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

[Tierney, Luke]

On Tue, 16 Jul 2019, 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.

Before going there I think we should reconsider whether allowing ==
comparisons on calls is a good idea. We already don't allow it for
expresison() objects. It is probably unavoidable to allow symbols
(there are probably lots of things that would break if quote(x) == "x"
did not work and return TRUE), but for calls it makes little sense to
have "f(x)" == quote(f(x)). These are very different objects that
happen to have identical string representations. For computing on the
language identical() is the right way to go (that is what is used in
the byte code compiler and codetools).

Best,

luke

>
> 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
>
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>

-- 
Luke Tierney
Ralph E. Wareham Professor of Mathematical Sciences
University of Iowa                  Phone:             319-335-3386
Department of Statistics and        Fax:               319-335-3017
    Actuarial Science
241 Schaeffer Hall                  email:   luke-tierney using uiowa.edu
Iowa City, IA 52242                 WWW:  http://www.stat.uiowa.edu