Parse YAML or TOML Front Matter — parse_front

Extract and parse YAML or TOML front matter from a file or a text string. Front matter is structured metadata at the beginning of a document, delimited by fences (--- for YAML, +++ for TOML). parse_front_matter() processes a character string, while read_front_matter() reads from a file. Both functions return a list with the parsed front matter and the document body.

Usage

parse_front_matter(text, parse_yaml = NULL, parse_toml = NULL)

read_front_matter(path, parse_yaml = NULL, parse_toml = NULL)

Arguments

text: A character string or vector containing the document text. If a vector with multiple elements, they are joined with newlines (as from readLines()).
parse_yaml, parse_toml: A function that takes a string and returns a parsed R object, or NULL to use the default parser. Use identity to return the raw string without parsing.
path: A character string specifying the path to a file. The file is assumed to be UTF-8 encoded. A UTF-8 BOM (byte order mark) at the start of the file is automatically stripped if present.

Value

A named list with two elements:

data: The parsed front matter as an R object, or NULL if no valid front matter was found.
body: The document content after the front matter, with leading empty lines removed. If no front matter is found, this is the original text.

Functions

parse_front_matter(): Parse front matter from text
read_front_matter(): Parse front matter from a file.

Custom Parsers

By default, the package uses yaml12::parse_yaml() for YAML and tomledit::parse_toml() for TOML. You can provide custom parser functions via parse_yaml and parse_toml to override these defaults.

Use identity to return the raw YAML or TOML string without parsing.

YAML Specification Version

The default YAML parser uses YAML 1.2 via yaml12::parse_yaml(). To use YAML 1.1 parsing instead (via yaml::yaml.load()), set either:

The R option frontmatter.parse_yaml.spec to "1.1"
The environment variable FRONTMATTER_PARSE_YAML_SPEC to "1.1"

The option takes precedence over the environment variable. Valid values are "1.1" and "1.2" (the default).

YAML 1.1 differs from YAML 1.2 in several ways, most notably in how it handles boolean values (e.g., yes/no are booleans in 1.1 but strings in 1.2).

Examples

# Parse YAML front matter
text <- "---
title: My Document
date: 2024-01-01
---
Document content here"

result <- parse_front_matter(text)
result$data$title  # "My Document"
#> [1] "My Document"
result$body        # "Document content here"
#> [1] "Document content here"

# Parse TOML front matter
text <- "+++
title = 'My Document'
date = 2024-01-01
+++
Document content"

result <- parse_front_matter(text)

# Get raw YAML without parsing
result <- parse_front_matter(text, parse_yaml = identity)

# Use a custom parser that adds metadata
result <- parse_front_matter(
  text,
  parse_yaml = function(x) {
    data <- yaml12::parse_yaml(x)
    data$parsed_at <- Sys.time()
    data
  }
)

# Or read from a file
tmpfile <- tempfile(fileext = ".md")
writeLines(text, tmpfile)

read_front_matter(tmpfile)
#> $data
#> $data$title
#> [1] "My Document"
#> 
#> $data$date
#> [1] "2024-01-01"
#> 
#> 
#> $body
#> [1] "Document content\n"
#>