davis

On Aug 15, 2013, at 01:46 , Gabriel Becker wrote:
> R-team,
> 
> The $value element of the return value of *withVisible* does not agree with
> the return value of *eval* when *withVisible* is passed a variable (symbol)
> containing an expression object or anonymous code/expressions which
> generates an expression object when evaluated (such as calls to *parse* or
*
> expression*).
> 
> I have attached a patch against the svn trunk which addresses this.
> 
> Example (under devel r63577):
> 
>> withVisible(parse(text="5+pi"))
> $value
> *expression(5+pi)*
> 
> $visible
> [1] TRUE
> 
>> eval(parse(text="5+pi"))
> *[1] 8.141593*
> 
I don't think that is a bug, it is by design. The comparison should be to
what happens if you just type the expression at the prompt:
> parse(text="5+pi")
expression(5+pi)

> With the attached patch this is no longer the case:
...so the patch introduces a bug since you can no longer withVisible() something
that returns a language object.
> 
>> withVisible(parse(text="5+pi"))
> $value
> *[1] 8.141593*
> 
> $visible
> [1] TRUE
> 
> The patch changes only the withVisible function in eval.c. I'm happy to
> work with / at the direction of an R-core member to get the patch into an
> different form/coding style/fix strategy/etc if its current form is not
> acceptable.
> 
> Thanks,
> ~G
> 
>> sessionInfo()
> R Under development (unstable) (2013-08-14 r63577)
> Platform: x86_64-unknown-linux-gnu (64-bit)
> 
> locale:
> [1] LC_CTYPE=en_US.UTF-8       LC_NUMERIC=C
> [3] LC_TIME=en_US.UTF-8        LC_COLLATE=en_US.UTF-8
> [5] LC_MONETARY=en_US.UTF-8    LC_MESSAGES=en_US.UTF-8
> [7] LC_PAPER=en_US.UTF-8       LC_NAME=C
> [9] LC_ADDRESS=C               LC_TELEPHONE=C
> [11] LC_MEASUREMENT=en_US.UTF-8 LC_IDENTIFICATION=C
> 
> attached base packages:
> [1] stats     graphics  grDevices utils     datasets  methods   base
> 
> 
> 
> -- 
> Gabriel Becker
> Graduate Student
> Statistics Department
> University of California, Davis
> ______________________________________________
> R-devel at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
-- 
Peter Dalgaard, Professor,
Center for Statistics, Copenhagen Business School
Solbjerg Plads 3, 2000 Frederiksberg, Denmark
Phone: (+45)38153501
Email: pd.mes at cbs.dk  Priv: PDalgd at gmail.com

On Thu, Aug 15, 2013 at 5:59 AM, Simon Urbanek
<simon.urbanek@r-project.org>wrote:
> Note that this is documented very explicitly:
>
> "eval’ evaluates its first argument in the current scope before
passing it
> to the evaluator"
>
But the same man page also describes "eval" like this:

 'eval' evaluates the 'expr' argument in the environment
specified
 by 'envir' and returns the computed value.

So, does "eval" perform one round of evaluation, or two? Does
"evaluates
[some] argument" refer to the ordinary process of obtaining the
argument's
value value, or to the feeding of that value back to the evaluator? Does
this "evaluation" take place in the environment passed as argument, or
the
calling environment?

I'm familiar with what 'eval' actually does, but its description in
the man
page is incoherent.

Peter

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