R devel - Dec 2013 - Proper way to drop 'srcref' from an expression created via substitute(function() ...)?

First, why does this expression have a 'srcref' element:
> exprA <- substitute(function(x) a*x, list(a=2))
> print(exprA)
function(x) 2 * x
> str(as.list(exprA))
List of 4
 $ : symbol function
 $ :Dotted pair list of 1
  ..$ x: symbol
 $ : language 2 * x
 $ :Class 'srcref'  atomic [1:8] 1 20 1 34 20 34 1 1
  .. ..- attr(*, "srcfile")=Classes 'srcfilecopy',
'srcfile'
<environment: 0x00000000111feaf8>

whereas this does not:
> exprB <- substitute(a*x, list(a=2))
> print(exprB)
2 * x
> str(as.list(exprB))
List of 3
 $ : symbol *
 $ : num 2
 $ : symbol x


Second, what is the proper way to drop that 'srcref' element in
'exprA'?  I can think of either

exprC <- exprA
exprC[[4L]] <- NULL

or

exprC <- parse(text=deparse(exprA))

Anything better/safer?


BACKGROUND:
The reason for this is that I wish to create a function dynamically
via variable substitution such that when printed, the function
displays the substituted values, e.g.
> fcnA <- eval(exprA)
> print(fcnA)
function(x) a*x

versus
> fcnC <- eval(exprC)
> print(fcnC)
function(x) 2 * x

Thanks,

Henrik

On 13-12-12 7:57 PM, Henrik Bengtsson wrote:
> First, why does this expression have a 'srcref' element:
>
>> exprA <- substitute(function(x) a*x, list(a=2))
>> print(exprA)
> function(x) 2 * x
>> str(as.list(exprA))
> List of 4
>   $ : symbol function
>   $ :Dotted pair list of 1
>    ..$ x: symbol
>   $ : language 2 * x
>   $ :Class 'srcref'  atomic [1:8] 1 20 1 34 20 34 1 1
>    .. ..- attr(*, "srcfile")=Classes 'srcfilecopy',
'srcfile'
> <environment: 0x00000000111feaf8>
>
> whereas this does not:
>
>> exprB <- substitute(a*x, list(a=2))
>> print(exprB)
> 2 * x
>> str(as.list(exprB))
> List of 3
>   $ : symbol *
>   $ : num 2
>   $ : symbol x
>
Function definitions get srcrefs.
>
> Second, what is the proper way to drop that 'srcref' element in
> 'exprA'?  I can think of either
>
> exprC <- exprA
> exprC[[4L]] <- NULL
That should be best.

Duncan Murdoch
>
> or
>
> exprC <- parse(text=deparse(exprA))
>
> Anything better/safer?
>
>
> BACKGROUND:
> The reason for this is that I wish to create a function dynamically
> via variable substitution such that when printed, the function
> displays the substituted values, e.g.
>
>> fcnA <- eval(exprA)
>> print(fcnA)
> function(x) a*x
>
> versus
>
>> fcnC <- eval(exprC)
>> print(fcnC)
> function(x) 2 * x
>
> Thanks,
>
> Henrik
>
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