R devel - Jul 2014 - Question on Code snippet semantics

I came across this code in library.R

package <- as.character(substitute(package))

where package is the first argument to the "library" function.

I've been racking my brains to understand why this is not just an 
elaborate (and ineffcient) way to write:

package <- "package"

E.g.

 > package <- as.character(substitute(package))
 > package
[1] "package"
 >

Thanks
Mick Jordan

On Jul 21, 2014, at 10:07 AM, Mick Jordan <mick.jordan at oracle.com>
wrote:
> I came across this code in library.R
> 
> package <- as.character(substitute(package))
> 
> where package is the first argument to the "library" function.
> 
> I've been racking my brains to understand why this is not just an
elaborate (and ineffcient) way to write:
> 
> package <- "package"
> 
> E.g.
> 
> > package <- as.character(substitute(package))
> > package
> [1] "package"
> >
> 
> Thanks
> Mick Jordan

Frequently used in a function body, where the function author wants the argument
to be passed as an object name, rather than a character vector, or perhaps both,
as is the case with library() and require().

For example:

test <- function(x) {as.character(substitute(x))}

# Quoted, passing "MyPackage" as a character
vector
> test("MyPackage")
[1] "MyPackage"


# Not quoted, passing the object MyPackage
> test(MyPackage)
[1] "MyPackage"


In both cases, the argument passed as 'x' can then be used within the
function as a character vector, rather than as the object itself.

Regards,

Marc Schwartz

subsitute(expr), with only one argument, is only useful inside of a
function and then only when the expression, expr, involves an argument
to the function.  Then the unevaluated actual arguments to the
function are substituted into the [unevaluated] expression.

E.g.,
   f <- function(x, y=stop("y is required"), z=log(-1)) {
      substitute(y + z)
   }
   f(1)
   # stop("y is required") + log(-1)
   f(1,sqrt(-1),log(1:10))
   # sqrt(-1) + log(1:10)

Bill Dunlap
TIBCO Software
wdunlap tibco.com


On Mon, Jul 21, 2014 at 8:07 AM, Mick Jordan <mick.jordan at oracle.com>
wrote:
> I came across this code in library.R
>
> package <- as.character(substitute(package))
>
> where package is the first argument to the "library" function.
>
> I've been racking my brains to understand why this is not just an
elaborate
> (and ineffcient) way to write:
>
> package <- "package"
>
> E.g.
>
>> package <- as.character(substitute(package))
>> package
> [1] "package"
>>
>
> Thanks
> Mick Jordan
>
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