Can I use R variables in quarto YAML?

[giabaio]

I may be missing something obvious, but I would like to do that. I'm working on a quarto slide presentation (formatted in revealjs) and I have defined a bunch of objects in the params variable in my YAML

---
...
params:
   title: "My title"
   author: Gianluca Baio
   date: 27 November 2022
   institute: "[My work](Their website)"
   conference: "Where I'm speaking"
   location: "What town"
...
---

HERE GOES THE REST OF MY SLIDES

What I would like to do is to use the elements of params to feed to the footer that I want to use for the slides, something like

...
format: 
   revealjs:
      footer: `r params$author` | `r params$title` | `r params$date`

but it seems like I can't use the

`r command`

tag inside the yaml, as it returns an error

bad indentation of a mapping entry at line 9, column 10:
       footer: `r params$author`

Like I said, it's possible I am not doing this right --- but can anyone explain why and how I can do it instead?

Thanks
Gianluca

[mcanouil]

Hi,

in theory, you have to use the keyword !expr as for the yaml chunk options and described in https://quarto.org/docs/computations/r.html#chunk-options

[giabaio]

I'll check again to make absolutely sure, but I think I have tried that, to no avail... Gianluca Baio Professor of Statistics & Health Economics Department of Statistical Science | University College London 1-19 Torrington Place, London WC1E 6BT, UK Twitter: @stats_UCL<https://twitter.com/stats_UCL> | Instagram: @stats_UCL<https://instagram.com/ucl.stats/> Telephone +44(0)20 7679 1248 (internal: 41248) Website: http://www.homepages.ucl.ac.uk/~ucakgba Blog: https://gianluca.statistica.it/blog/ Book a meeting with me: https://book.morgen.so/gianlucabaio/book-me [Twitter]<https://twitter.com/gianlubaio> [LinkedIn (Personal)] <https://linkedin.com/in/gianluca-baio-b893879/> [Github] <https://github.com/giabaio> [Zoom] <https://ucl.zoom.us/j/9904462616> [ORCID] <https://orcid.org/0000-0003-4314-2570> [Soundcloud] <https://soundcloud.com/uclsound/sets/sample-space>

…

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[mcanouil]

I tried it and it does not work as for rmarkdown ...

---
params:
  title: "My title"
  author: Gianluca Baio
  date: 27 November 2022
  institute: "[My work](Their website)"
  conference: "Where I'm speaking"
  location: "What town"
title: !expr params$title
format: revealjs
---

produced an error:

Error: object 'params' not found
Error in yaml::yaml.load(..., eval.expr = TRUE) : 
  Could not evaluate expression: params$title
Calls: .main ... parse_yaml_front_matter -> yaml_load -> <Anonymous>
Execution halted

[giabaio]

Nope --- I've tried again. I can specify in my YAML

---
params:
   title: Title here
   shorttitle: Short title here
   author: Gianluca Baio
   date: 27 November 2022
   institute: "[My department](the website) | My institution"
   conference: "Some conference"
   location: "Some place"
   shortlocation: "A shorter version of the place"
format: 
   revealjs:
     header-includes: |
      <link href="https://cdnjs.cloudflare.com/ajax/libs/font-awesome/6.1.1/css/all.min.css" rel="stylesheet"/>
     theme: [ucl-stats.scss]
     slide-number: true
     chalkboard: 
      buttons: false
      theme: whiteboard
     preview-links: auto
     width: 1800
     height: 1000
     footer: |
        &copy; !expr params$author
# Includes various LaTeX macros
     include-in-header: "latex_macros.html"
     css: styles.css
from: markdown+emoji
---

## Just to check that the text gets parsed OK
`r rmarkdown::metadata$params$author`

but what happens is that the footer is parsed "verbatim", so it appears as

© !expr params$author

but the text is parsed correctly in the slide body (so it turns the object params$author into my name, as given in the YAML).

