Package {tera}

Title: Generate Text and Documents with the Tera Templating Engine
Version: 0.1.0
Description: The 'tera' package uses 'extendr' to provide access to Vincent Prouillet's 'Tera' templating engine in Rust. Users mainly interact with a Tera R6 object, which serves as a template library with encapsulated methods for rendering templates with a given context. Template syntax supports additional logic, including built-in filters, tests, and functions, as well as loops, conditions, and inheritance. Documentation for Tera's templating syntax can be found at https://keats.github.io/tera/.
License: MIT + file LICENSE
Encoding: UTF-8
Config/rextendr/version: 0.5.0
SystemRequirements: Cargo (Rust's package manager), rustc
Depends: R (≥ 4.2)
Imports: cli, R6, rlang (≥ 1.1.0), yyjsonr
Suggests: knitr, quarto, rmarkdown, testthat (≥ 3.0.0), withr
URL: https://github.com/kbvernon/tera-r, https://kbvernon.github.io/tera-r/
VignetteBuilder: quarto, knitr
Config/testthat/edition: 3
Config/roxygen2/version: 8.0.0
NeedsCompilation: yes
Packaged: 2026-09-14 14:39:10 UTC; kenne
Author: Kenneth Blake Vernon ORCID iD [aut, cre, cph]
Maintainer: Kenneth Blake Vernon <kenneth.b.vernon@gmail.com>
Repository: CRAN
Date/Publication: 2026-09-28 14:10:02 UTC

tera: Generate Text and Documents with the Tera Templating Engine

Description

The 'tera' package uses 'extendr' to provide access to Vincent Prouillet's 'Tera' templating engine in Rust. Users mainly interact with a Tera R6 object, which serves as a template library with encapsulated methods for rendering templates with a given context. Template syntax supports additional logic, including built-in filters, tests, and functions, as well as loops, conditions, and inheritance. Documentation for Tera's templating syntax can be found at https://keats.github.io/tera/.

Author(s)

Maintainer: Kenneth Blake Vernon kenneth.b.vernon@gmail.com (ORCID) [copyright holder]

Authors:

See Also

Useful links:

R6 class for Tera templating engine

Description

A Tera templating engine for rendering templates with contexts consisting of variables and values. Similar to glue, but with additional template logic.

The glob pattern ⁠templates/*.html⁠ will match all files with the .html extension located directly inside the templates folder, while ⁠templates/**/*.html⁠ will match all files with the .html extension directly inside or in a subdirectory of templates. The default naming convention is to give each template their full relative path from templates (or whatever the template directory is called).

When initialized with a glob, template names take the path of the file template relative to the base of the template directory. For example, if you pass ⁠templates/**/*.html⁠, the template file templates/blog/post.html would have the name blog/post.html. When specified manually, all templates must be named. The same is true of context variables.

It is possible to add variables to a global context that can be accessed in any template. See the set_globals() method.

Methods

Public methods

Tera$new()

Initialize a Tera template engine

Usage
Tera$new(glob = NULL)
Arguments
glob

character scalar, a glob pattern with * wildcards indicating a potentially nested directory containing multiple file templates. If NULL (the default), a Tera engine with an empty library is initialized.

Returns

an R6 Tera template engine.

Tera$print()

Print Tera

Usage
Tera$print(n = 10L, ...)
Arguments
n

integer scalar, number of templates to print (default is 10L)

...

ignored

Returns

Self (invisibly)

Tera$add_file_templates()

Add file templates to library from file paths.

Usage
Tera$add_file_templates(...)
Arguments
...

specify list of templates as key = value pairs where key is the name of the template and value is the path to the template on file.

Returns

Self (invisibly)

Tera$add_string_templates()

Add templates to library from character strings.

Usage
Tera$add_string_templates(...)
Arguments
...

specify list of templates as key = value pairs where key is the name of the template and value is a string template.

Returns

Self (invisibly)

Tera$reload()

Re-parse all file templates found in the glob given to Tera, including any new file templates that were added. Templates added manually are preserved.

Usage
Tera$reload()
Returns

Self (invisibly)

Tera$render_template()

Render specified template file to string or file if outfile is specified.

