How to include an R instruction as a parameter in a quarto document

[ddotta]

Description

Hello !

Is it possible to use an R instruction as a parameter in a quarto document?

With an .Rmd file, I used this syntax but it doesn't work :

params:
  com : !r c("01001", "01002", "01003")

# params$com

I got this error

ERROR: YAMLError: unknown tag !<!r> at line 6, column 43:
     ...  !r c("01001", "01002", "01003")

Many thanks in advance !

[cderv]

Is it possible to use an R instruction as a parameter in a quarto document?

No you can't do that anymore. You need to initialize the values using YAML syntax. It was possible in R Markdown because all happened in R, including the YAML parsing of options, whereas in Quarto R is used only for computation with knitr, and everything else does not use tools like R. This includes parsing parameters to pass them the engine, or allow auto completion, and YAML validation.

If you really need to use R syntax, you can use the quarto R package to pass parameters, but this will be converted by the package to YAML equivalent.

You can know what to write in YAML using yaml package

> cat(yaml::as.yaml(list(com = c("01001", "01002", "01003"))))
com:
- '01001'
- '01002'
- '01003'

[cderv]

To follow up on @mcanouil comment below, let me give more details

!r and !expr are specific YAML tag handler defined in knitr. As I said, the YAML header in a .qmd is first handled by Quarto in all situation. We taught Quarto about !expr so that Quarto's YAML validation does not fail on this, as it could be handled by knitr. We did not do it for !r. So that is why you get an error message with !r and that !expr would work like shown below #10991 (comment)

So technically, you could be using !expr inside your document instead of !r, but you can't use !r directly. However, as you are migrating, I would advice not using it when not needed and that is why I did not mention it. It is knitr specific and could be removed in the future (when dealing with a common interface for all engine #9197 and also because executing code in YAML is a bit of a security problem (eval.expr = false has been made the default in yaml R package now BTW)

Also if you are passing parameters at command line (which usually you would like to do), you will use --execute-param with a YAML file, and this one can't have !expr in it, because it will not be read by knitr but by Quarto.

So overall, this is why I suggest not using !expr for params.

[ddotta]

Thanks for your great answer @cderv and @mcanouil !

[mcanouil]

See the following "knitr" example.
Note that Quarto discourages the use of !expr as it is not something working across engines (i.e., it is a knitr feature) and is not working everywhere.

---
title: "Quarto Playground"
format: html
params:
  key: !expr "
    list(
      A = 1+1,
      B = letters
    )"
---

```{r}
params[["key"]][["A"]]
```

@ddotta Please edit the title to a meaningful one.