[Rd] Unexpected behaviour when comparing (==) long quoted expressions

[Daniel Chen]

Hi Martin:

Yes, I totally made things worse (and blundered my first listserv post) 
when things got converted from markdown...

For posterity (and clarity), I've reproduced the examples that show the 
unexpected behaviour and the current workaround we've used.

Example 1: expected u == s to return FALSE, but it return TRUE instead.

u <- quote(tidyr::gather(key = key, value = value, 
new_sp_m014:newrel_f65, na.rm = TRUE))
s <- quote(tidyr::gather(key = key, value = value, 
new_sp_m014:newrel_f65, na.rm = FALSE))
u == s # TRUE

u <- quote(tidyr::gather(key = key, value = value, na.rm = TRUE))
s <- quote(tidyr::gather(key = key, value = value, na.rm = FALSE))
u == s # FALSE

Example 2: seems more to do with length of the quoted expression than 
function arguments.

u <- 
quote(f(x123456789012345678901234567890123456789012345678901234567890, 1))
s <- 
quote(f(x123456789012345678901234567890123456789012345678901234567890, 2))
u == s
#> [1] TRUE

Workaround:

You can get around this issue by using all.equal or identical

u <- quote(tidyr::gather(key = key, value = value, 
new_sp_m014:newrel_f65, na.rm = TRUE))
s <- quote(tidyr::gather(key = key, value = value, 
new_sp_m014:newrel_f65, na.rm = FALSE))
u == s # TRUE
all.equal(u, s) # "target, current do not match when deparsed"
identical(u, s) # FALSE


The snippet for the implementation shows only the first element from the 
resulting deparse character vector is used 
(https://github.com/wch/r-source/blob/e647f78cb85282263f88ea30c6337b77a30743d9/src/main/relop.c#L140-L155):


     SET_STRING_ELT(tmp, 0, (iS) ? PRINTNAME(x) :
                STRING_ELT(deparse1(x, 0, DEFAULTDEPARSE), 0));


- Dan



On 7/16/19 1:35 AM, Martin Maechler wrote:
>>>>>> Daniel Chen
>>>>>>      on Fri, 12 Jul 2019 13:53:21 -0500 writes:
>      > Hi everyone:
>      > I’m one of the interns at RStudio this summer working on a project that
>      > helps teachers grade student code. I found an unexpected behaviour with
>      > the |==| operator when comparing |quote|d expressions.
>
>      > Example 1:
>
>      > |u <- quote(tidyr::gather(key = key, value = value,
>      > new_sp_m014:newrel_f65, na.rm = TRUE)) s <- quote(tidyr::gather(key =
>      > key, value = value, new_sp_m014:newrel_f65, na.rm = FALSE)) u == s #
>      > TRUE u <- quote(tidyr::gather(key = key, value = value, na.rm = TRUE)) s
>      > <- quote(tidyr::gather(key = key, value = value, na.rm = FALSE)) u == s
>      > # FALSE |
>
> Unfortunately the above is almost unreadable, as you "forgot" to
> click (in the lower right corner of your Gmail interface with
> the three vertical dots) "plain text mode".
>
>      > Example 2:
>
>      > |u <-
>      > quote(f(x123456789012345678901234567890123456789012345678901234567890,
>      > 1)) s <-
>      > quote(f(x123456789012345678901234567890123456789012345678901234567890,
>      > 2)) u == s #> [1] TRUE |
>
> this is even readable after html - de-html mangling
>
>      > Winston Chang pointed out in the help page for |==|:
>
>      > Language objects such as symbols and calls are deparsed to character
>      > strings before comparison.
>
>      > and in the source code that does the comparison [1] shows that It
>      > deparses each language object and then only extracts the first element
>      > from the resulting character vector:
>
>      > |SET_STRING_ELT(tmp, 0, (iS) ? PRINTNAME(x) : STRING_ELT(deparse1(x, 0,
>      > DEFAULTDEPARSE), 0)); |
>
>      > Is this a fix that needs to happen within the |==| documentation? or an
>      > actual bug with the operator?
>
> This a good question.
>
> Thank you, Daniel, for providing the link to the source code in
> <R>/src/main/relop.c .
>
> Looking at that and its context, I think we (R core) should
> reconsider that implementation of '=='  which indeed does about
> the same thing as deparse {which also truncates at some point by
> default; something very very reasonable for error messages, but
> undesirable in other cases}.
>
> But I think it's fair expectation that comparing  calls  ["language"]
> with '==' should compare the full call's syntax even if that may
> occasionally be very long.
>
> Martin
>
>      > For more context the original issue we had is here:
>      > https://github.com/rstudio-education/grader/issues/28
>
>      > Workaround:
>
>      > You can get around this issue by using |all.equal| or |identical|
>
>      > |u <- quote(tidyr::gather(key = key, value = value,
>      > new_sp_m014:newrel_f65, na.rm = TRUE)) s <- quote(tidyr::gather(key =
>      > key, value = value, new_sp_m014:newrel_f65, na.rm = FALSE)) u == s #
>      > TRUE all.equal(u, s) # "target, current do not match when deparsed"
>      > identical(u, s) # FALSE |
>
>      > Thanks,
>
>      > Dan
>
>      > [1] https://github.com/wch/r-source/blob/e647f78cb85282263f88ea30c6337b77a30743d9/src/main/relop.c#L140-L155