Allow hashtags in single-line input strings

[gadenbuie]

I've been playing with something akin to the collapse_transformer() example, but I'd like to be able to include # in one-line input strings.

What I want to do is essentially the following, where I would parse the template string in the transformer, but unfortunately it doesn't make it past the parser.

glue::glue("{#b a}")
# no output due to #b

Here are a few examples using an identity transformer that just returns the text that reaches the transformer.

identity <- function(x, ...) x

glue::glue("{a}", .transformer = identity)
#> a

"{a*}" works and the transformer receives "a*", but "{a#}" does not work: the transformer isn’t called …

glue::glue("{a*}", .transformer = identity)
#> a*
glue::glue("{a#}", .transformer = identity)

…unless the # is followed by a new line.

glue::glue("{a#\n}", .transformer = identity)
#> a#

Another example without the identity transformer.

a <- "apple"
glue::glue("{a#}")
glue::glue("{a#\n}")
#> apple

I can see where problems could arise with incomplete expressions due to comments, but it also makes sense that such problems would throw parsing errors.

glue::glue("{c(a,#\na)}")
#> apple
#> apple
glue::glue("{c(a,#a)}")
glue::glue("{c(a,#a)\n}")
#> Error in parse(text = text, keep.source = FALSE): <text>:3:0: unexpected end of input
#> 1: c(a,#a)
#> 2: 
#>   ^

That said, one place where changing the parser might break current behavior is that currently empty expressions cause the entire result to be zero-length. Except for the last, the following all return character(0).

glue::glue("{#a} {'banana'}")
glue::glue("{#a\n} {'banana'}")
glue::glue("{a#a} {'banana'}")
glue::glue("{a#a\n} {'banana'}")
#> apple banana

^{Created on 2020-06-04 by the reprex package (v0.3.0)}

[jimhester]

You can now control the comment character with the .comment argument, so what you are trying to do should now be possible.