在自定义引擎中匹配 knitr 文档环境

Question

我有兴趣为 knitr 制作一个引擎，该引擎将在发送代码块进行评估之前通过以下步骤对代码块进行预处理：

1. 预处理options$code
2. 在父环境中使用evaluate::evaluate() 评估代码
3. 用knitr::engine_output()格式化输出

我发现在这些自定义块中创建的任何变量都不会在文档的其余部分中可用。经过一番修改，我发现如果我遍历调用堆栈并找到调用 knitr 的最后一个位置，我可以将envir 参数的值用作evaluate::evaluate() 的环境。不过，这感觉很hacky。 有没有更好的方法来匹配自定义块引擎中的文档环境？

示例

在所有 cmets 上附加一个 Flippy 的引擎

knitr::knit_engines$set(flip =
  function(options) {
    # pre-process code
    code <- gsub("(#+?)", "\\1 (╯°□°）╯︵", options$code)

    # Find environment <--------------- IS THERE A BETTER WAY?
    #
    # grabbing the call stack
    cstack <- vapply(sys.calls(), function(i) paste(as.character(deparse(i)), collapse = "\n"), character(1))
    fstack <- sys.frames()
    # Finding the last instance of the knit function and grabbing the envir variable
    knitting <- rev(grep("knit(", cstack, fixed = TRUE))[1]
    e <- get("envir", fstack[[knitting]])

    OUT <- evaluate::evaluate(code, envir = e)
    knitr::engine_output(options, out = OUT)
  }
)

tmp <- tempfile(fileext = ".Rmd")
tmpout <- tempfile(fileext = ".md")
txt <- "---\noutput: md_document\n---\n\n```{r}\na <- 'A'\na\n```\n\n```{flip}\nb <- paste(a, 'and B') # FLIPPIN\nb\n```\n\nSponsored by the letters `r try(b)`\n"
cat(txt, file = tmp)

rmarkdown::render(tmp, output_file = tmpout, envir = new.env())
#> processing file: file3230dc4500b.Rmd
#> output file: file3230dc4500b.knit.md
#> /usr/bin/pandoc +RTS -K512m -RTS file3230dc4500b.utf8.md --to markdown_strict --from markdown+autolink_bare_uris+tex_math_single_backslash --output /tmp/Rtmpzc5qWO/file32306aeaf291.md --standalone
#> 
#> Output created: /tmp/Rtmpzc5qWO/file32306aeaf291.md
cat(readLines(tmp), sep = "\n")
#> ---
#> output: md_document
#> ---
#> 
#> ```{r}
#> a <- 'A'
#> a
#> ```
#> 
#> ```{flip}
#> b <- paste(a, 'and B') # FLIPPIN
#> b
#> ```
#> 
#> Sponsored by the letters `r try(b)`
cat(readLines(tmpout), sep = "\n")
#>     a <- 'A'
#>     a
#>     #> [1] "A"
#> 
#>     b <- paste(a, 'and B') # (╯°□°）╯︵ FLIPPIN
#>     b
#>     #> [1] "A and B"
#> 
#> Sponsored by the letters A and B

^{由reprex package (v0.3.0) 于 2020 年 6 月 16 日创建}

Answer 1

事实证明，{knitr} 中有一个函数正是针对这种情况：knitr::knit_global()，恰好是评估内联代码时的环境：https://github.com/yihui/knitr/blob/0daf31be36eed8d8ec0ba51eedee909283afc45d/R/hooks.R#L13

knitr::knit_engines$set(flip = 
  function(options) {
    # pre-process code
    code <- gsub("(#+?)", "\\1 (╯°□°）╯︵", options$code)

    e <- knitr::knit_global() # <----- SOLUTION ᕕ( ᐛ )ᕗ

    OUT <- evaluate::evaluate(code, envir = e)
    knitr::engine_output(options, out = OUT)
  }
)

tmp <- tempfile(fileext = ".Rmd")
tmpout <- tempfile(fileext = ".md")
txt <- "---\noutput: md_document\n---\n\n```{r}\na <- 'A'\na\n```\n\n```{flip}\nb <- paste(a, 'and B') # FLIPPIN\nb\n```\n\nSponsored by the letters `r try(b)`\n"
cat(txt, file = tmp)

rmarkdown::render(tmp, output_file = tmpout, envir = new.env())
#> processing file: file3b0117b9cb1f.Rmd
#> output file: file3b0117b9cb1f.knit.md
#> /usr/bin/pandoc +RTS -K512m -RTS file3b0117b9cb1f.utf8.md --to markdown_strict --from markdown+autolink_bare_uris+tex_math_single_backslash --output /tmp/Rtmp1KgoUj/file3b01702d22f0.md --standalone
#> 
#> Output created: /tmp/Rtmp1KgoUj/file3b01702d22f0.md
cat(readLines(tmp), sep = "\n")
#> ---
#> output: md_document
#> ---
#> 
#> ```{r}
#> a <- 'A'
#> a
#> ```
#> 
#> ```{flip}
#> b <- paste(a, 'and B') # FLIPPIN
#> b
#> ```
#> 
#> Sponsored by the letters `r try(b)`
cat(readLines(tmpout), sep = "\n")
#>     a <- 'A'
#>     a
#>     #> [1] "A"
#> 
#>     b <- paste(a, 'and B') # (╯°□°）╯︵ FLIPPIN
#>     b
#>     #> [1] "A and B"
#> 
#> Sponsored by the letters A and B

^{由reprex package (v0.3.0) 于 2020 年 6 月 16 日创建}