Rmarkdown child documents do not detect their `params`

Question

I have a master Rmarkdown document into which I am including my individual chapters using knitr's child option. Each chapter makes use of rmarkdown parameters in its own YAML. Each chapter compiles well individually, but when put in this master document, I get the error

object: 'params' not found

I believe it is because when the child is knitted, knitr does not read the parameters in the YAML (which is an rmarkdown feature, not a knitr feature).

Is there some way I can make these available to knitr? Is there an "rmarkdown"-way of putting in child documents?

---
title: My thesis
---

blah blah.

# Introduction

```{r child='01-introduction.rmd'}
```

# Mathematical background

```{r child='02-mathsy-maths.rmd'}
```

Example 01-introduction.rmd

---
title: Introduction
params:
    dataset: ABC
---

Answer 1

As I understand knitr, when you knit a child document, this document is evaluated in the context (ie environment) of the parent document.

So, I see 4 solutions.

Set the params in the main document

With this solution, params are controlled inside the YAML front-matter of the main document. I think it is the natural solution.

---
title: My thesis
params:
  dataset: ABC
---

blah blah.

# Introduction

```{r child='01-introduction.rmd'}
```

# Mathematical background

```{r child='02-mathsy-maths.rmd'}
```

Assign the params in the global environment

With this solution, params are controlled with R code inside the main document.

---
title: My thesis
---

blah blah.

# Introduction
```{r set-params, include=FALSE}
params <- list(dataset = "ABC")
```

```{r child='01-introduction.rmd'}
```

# Mathematical background

```{r child='02-mathsy-maths.rmd'}
```

Retrieve params of the child document

With this solution, params are controlled inside each child document. It is a variant of the previous solution.
In the main document, child document's params are read using knitr::knit_params() and then assigned in the global environment.

---
title: My thesis
---

blah blah.

```{r def-assign-params, include=FALSE}
assign_params <- function(file) {
  text <- readLines(file)
  knit_params <- knitr::knit_params(text)
  params <<- purrr::map(knit_params, "value")
}
```

# Introduction

```{r, include=FALSE}
assign_params('01-introduction.rmd')
```

```{r child='01-introduction.rmd'}
```

# Mathematical background

```{r child='02-mathsy-maths.rmd'}
```

Use a (hacky) hook to assign params temporarily

Here, I define a hook for a new use.params chunk option: this solution extends the previous one. When use.params=TRUE is used, this hook is run for each chunk of the child document.
Note that with this solution, you cannot use params in inline code.

---
title: "Main document"
---

```{r hook-def, include=FALSE}
params_cache <- new.env(parent = emptyenv())

knitr::knit_hooks$set(use.params = function(before, options, envir) {
  if (before && options$use.params) {
    if (exists("params", envir = envir)) {
      params_cache$params <- envir$params
    }
    text <- readLines(knitr::current_input(dir = TRUE))
    knit_params <- knitr::knit_params(text)
    envir$params <- purrr::map(knit_params, "value")
  }
  if (!before && options$use.params) {
    if (exists("params", envir = params_cache)) {
      envir$params <- params_cache$params
      rm("params", envir = params_cache)
    } else {
      rm("params", envir = envir)
    }
  }
})
```

blah blah.

# Introduction

```{r child='01-introduction.rmd', use.params=TRUE}
```

# Mathematical background

```{r child='02-mathsy-maths.rmd', use.params=TRUE}
```