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，它可以在发送代码块进行评估之前预处理代码块，步骤如下：

1. 预处理options$code
2. 在父环境
中使用evaluate::evaluate()评估代码
3. 使用 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在文档的其余部分都不可用。经过一些修补，我发现如果我遍历调用堆栈并找到最后调用 knitr 的地方，我可以获取 envir 参数的值并将其用作 evaluate::evaluate() 的环境。不过，这感觉很老套。 有没有更好的方法来匹配自定义块引擎中文档的环境？

例子

一个在所有评论上附加 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 2020-06-16 创建

Answer 1

原来{knitr}中有一个函数正是针对这种情况的：knitr::knit_global(), which happens to be the environment when evaluating inline code: 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