Create a data.frame where a column is a list

Question

I know how to add a list column:

> df <- data.frame(a=1:3)
> df$b <- list(1:1, 1:2, 1:3)
> df
  a       b
1 1       1
2 2    1, 2
3 3 1, 2, 3

This works, but not:

> df <- data.frame(a=1:3, b=list(1:1, 1:2, 1:3))
Error in data.frame(1L, 1:2, 1:3, check.names = FALSE, stringsAsFactors = TRUE) : 
  arguments imply differing number of rows: 1, 2, 3

Why?

Also, is there a way to create df (above) in a single call to data.frame?

Answer

Slightly obscurely, from ?data.frame:

If a list or data frame or matrix is passed to ‘data.frame’ it is as if each component or column had been passed as a separate argument (except for matrices of class ‘"model.matrix"’ and those protected by ‘I’ ).

(emphasis added).

So

data.frame(a=1:3,b=I(list(1,1:2,1:3)))

seems to work.