R devel - Jan 2017 - problem with print.generic(x)deparse(substitute(x))

Hi, All:


       I'm having trouble getting deparse(substitute(x)) inside 
print.generic to consistently


       I'm having trouble getting a print.something to work 
consistently.  Consider the following toy example:


# Define an object of class 'dum'
k <- 1
class(k) <- 'dum'
str(k) # as expected

# Define print.dum
print.dum <- function(x, ...)
   deparse(substitute(x))

print(k) # Prints "k" as expected
#####**** THE FOLLOWING PRINTS NOTHING:
k # Why?


### Thanks,
### Spencer Graves

sessionInfo()

R version 3.3.2 (2016-10-31)
Platform: x86_64-apple-darwin13.4.0 (64-bit)
Running under: macOS Sierra 10.12.2

locale:
[1] en_US.UTF-8/en_US.UTF-8/en_US.UTF-8/C/en_US.UTF-8/en_US.UTF-8

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

loaded via a namespace (and not attached):
[1] tools_3.3.2

On 09 Jan 2017, at 10:53 , Spencer Graves <spencer.graves at prodsyse.com>
wrote:
> # Define an object of class 'dum'
> k <- 1
> class(k) <- 'dum'
> str(k) # as expected
> 
> # Define print.dum
> print.dum <- function(x, ...)
>  deparse(substitute(x))
> 
> print(k) # Prints "k" as expected
> #####**** THE FOLLOWING PRINTS NOTHING:
> k # Why?

Because it doesn't work that way...

First of all, your print.dum relies on autoprinting of its return value, it
doesn't print anything itself. That's not how one usually writes print
methods: You should print something and (usually) return the argument invisibly.

Autoprinting calls the print method to do the actual printing and returns the
object invisibly, irrespective of the return value from the print method. To
wit:
> k
> dput(.Last.value)
structure(1, class = "dum")

(Trying to print the return value would invite infinite looping.)

However, there's another catch: deparse(substitute(...)) relies on knowing
the argument to print() before evaluation, but autoprinting does not retain that
information, it just looks at the object that has been computed and passes it to
the relevant print method, so you get this effect:
> print.dum <- function(x, ...)
+  print(deparse(substitute(x)))
> k
[1] "x"

-pd



-- 
Peter Dalgaard, Professor,
Center for Statistics, Copenhagen Business School
Solbjerg Plads 3, 2000 Frederiksberg, Denmark
Phone: (+45)38153501
Office: A 4.23
Email: pd.mes at cbs.dk  Priv: PDalgd at gmail.com

Hi, Peter et al.:


On 2017-01-09 4:24 AM, peter dalgaard wrote:
> On 09 Jan 2017, at 10:53 , Spencer Graves <spencer.graves at
prodsyse.com> wrote:
>
>> # Define an object of class 'dum'
>> k <- 1
>> class(k) <- 'dum'
>> str(k) # as expected
>>
>> # Define print.dum
>> print.dum <- function(x, ...)
>>   deparse(substitute(x))
>>
>> print(k) # Prints "k" as expected
>> #####**** THE FOLLOWING PRINTS NOTHING:
>> k # Why?
>
> Because it doesn't work that way...
>
> First of all, your print.dum relies on autoprinting of its return value, it
doesn't print anything itself. That's not how one usually writes print
methods: You should print something and (usually) return the argument invisibly.
>
> Autoprinting calls the print method to do the actual printing and returns
the object invisibly, irrespective of the return value from the print method. To
wit:
>
>> k
>> dput(.Last.value)
> structure(1, class = "dum")
>
> (Trying to print the return value would invite infinite looping.)
>
> However, there's another catch: deparse(substitute(...)) relies on
knowing the argument to print() before evaluation, but autoprinting does not
retain that information, it just looks at the object that has been computed and
passes it to the relevant print method, so you get this effect:
>
>> print.dum <- function(x, ...)
> +  print(deparse(substitute(x)))
>> k
> [1] "x"
       Thanks.  Permit me to ask my real question:  Is there a way to 
modify print.findFn{sos} so I can print "k" in the second line of the 
HTML from the following:


(k <- findFn('spline', 1))


       Currently, the HTML starts as follows:


findFunction Results
call: findFn(string = "spline", maxPages = 1)


       I want it to start as follows:


findFn Results

call: (k <- findFn(string = "spline", maxPages = 1))
For a package summary:  installPackages(k); writeFindFn2xls(k)


       When I added "ds <- deparse(substitute(x))" to
"print.findFn", I
got ds = "x", not "k" from "(k <-
findFn('spline', 1))" and from "k" by
itself (autoprint) but "x" from "print(k)".  Thanks to Peter
for
explaining this problem.  Is there a work around?


       Spencer
> -pd
>
>
>