Usage
Tera$render_template(template, ..., block = NULL, outfile = NULL)
Arguments
template

character scalar, the name of the template to render.

...

specify context as key = value pairs where key is the template variable and value is the data to inject.

block

character scalar, if not NULL, interpolation will be restricted to the specified block in template (default is NULL).

outfile

character scalar, the path to file where template is to be rendered (default is NULL).

Returns

if outfile = NULL (default), the rendered string; otherwise, outfile (invisibly).

Tera$render_string()

Render specified template string to string or file if outfile is specified.

Usage
Tera$render_string(string, ..., outfile = NULL, autoescape = TRUE)
Arguments
string

character scalar, the template string to render.

...

specify context as key = value pairs where key is the template variable and value is the data to inject.

outfile

character scalar, the path to file where template is to be rendered (default is NULL).

autoescape

boolean scalar, whether to autoescape HTML (default is TRUE).

Returns

if outfile = NULL (default), the rendered string; otherwise, outfile (invisibly).

Tera$render_component()

Render specified component to string or file if outfile is specified.

Usage
Tera$render_component(
  component,
  ...,
  body = NULL,
  outfile = NULL,
  autoescape = TRUE
)
Arguments
component

character scalar, the name of the component to render.

...

specify context as key = value pairs where key is the template variable and value is the data to inject.

body

character scalar, optional, passed to component {{ body }} variable.

outfile

character scalar, the path to file where template is to be rendered (default is NULL).

autoescape

boolean scalar, whether to autoescape HTML (default is TRUE).

Returns

if outfile = NULL (default), the rendered string; otherwise, outfile (invisibly).

Tera$templates()

List current templates in library.

Usage
Tera$templates()
Returns

a sorted character vector of template names

Tera$components()

List current components in library.

Usage
Tera$components()
Returns

a sorted character vector of component names

Tera$variables()

List all variables in template.

Usage
Tera$variables(template)
Arguments
template

character scalar, the name of the template.

Returns

a sorted character vector of variable names

Tera$autoescape_on()

Turn on autoescaping of HTML. This only applies to templates whose names end with ".html", ".htm", or ".xml". Autoescaping is on by default.

Usage
Tera$autoescape_on()
Returns

Self (invisibly)

Tera$autoescape_off()

Turn off autoescaping of HTML. This only applies to templates whose names end with ".html", ".htm", or ".xml". Autoescaping is on by default.

Usage
Tera$autoescape_off()
Returns

Self (invisibly)

Tera$set_globals()

Add a set of variables to a global context that can then be accessed by every template. If a variable with the same name already exists, it is overridden.

Usage
Tera$set_globals(...)
Arguments
...

specify context as key = value pairs where key is the template variable and value is the data to inject.

Returns

Self (invisibly)

Tera$globals()

List current variables in global context.

Usage
Tera$globals()
Returns

a sorted character vector of variable names

Tera$clear_globals()

Remove all variables from global context. Resets to an empty context.

Usage
Tera$clear_globals()
Returns

Self (invisibly)

Tera$set_delimiters()

Set the delimiters used to mark up templates, which is useful for templating files that themselves contain ⁠{{⁠, such as LaTeX. Each delimiter must be exactly two bytes long, the three start delimiters must differ from each other, and any delimiter left NULL is unchanged.

Delimiters must be set before any templates are added, so this can only be called on an engine with an empty library.

Usage
Tera$set_delimiters(
  block_start = NULL,
  block_end = NULL,
  variable_start = NULL,
  variable_end = NULL,
  comment_start = NULL,
  comment_end = NULL
)
Arguments
variable_start, variable_end

character scalar, delimiters for variables (defaults are "{{" and "}}").

comment_start, comment_end

character scalar, delimiters for comments (defaults are "{#" and "#}").

Returns

Self (invisibly)

Tera$delimiters()

List the delimiters currently used to mark up templates.

