[Rd] substitute creates an object which prints incorrectly (PR#9427)

[Peter Dalgaard]

Bill.Venables at csiro.au wrote:
> The function "substitute" seems to fail to make a genuine
> substitution, although the printed verision seems fine.  Here is an
> example.
>
>   
>> m <- substitute(Y <- function(x) FUN(x+1), 
>>     
> + list(Y = as.name("y"), FUN = as.name("sin")))
>   
>> m
>>     
> y <- function(x) sin(x + 1)
>   
>> eval(m)
>> y
>>     
> function(x) FUN(x+1)
>
> However the story doesn't end there.  The substitution appears to have
> been made, even though the printed version, this time, suggests
> otherwise.
>
>   
>> y(pi)
>>     
> [1] -0.841471
>   
>> sin(pi+1)
>>     
> [1] -0.841471
>   
>
>   
Yes, this is (fairly) well known. It has to do with the retention of 
function source.

The thing to notice is that it is only the printing of y that is really 
confused. If you do

dput(y)
attr(y, "source")
attr(y, "source") <- NULL
y

then you should see the point. The tricky bit is that the "source" 
attribute exists in an intermediate form inside m. Notice that m 
contains, not the function itself, but a call to the function `function` 
which creates the function when eval'ed. This call contains the function 
source as its 4th element (look at m[[3]][[4]] in your example), and you 
might try setting it to NULL and see how things will clear up.

The issue with substitute is that it cannot sensibly substitute into 
character vectors, so it just leaves  the source as is, which gives the 
symptoms you see. It could, however, and probably should, recognize 
calls to `function` and NULL out their 4th element. It cannot be done 
completely failsafe though (`function` could result from a computation, 
or even be part of the substitution), so one has to decide that the 
extreme cases are too extreme worry about them.
 
-pd

> Bill Venables 
> CMIS, CSIRO Laboratories, 
> PO Box 120, Cleveland, Qld. 4163 
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