Need to clarify how plain lists are supposed to be treated by as.tags

[wch]

The handling of plain lists in htmltools is inconsistent and kind of confusing. If there's a list at the top level, it causes problems. But if it's inside of another tag/tagList object, it seems to behave almost like a tagList.

We should clarify where a plain list is acceptable and where it's not.

library(htmltools)

# Here's a tagList and a list with HTML inside
tl <- tagList(div("ab"), "cd")
tl
#> <div>ab</div>
#> cd

l <- list(div("ab"), "cd")
l
#> [[1]]
#> <div>ab</div>
#> 
#> [[2]]
#> [1] "cd"


# as.tags() works on the tagList, but errors on the list. The error is actually
# due to the printing.
as.tags(tl)
#> <div>ab</div>
#> cd
as.tags(l)
#> Error in writeImpl(text): Text to be written must be a length-one character vector


# We can looks at the str() of the output of as.tags(). For the plain list, it
# doesn't look right -- it's basically the deparsed code.
str(as.tags(tl))
#> List of 2
#>  $ :List of 3
#>   ..$ name    : chr "div"
#>   ..$ attribs : Named list()
#>   ..$ children:List of 1
#>   .. ..$ : chr "ab"
#>   ..- attr(*, "class")= chr "shiny.tag"
#>  $ : chr "cd"
#>  - attr(*, "class")= chr [1:2] "shiny.tag.list" "list"
str(as.tags(l))
#> List of 1
#>  $ : chr [1:2] "list(name = \"div\", attribs = list(), children = list(\"ab\"))" "cd"
#>  - attr(*, "class")= chr [1:2] "shiny.tag.list" "list"


# Now, if we wrap either of them in another tag, they produce the same text
# output from as.tags().
as.tags(div(tl))
ab
cd
#> <div>
#>   <div>ab</div>
#>   cd
#> </div>
as.tags(div(l))
ab
cd
#> <div>
#>   <div>ab</div>
#>   cd
#> </div>

# Structure of as.tags is almost identical, except for shiny.tag.list attribute
str(as.tags(div(tl)))
#> List of 3
#>  $ name    : chr "div"
#>  $ attribs : Named list()
#>  $ children:List of 1
#>   ..$ :List of 2
#>   .. ..$ :List of 3
#>   .. .. ..$ name    : chr "div"
#>   .. .. ..$ attribs : Named list()
#>   .. .. ..$ children:List of 1
#>   .. .. .. ..$ : chr "ab"
#>   .. .. ..- attr(*, "class")= chr "shiny.tag"
#>   .. ..$ : chr "cd"
#>   .. ..- attr(*, "class")= chr [1:2] "shiny.tag.list" "list"
#>  - attr(*, "class")= chr "shiny.tag"
str(as.tags(div(l)))
#> List of 3
#>  $ name    : chr "div"
#>  $ attribs : Named list()
#>  $ children:List of 1
#>   ..$ :List of 2
#>   .. ..$ :List of 3
#>   .. .. ..$ name    : chr "div"
#>   .. .. ..$ attribs : Named list()
#>   .. .. ..$ children:List of 1
#>   .. .. .. ..$ : chr "ab"
#>   .. .. ..- attr(*, "class")= chr "shiny.tag"
#>   .. ..$ : chr "cd"
#>  - attr(*, "class")= chr "shiny.tag"


# Note that the structure of both above adds a layer of nesting in the children.
# Compare to case where there's no list or tagList, just a div wrapping the
# inner content.
str(as.tags(div(div("ab"), "cd")))
#> List of 3
#>  $ name    : chr "div"
#>  $ attribs : Named list()
#>  $ children:List of 2
#>   ..$ :List of 3
#>   .. ..$ name    : chr "div"
#>   .. ..$ attribs : Named list()
#>   .. ..$ children:List of 1
#>   .. .. ..$ : chr "ab"
#>   .. ..- attr(*, "class")= chr "shiny.tag"
#>   ..$ : chr "cd"
#>  - attr(*, "class")= chr "shiny.tag"

[cpsievert]

The weird result of str(as.tags(l)) is because as.character() gets called on the list before turning into a tag list here:

htmltools/R/tags.R

Lines 909 to 914 in f589c77

	as.tags.default <- function(x, ...) {
	if (is.list(x) && !isTagList(x))
	unclass(x)
	else
	tagList(as.character(x))
	}

Seems like that method should instead be:

as.tags.default <- function(x, ...) {
  if (is.list(x) && !isTagList(x))  return(unclass(x))
  if (is.list(x)) return(tagList(x))
  tagList(as.character(x))
}