Usage
Tera$delimiters()
Returns

a named list of delimiters

Examples

# - setup -----
# initialize empty Tera engine
tera <- Tera$new()
tera

# initialize Tera engine from directory with glob
template_dir <- system.file("templates", package = "tera")
glob <- "**/*.html"
tera <- Tera$new(file.path(template_dir, glob))
tera

# add templates manually from file
tera <- Tera$new()
tera$add_file_templates(
  "base.html" = file.path(template_dir, "base.html"),
  "index.html" = file.path(template_dir, "index.html")
)
tera

# add templates manually from string
tera$add_string_templates(
  img = '<img src="{{ img_src }}">'
)
tera

# reload templates from glob
example_dir <- file.path(tempdir(), "tera-templates")
dir.create(example_dir)

file.copy(
  from = file.path(template_dir, "base.html"),
  to = file.path(example_dir, "base.html")
)

tera <- Tera$new(file.path(example_dir, "*.html"))
tera

file.copy(
  from = file.path(template_dir, "index.html"),
  to = file.path(example_dir, "index.html")
)

tera$reload()

# - rendering -----
tera <- Tera$new(file.path(template_dir, glob))

cat(
  readLines(file.path(template_dir, "index.html")),
  sep = "\n"
)

# render a template file
tera$render_template(
  "index.html",
  title = "Index",
  p = "Welcome to my awesome homepage.",
  owner = "Blake"
)

# render a template block
tera$render_template(
  "index.html",
  title = "Index",
  p = "Welcome to my awesome homepage.",
  owner = "Bob",
  block = "content"
)

# render a template string
tera$render_string(
  '<img src="{{ img_src }}">',
  img_src = "foo/bar.svg"
)

# render a component
cat(
  readLines(file.path(template_dir, "components.html")),
  sep = "\n"
)

tera$render_component(
  "button",
  label = "Primary Button",
  variant = "primary"
)

# render a component with body
tera$render_component(
  "widget",
  title = "foo",
  body = "<p>This is a widget!</p>"
)

# - inspection -----
# list all templates in library
tera$templates()

# list all variables in template
tera$variables("index.html")

# list all components in library
tera$components()

# - global context -----
# add variables to global context
tera$set_globals(
  base_url = "https://www.example.com",
  site_title = "My Website",
  description = "This is my awesome website!"
)

# list all variables in global context
tera$globals()

# clear global context
tera$clear_globals()
tera$globals()

# - autoescape -----
tera <- Tera$new()

template_dir <- system.file("templates", package = "tera")
tera$add_file_templates(
  "hello" = file.path(template_dir, "hello.html"),
  "hello.html" = file.path(template_dir, "hello.html")
)

# not recognized as html
tera$render_template(
  "hello",
  x = "&world",
  y = "an apostrophe, '"
)

# html
tera$render_template(
  "hello.html",
  x = "&world",
  y = "an apostrophe, '"
)

# turn off autoescape
tera$autoescape_off()

tera$render_template(
  "hello.html",
  x = "&world",
  y = "an apostrophe, '"
)

# - delimiters -----
# must be set before any templates are added
tera <- Tera$new()

tera$set_delimiters(
  variable_start = "<<",
  variable_end = ">>"
)

tera$delimiters()

tera$render_string("<< greeting >>, world!", greeting = "Hello")

One-Off Template Rendering

Description

For rendering a single template file or string, it may be preferable to use these one-off rendering options.

Usage

render_template(path, ..., outfile = NULL, autoescape = TRUE)

render_string(string, ..., outfile = NULL, autoescape = TRUE)

Arguments

path

character scalar, path to a template file

...

specify context as key-value pairs where key is the template variable and value is the data to inject.

outfile

character scalar, the path to file where template is to be rendered. If NULL (the default), it will render the template file to a string in the current R session.

autoescape

boolean scalar, whether to autoescape HTML (default is TRUE).

string

character scalar, the template string to render.

Details

Requires a path to a template file, not a template string.

Value

'outfile“ (invisibly)

Examples

tmp <- system.file("templates/hello.html", package = "tera")

# render to string
render_template(
  tmp,
  x = "world",
  y = "tera"
)

render_string(
  "<p>Hello {{ x }}. This is {{ y }}.</p>",
  x = "world",
  y = "tera"
)

# render to file
outfile <- file.path(tempdir(), "hello-rendered.html")
render_template(
  tmp,
  x = "world",
  y = "tera",
  outfile = outfile
)

readLines(outfile, warn = FALSE)