Unexpected behavior of identical() with language objects

[See below for full email trail-- Outlook has beaten me into submission]

I ran into this and found the result very surprising:

identical( quote({ a }),  quote({ a }) ) # FALSE
<<...>>
-Winston

Yes, looks like srcrefs are to blame:

x <- quote({ a })
y <- quote({ a })

identical(x, y)
# [1] FALSE

<<...>>

Maybe identical() needs an ignore.srcref option? Normally when
comparing expressions or functions, you want to compare the code, not
it's textual representation.

Hadley

What a great gotcha!

Seems to me it would be better to leave 'identical' alone and have a wrapper that you can call to strip any srcrefs. There is already 'utils::removeSource' but it doesn't work as-is with non-functions. The 5-minute hack below solves the particular example, but may well fall over with other cases--- not tested.

(This sort of thing reinforces my own feelings about 'srcref' as opposed to nice simple 'source'...)

Mark Bravington
CSIRO/Marine Lab/Hobart/Tas 7000/Australia

rmsrc <- function (fn) {
# based on utils::removeSource
    is.fun <- is.function( fn)
    if( (!is.fun && !is.language(fn)) || is.primitive(fn)) {
return(fn)
    }
    
    attr(fn, "source") <- NULL
    attr(fn, "srcref") <- NULL

    if( !is.fun) {
      fn <- as.function( list( fn))
    }

    attr(body(fn), "wholeSrcref") <- NULL
    attr(body(fn), "srcfile") <- NULL
    recurse <- function(part) {
        attr(part, "srcref") <- NULL
        if (is.language(part) && is.recursive(part)) {
            for (i in seq_along(part)) part[[i]] <- recurse(part[[i]])
        }
        part
    }
    body(fn) <- recurse(body(fn))
    
    if( !is.fun) {
return( body( fn))
    } else {
return( fn)
    }
}
#> identical( rmsrc( quote({a})), rmsrc( quote({a})))
#[1] TRUE 

-----Original Message-----
From: r-devel-bounces at r-project.org [mailto:r-devel-bounces at r-project.org]
On Behalf Of Winston Chang
Sent: Thursday, 30 October 2014 7:59 AM
To: Hadley Wickham
Cc: R Devel List
Subject: Re: [Rd] Unexpected behavior of identical() with language objects

Ah, I was using identical() to compare two function bodies. It returns
FALSE even when you remove srcrefs from the body:

f1 <- function(x) {
  if (TRUE) { x }
}
f2 <- function(x) {
  if (TRUE) { x }
}
f1b <- body(f1)
f2b <- body(f2)
attributes(f1b) <- NULL
attributes(f2b) <- NULL

# The bodies look the same with str()
str(f1b)
#  language {  if (TRUE) {; x; } }
str(f2b)
#  language {  if (TRUE) {; x; } }

identical(f1b, f2b)
# FALSE



What I didn't realize was that the curly brace inside the body also
independently captures srcrefs, but this isn't printed with str(f1b).
However, str() on a more targeted part of the object reveals them:
str(f1b[[2]][[3]])
# length 2 {  x }
#  - attr(*, "srcref")=List of 2
#   ..$ :Class 'srcref'  atomic [1:8] 2 13 2 13 13 13 2 2
#   .. .. ..- attr(*, "srcfile")=Classes 'srcfilecopy', 'srcfile'
<environment: 0x452b2c0>
#   ..$ :Class 'srcref'  atomic [1:8] 2 15 2 15 15 15 2 2
#   .. .. ..- attr(*, "srcfile")=Classes 'srcfilecopy', 'srcfile'
<environment: 0x452b2c0>
#  - attr(*, "srcfile")=Classes 'srcfilecopy', 'srcfile' <environment:
0x452b2c0>
#  - attr(*, "wholeSrcref")=Class 'srcref'  atomic [1:8] 1 0 2 17 0 17 1 2
#   .. ..- attr(*, "srcfile")=Classes 'srcfilecopy', 'srcfile'
<environment: 0x452b2c0>

-Winston

On Wed, Oct 29, 2014 at 3:46 PM, Hadley Wickham <h.wickham at gmail.com>
wrote:

Is this expected behavior? I can't seem to find anything in the help
for identical that relates to this.

It's not in ?identical, but ?Paren gives you some pointers.
str(quote((a))) and str(quote({a})) are also informative.

Yes, looks like srcrefs are to blame:

x <- quote({ a })
y <- quote({ a })

identical(x, y)
# [1] FALSE

attr(x, "srcref") <- NULL
attr(x, "srcfile") <- NULL
attr(x, "wholeSrcref") <- NULL

attr(y, "srcref") <- NULL
attr(y, "srcfile") <- NULL
attr(y, "wholeSrcref") <- NULL
identical(x, y)
# [1] TRUE

Maybe identical() needs an ignore.srcref option? Normally when
comparing expressions or functions, you want to compare the code, not
it's textual representation.

Hadley

--
http://had.co.nz/