I don't know if there's an issue with how the footer is defined --- in that it doesn't allow Rmarkdown, but only markdown? If so, would it be possible to implement a different strategy? I can, for example, create a footer for a specific slide, something like

...
::: footer
© `r rmarkdown::metadata$params$author | 
`r some_function_to_add_my_twitter()` `r some_function_to_add_my_github()` | 
`r rmarkdown::metadata$params$shorttitle` |
`r rmarkdown::metadata$params$shortlocation` |
`r rmarkdown::metadata$params$date`
:::

and that works perfectly. But then I would need to copy the same text for each slide, which is obviously not ideal...

Any suggestions?

Many thanks again!
Gianluca

[mcanouil]

You can't mix expression and string, it's one or the other.
For your last example, it should be: footer: !expr paste("©", params$author).
But anyway, currently it seems params can't be used in the yaml header of a Quarto document.

[giabaio]

I tried it and it does not work as for rmarkdown ...

---
params:
  title: "My title"
  author: Gianluca Baio
  date: 27 November 2022
  institute: "[My work](Their website)"
  conference: "Where I'm speaking"
  location: "What town"
title: !expr params$title
format: revealjs
---

produced an error:

Error: object 'params' not found
Error in yaml::yaml.load(..., eval.expr = TRUE) : 
  Could not evaluate expression: params$title
Calls: .main ... parse_yaml_front_matter -> yaml_load -> <Anonymous>
Execution halted

Interesting --- it works if you do

title: "`r params$title`"

(so enclosed in quotes). But it fails if you try with !expr...

[giabaio]

You can't mix expression and string, it's one or the other. For your last example, it should be: footer: !expr paste("©", params$author). But anyway, currently it seems params can't be used in the yaml header of a Quarto document.

If I try your suggestion, the resulting footer is processes but not rendered as I would like, ie it is like this:

!expr paste(‘©’, params$author)

(It does also work with

"`r paste('©', params$title)`"

This makes me think that the problem is with footer (and that actually you can use

`r ...`

in the yaml)?

[mcanouil]

This is confusing ... 😅 i.e., having to switch between !expr ... and "`r ...`" depending on if you are modifying document yaml or yaml chunk option.
It's related to #606 and #1334

It seems that all the yaml options are not parsed the same way ...

[cderv]

In a way, the !expr vs `r ...` difference is the same as with R Markdown. It was already the case where

• YAML fields processed by rmarkdown before knitting required using !expr as fields are loading in R using yaml package for rmarkdown process to use them.
• YAML fields only processed by Pandoc (as pandoc variable) like title or date could use knitr inline code chunk, because the YAML fields were not processed before knitting, and not processed in R.

The latter is why title: "`r params$title`" works but not with !expr as not loaded by R.

With Quarto, the logic is quite similar: knitr inline code chunk can't be used as fields will be processed by Quarto directly.
Summary for here is quite good: #606 (comment)

So yes, for now, I am not sure it is possible to use params in other YAML configuration, probably because of the the order or evaluation. params are really designed to be used computation engine (Jupyter and Knitr) as the doc located in computation engine section shows: https://quarto.org/docs/computations/parameters.html

Quarto introduces Variables (https://quarto.org/docs/authoring/variables.html#var) that can be used in YAML fields too, only in projects for now though: #1252

Did you already tried using variables ?

But in short, to answer your issue question: "Can I use R variables in quarto YAML?", the answer is no you cannot use R variables as Quarto will need the value of the variable and the YAML header is not processed by knitr.

Maybe we can find a way to make params accessible as variables though ? Or at least improve using parameterized YAML value .

[giabaio]

Thank you for this. I'll look at the summary you mention and the other bits and pieces. I guess the specific question is probably a bit wider than the reason why I asked it, which is in fact: "can I construct the footer in a quarto presentation by combining elements from the yaml?"

In a way, the answer to this is yes --- I have done it, but this means having to replicate the footer on each slide... It is probably beyond my understanding/knowledge of reveal/javascript/html/css/scss etc, but I guess there probably are ways to "inject" this compiled code onto every slide?

I made a comment on #809 last night --- I was trying to hack a function that can be used for xaringan presentations to specify an image to be used as logo, as well as its features and where to position it, and then use some javascript (I think!) to make sure that that code is replicated on every slide. You pass that as an include_after_body and then you've basically created a shortcut that avoids having to replicate that same code for every slide. In addition, I haven't quite figured out how to do it with reveal/quarto slides, but I think you can define classes (=slides) over which you don't want the code/logo to be included, which may be handy...

