Bug in comparison of language objects?

I noticed the following odd behaviour today:

    exprs <- expression( mean(a), mean(b), { a }, { b } )

    exprs[[1]] == exprs[[2]]
    #> [1] FALSE

    exprs[[3]] == exprs[[4]]
    #> [1] TRUE

Does it make sense to anyone that the argument passed to `mean` matters,
but the expression contained in braces doesn't?

I have done some debugging, and found the cause:  for the comparison of 
language objects, R deparses them to strings using C function 
deparse1(), and looks at only the first line.  "mean(a)" deparses as is, 
but "{ a }" deparses to 3 lines

{
   a
}

and the first line is the same as for "{ b }", so they compare equal.

I think it would make more sense to deparse them to one long string, and 
compare those, i.e. to replace deparse1() with deparse1line() (which may 
have been the intention).

Duncan Murdoch

Duncan Murdoch 
    on Tue, 20 Feb 2024 08:47:30 -0500 writes:

>> I noticed the following odd behaviour today:
    >> 
    >> exprs <- expression( mean(a), mean(b), { a }, { b } )
    >> 
    >> exprs[[1]] == exprs[[2]] #> [1] FALSE
    >> 
    >> exprs[[3]] == exprs[[4]] #> [1] TRUE
    >> 
    >> Does it make sense to anyone that the argument passed to
    >> `mean` matters, but the expression contained in braces
    >> doesn't?

    > I have done some debugging, and found the cause: for the
    > comparison of language objects, R deparses them to strings
    > using C function deparse1(), and looks at only the first
    > line.  "mean(a)" deparses as is, but "{ a }" deparses to 3
    > lines

    > { a }

    > and the first line is the same as for "{ b }", so they
    > compare equal.

    > I think it would make more sense to deparse them to one
    > long string, and compare those, i.e. to replace deparse1()
    > with deparse1line() (which may have been the intention).

    > Duncan Murdoch

I agree ... (and more do).
Thank you for  adding it as formal report to R's bugzilla,
https://bugs.r-project.org/show_bug.cgi?id=18676

Unfortunately, it triggers something in the (byte) compiler test
suite, and (also/hence) will probably be too late for R 4.3.3.

Martin

Martin Maechler