Allow custom delimiters

[baptiste]

{ } would make it hard to work with some languages such as TeX.

[egnha]

Here's a simple DIY solution: substitute custom delimiters by glue's delimiters, then apply glue.

• First, make a delimiter converter (which ”escapes” curly braces):

  redelimiter <- function(l, r) {
    redelim <- list(c("\\{", "{{"), c("\\}", "}}"), c(l, "{"), c(r, "}"))
    function(text)
      Reduce(function(x, .) gsub(.[1], .[2], x), redelim, text)
  }

  For example, to convert <<, >> delimiters to glue's delimiters, apply redelimiter("<<", ">>") to your string:

  redelimiter("<<", ">>")("The value of $e^{2\\pi i}$ is $<<one>>$.")
  #> [1] "The value of $e^{{2\\pi i}}$ is ${one}$."

• Then extend glue() to handle custom delimiters, by “redelimiting” before calling glue. (One has to be careful to pass the interpolation arguments to glue() unevaluated.)

  glue_with_delim <- function(l, r) {
    redelim <- redelimiter(l, r)
    # derived from glue_data()
    restring <- function(..., env) {
      strings <- eval(substitute(alist(...)))
      is_text <- !glue:::has_names(strings)
      strings[is_text] <- redelim(lapply(strings[is_text], eval, envir = env))
      strings
    }
    # same formals as glue()
    function(..., .sep = "", .envir = parent.frame()) {
      strings <- restring(..., env = parent.frame())
      eval(as.call(c(glue::glue, strings, .sep = .sep, .envir = .envir)))
    }
  }

• With glue_with_delim(), you can make a version of glue() adapted to custom delimiters:

  gum <- glue_with_delim("<<", ">>")
  
  text1 <- "The value of $e^{2\\pi i}$ is $<<one>>$."
  text2 <- "The value of $e^{2\\pi i}+1$ is $<<two>>$."
  gum(text1, text2, one = Re(exp(pi * 2i)), two = one + 1, .sep = "\n")
  #> The value of $e^{2\pi i}$ is $1$.
  #> The value of $e^{2\pi i}+1$ is $2$.

  (gum(), as defined, won't allow you to insert literal <<, >>. But you can lift this limitation by calling glue_with_delim() with appropriate regular expressions, in place of "<<", ">>".)

@jimhester Do you think custom delimiters are better off as DIY?

A disadvantage of the DIY solution presented here is its inefficiency: aside from the call to redelim(), restring() duplicates computations in glue_data().

[jimhester]

I think this is an interesting solution. I think modifying the C code to handle this case is probably a better way to go in the package, but you can definitely use something like this until that is implemented.