[bug] Printing error when assigning to table nested in structure

[fruce-ki]

Following discussion on StackOverflow ([http://stackoverflow.com/questions/38016170/getting-a-printing-error-while-assigning-vector-to-data-table-column/38073502#38073502]) I seem to have encountered a bug. This occurs when assigning to a pre-allocated table column.

Error message:

Error in SYS[[length(SYS) - 3L]][[1L]] == "knit_print.default" : 
  comparison (1) is possible only for atomic and list types

The code does not crash and the error is not printed to the console until I attempt to display the value of the table or of its containing structure. The error occurs only when the assignment is executed from source code. If I interrupt the execution and paste the offending assignment into the console, it executes without problem.

Searching the internet for the error, I found literally nothing, except for the source code of data.table here on github. The relevant code is in lines 45-50, and seems to regard supressing knitr print output for in-place assignments.

The error still exists in the latest dev version of data.tables from github.

Minimal reproducible example:

alloc_out <- function(annot = c("a", "b")) {
  Genes <- data.table("parent" = annot, flag = NA)
  ro <- structure(list("Genes" = Genes), class=c("dtu"))  # This sets up the error.
#  ro <- structure(list("Genes" = Genes))  # This alternative does not lead to the error.
  ro$Genes[, flag := TRUE]  # This causes error in source, but works in console.
  return(ro)
}

The error seems to be triggered by the line encapsulating the table into a strucutre. Specifically my use of a class attribute. Omitting the class attribute, leads to no error.

Session info:

R version 3.2.4 (2016-03-10)
Platform: x86_64-apple-darwin13.4.0 (64-bit)
Running under: OS X 10.11.5 (El Capitan)

locale:
[1] en_GB.UTF-8/en_GB.UTF-8/en_GB.UTF-8/C/en_GB.UTF-8/en_GB.UTF-8

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
[1] rats_0.1         data.table_1.9.6

loaded via a namespace (and not attached):
 [1] Rcpp_0.12.4        matrixStats_0.50.2 digest_0.6.9       withr_1.0.1        chron_2.3-47       jsonlite_0.9.19    magrittr_1.5       stringi_1.0-1      devtools_1.11.0    tools_3.2.4        stringr_1.0.0      htmlwidgets_0.6    yaml_2.1.13       
[14] parallel_3.2.4     memoise_1.0.0      htmltools_0.3.5  

(rats_0.1 is my package in development, where this bug became apparent)