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*

parse(text="5+pi")

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 Wed, Aug 14, 2013 at 11:14 PM, Peter Meilstrup <peter.meilstrup at gmail.com

wrote:

I agree that the present behavior of withVisible is the Right Thing, but
the documentation is confusing. The documentation claims that withVisible
"evaluates an expression."

This may capture an inside view of how the .Internal function is
implemented, but is nonsense from the R user's standpoint, where
"expression" is a particular type of R object and "evaluate an expression"
means to do something like eval().

And particularly, if you pass withVisible a variable containing an
expression, said expression is not evaluated. The expression/symbol
pointing to the object where it is stored is, I know, but the expression
itself is not.

Furthermore, the exact same language ("evaluate an expression") appears in
the eval documentation, and that function DOES evaluate the actual
expression. Thus my (apparently incorrect) assumption that they should have
the same behavior.

~G

Peter


On Wed, Aug 14, 2013 at 11:04 PM, peter dalgaard <pdalgd at gmail.com> wrote:

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

--
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

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

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