On 29/10/2014, 6:22 PM, mark.braving...@csiro.au wrote:
> [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.
Sounds like utils::removeSource may need to be fixed, but until today
I'd never heard any complaints about it.
> (This sort of thing reinforces my own feelings about 'srcref' as opposed to
> nice simple 'source'...)
Nice simple 'source' can't do what srcrefs can do, for example report on
locations during debugging, or when run-time errors occur.
Duncan Murdoch
>
> 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-boun...@r-project.org [mailto:r-devel-boun...@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.wick...@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/
>>
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>
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