[Rd] R 2.5.0 devel try issue in conjuntion with S4 method dispatch

[Peter Dalgaard]

Seth Falcon wrote:
> ml-it-r-devel at epigenomics.com writes:
>   
>> Investigating this new implementation I come across an issue in
>> conjuntion with using S4 classes and methods. try(expr) does not return an
>> object with attribute 'try-error' in case of method dispatch failure
>> in the wrapped expression which to me seems not
>> quite correct.
>>     
>
> We've seen some similar issues but had not had time to track them
> down.
>
>   
>> Examples to reproduce the observation:
>>     
>
> It isn't S4 specific.  The issue seems more about anonymous
> calls/functions.
>
>    ll = list()
>    ll[[1]] = function(x) stop("died")
>    
>    v = try(do.call(ll[[1]], list(1)), silent=TRUE)
>    Error in as.list(call)[[1]] == "doTryCatch" : 
>    	comparison (1) is possible only for atomic and list types
>    > v
>    Error: object "v" not found
>
> I don't fully understand why the call is being computed, but the
> following seems to get things going.
>
> + seth
>
> --- a/src/library/base/R/New-Internal.R
> +++ b/src/library/base/R/New-Internal.R
> @@ -7,7 +7,8 @@ try <- function(expr, silent = FALSE) {
>              ## Patch up the call to produce nicer result for testing as
>              ## try(stop(...)).  This will need adjusting if the
>              ## implementation of tryCatch changes.
> -            if (as.list(call)[[1]] == "doTryCatch")
> +            callFun <- as.list(call)[[1]]
> +            if (is.name(callFun) && callFun == "doTryCatch")
>                  call <- sys.call(-4)
>              dcall <- deparse(call)[1]
>              prefix <- paste("Error in", dcall, ": ")
>
>   
Good catch, Seth. The code still looks a bit clunky though, and I wonder 
if this wouldn't be nicer:

if (identical(call[[1]], quote(doTryCatch))) call <- sys.call(-4)

I.e., the thing that is clearly wrong is the use of "==" on something 
that is not necessarily a simple name, but the use of as.list seems 
unnecessary and comparisons between strings and names is a bit awkward too.