[R-Forge #2611] := shouldn't print in knitr

[arunsrinivasan]

Submitted by: Matt Dowle; Assigned to: Nobody; R-Forge link

As raised on SO here

[rubenarslan]

In conjunction with using dplyr on a data.table, this still happens. Presumably the error can also occur when other classes remove data.table from being the pole position class.

Here's a reproducible Rmd file:

`:=` prints in conjunction with dplyr, because dplyr adds all kind of classes to data.tables

```{r}
library(dplyr); library(data.table)
mtcars = data.table(mtcars)

grouped = mtcars %>% group_by(cyl)
grouped[, mpg := 2]
class(grouped)

class(grouped) = c("data.table", "grouped_dt", "tbl_dt", "tbl", "data.frame") // reorder classes
grouped[, mpg := 2]

grouped = mtcars %>% group_by(cyl) %>% data.table()
grouped[, mpg := 2]
class(grouped)
```

This is with data.table_1.9.5, dplyr_0.4.1 and knitr_1.9.