I wonder whether something similar could be used for the footer issue?
Gianluca

[mcanouil]

With shortcodes and meta (https://quarto.org/docs/authoring/variables.html#meta) you can almost do the footer part, but I can't retrieve the author name ...
@cderv Is it a bug in the meta shortcode for author (i.e., author and any subkey including the author.affiliation from the documentation) or am I missing something?

[cderv]

In a way, the answer to this is yes --- I have done it, but this means having to replicate the footer on each slide... It is probably beyond my understanding/knowledge of reveal/javascript/html/css/scss etc, but I guess there probably are ways to "inject" this compiled code onto every slide?

You mean the div solution shown in #1391 (comment) ?
Regarding reducing copy pasting, Quarto has an "includes" feature: https://quarto.org/docs/authoring/includes.html
You can set the content in an external file, and you should be able to include it in all slide using the shortcode.
Leveraging knitr chunks and its option would be a way to also only copy paste the code for insertion and not the whole block.

Are the value in params you are setting only useful for the YAML footer ? and not used in R code chunk ?
Because, maybe it would be more suited to use https://quarto.org/docs/authoring/variables.html#var instead.
You should try it.

You could also leverage Lua filters to add some component in each slides (https://quarto.org/docs/extensions/filters.html).
include_in_header strategy may not work for footer because of the way it created in Quarto.

In addition, I haven't quite figured out how to do it with reveal/quarto slides, but I think you can define classes (=slides) over which you don't want the code/logo to be included, which may be handy...

Lua filters could be the solution, otherwise some JS inserted in header to remove the logo when class is detected. But the latter would be done in browser and not before the documented is rendered to HTML.

[cderv]

@mcanouil this is because author keys is processed specifically in Quarto (we do some normalization) and the metadata key will not be the one you used at the end. We don't have documentation ready on that yet but will be soon.

Maybe this is a bug also and we don't put back the author field to be accessible in the meta shortcode. If you build a simple reproducible example with html format, you can open a new issue.

[giabaio]

With shortcodes and meta (https://quarto.org/docs/authoring/variables.html#meta) you can almost do the footer part, but I can't retrieve the author name ... @cderv Is it a bug in the meta shortcode for author (i.e., author and any subkey including the author.affiliation from the documentation) or am I missing something?

Thanks --- that's helpful. I think the issue is that you can only use "standard" fields in the meta shortcodes? I know this is getting a bit more complex/specific than the original question let out (and apologies for this!), but basically, I think the reason for wanting to use params is so that I can create a separate title page, which I can then process and format the way I want to.

For instance, I've created a separate title-page.qmd file, reading like this:

:::: {.title-slide}
::: {.title}
`r rmarkdown::metadata$params$title`
:::

::: {.author}
`r rmarkdown::metadata$params$author`
:::

::: {.institute}
`r rmarkdown::metadata$params$institute`
:::

::: {.social}
A bunch of `html`/`R` code to produce icons + links to my social accounts
:::

::: {.conference}
`r rmarkdown::metadata$params$conference`, `r rmarkdown::metadata$params$location` 
:::

::: {.date}
`r if(!is.null(rmarkdown::metadata$params$date)){rmarkdown::metadata$params$date} else {format(Sys.Date(),"%e %B %Y")}`
:::

A bunch of `R` code that defines the sticky post-its with extra icons & text

::: {.logo-stats}
:::

::: footer
# Because I don't want the footer in the title page --- this doesn't work perfectly because the first time you open the slides
# the "overall" footer still shows...
:::

::::

and defined a suitable class .title-slide with various components (.logo-stats, .date, .conference, .social, etc) that define how each should be formatted/typeset. In my main .qmd file, after the yaml (which does not contain the fields title and author so as to not create the default title page), I then load this as

{r include=FALSE}
source("setup.R")

{r child="title-slide.qmd"}

(setup.R contains a bunch of functions I have written to generate the "social" text + other utilities I use in other parts of the slides). This process generates the following title page

which is what I want.

@cderv: yes I meant #1391 (comment). When I create that footer, all works well. I haven't played with vars yet, but can have a go...