Topiary

• Topiary web site
• Topiary playground

Topiary aims to be a uniform formatter for simple languages, as part of the Tree-sitter ecosystem. It is named after the art of clipping or trimming trees into fantastic shapes.

Topiary is designed for formatter authors and formatter users. Authors can create a formatter for a language without having to write their own formatting engine or even their own parser. Users benefit from uniform code style and, potentially, the convenience of using a single formatter tool, across multiple languages over their codebases, each with comparable styles applied.

Motivation

The style in which code is written has, historically, been mostly left to personal choice. Of course, this is subjective by definition and has led to many wasted hours reviewing formatting choices, rather than the code itself. Prescribed style guides were an early solution to this, spawning tools that lint a developer's formatting and ultimately leading to automatic formatters. The latter were popularised by gofmt, whose developers had the insight that "good enough" uniform formatting, imposed on a codebase, largely resolves these problems.

Topiary follows this trend by aspiring to be a "universal formatter engine", which allows developers to not only automatically format their codebases with a uniform style, but to define that style for new languages using a simple DSL. This allows for the fast development of formatters, providing a Tree-sitter grammar is defined for that language.

Design Principles

Topiary has been created with the following goals in mind:

• Use Tree-sitter for parsing, to avoid writing yet another grammar for a formatter.

• Expect idempotency. That is, formatting of already-formatted code doesn't change anything.

• For bundled formatting styles to meet the following constraints:

  • Be compatible with attested formatting styles used for that language in the wild.

  • Be faithful to the author's intent: if code has been written such that it spans multiple lines, that decision is preserved.

  • Minimise changes between commits such that diffs focus mainly on the code that's changed, rather than superficial artefacts. That is, a change on one line won't influence others, while the formatting won't force you to make later, cosmetic changes when you modify your code.

  • Be well-tested and robust, so that the formatter can be trusted in large projects.

• For end users -- i.e., not formatting style authors -- the formatter should:

  • Prescribe a formatting style that, while customisable, is uniform and "good enough" for their codebase.

  • Run efficiently.

  • Afford simple integration with other developer tools, such as editors and language servers.

Language Support

For now, the Tree-sitter grammars for the languages that Topiary targets are statically linked. The formatting styles for these languages come in two levels of maturity: supported and experimental.

Supported

These formatting styles cover their target language and fulfil Topiary's stated design goals. They are exposed, in Topiary, through a command line flag.

• OCaml (both implementations and interfaces)
• Nickel
• JSON
• TOML

Experimental

These languages' formatting styles are subject to change and/or not yet considered production-ready. They can be accessed in Topiary by specifying the path to their query files.

• Rust
• Bash
• Tree Sitter Queries

Getting Started

Installing

The project can be built and installed with Cargo from the repository directory:

cargo install --path topiary-cli

The TOPIARY_LANGUAGE_DIR variable can be set either at build time or runtime. It should point to the directory where Topiary's language query files (.scm) are located. Otherwise, Topiary will fall back to the languages directory in the current working directory.

See CONTRIBUTING.md for details on setting up a development environment.

Setting up as pre-commit hook

Topiary integrates seamlessly with pre-commit-hooks.nix: add Topiary as input to your flake and, in pre-commit-hooks.nix's setup, use:

pre-commit-check = nix-pre-commit-hooks.run {
  hooks = {
    nixfmt.enable = true; ## keep your normal hooks
    ...
    ## Add the following:
    topiary = topiary.lib.${system}.pre-commit-hook;
  };
};

Usage

topiary [OPTIONS] <--language <LANGUAGE>
                  |--input-file <INPUT_FILE>
                  |--query <QUERY>>

Options:

• -l, --language <LANGUAGE>
  Which language to parse and format [possible values: json, nickel, ocaml, ocaml-implementation, ocaml-interface, toml].

• -f, --input-file <INPUT_FILE>
  Path to an input file. If omitted, or equal to "-", read from standard input.

• -q, --query <QUERY>
  Which query file to use.

• -o, --output-file <OUTPUT_FILE>
  Path to an output file. If omitted, or equal to "-", write to standard output.

• -i, --in-place
  Format the input file in place. (This has the effect of setting the output file equal to the input file.)

• -v, --visualise
  Visualise the syntax tree, rather than format [possible values: json (default), dot].

• -s, --skip-idempotence
  Do not check that formatting twice gives the same output.

• -h, --help
  Print help information.

• -V, --version
  Print version information.

Language selection is based on precedence, in the following order:

• A specified language
• Detected from the input file's extension
• A specified query file

Exit Codes

The Topiary process will exit with a zero exit code upon successful formatting. Otherwise, the following exit codes are defined:

Reason	Code
Unspecified error	1
CLI argument parsing error	2
I/O error	3
Topiary query error	4
Source parsing error	5
Language detection error	6
Idempotency error	7
Unspecified formatting error	8

Example

Once built, the program can be run like this:

echo '{"foo":"bar"}' | topiary --language json

topiary can also be built and run from source via either Cargo or Nix, if you have those installed:

echo '{"foo":"bar"}' | cargo run -- --language json
echo '{"foo":"bar"}' | nix run . -- --language json

It will output the following formatted code:

{ "foo": "bar" }

Set the RUST_LOG=debug environment variable if you want to enable debug logging.

Design

As long as there is a Tree-sitter grammar defined for a language, Tree-sitter can parse it and provide a concrete syntax tree (CST). Tree-sitter will also allow us to run queries against this tree. We can make use of that to define how a language should be formatted. Here's an example query:

[
  (infix_operator)
  "if"
  ":"
] @append_space

This will match any node that the grammar has identified to be an infix_operator, as well as any anonymous node containing if or :. The match will be captured with the name @append_space. Our formatter runs through all matches and captures, and when we process any capture called @append_space, we will append a space after the matched node.

The formatter goes through the CST nodes and detects all that are spanning more than one line. This is interpreted to be an indication from the programmer who wrote the input that the node in question should be formatted as multi-line. Any other nodes will be formatted as single-line. Whenever a query match has inserted a softline, it will be expanded to a newline if the node is multi-line, or to a space or nothing if the node is single-line, depending on whether @append_spaced_softline or @append_empty_softline was used.

Before rendering the output, the formatter will do a number of cleanup operations, such as reducing consecutive spaces and newlines to one, trimming spaces at end of lines and leading and trailing blanks lines, and ordering indenting and newline instructions consistently.

This means that you can for example prepend and append spaces to if and true, and we will still output if true with just one space between the words.

Supported capture instructions

This assumes you are already familiar with the Tree-sitter query language.

Note that a capture is put after the node it is associated with. If you want to put a space in front of a node, you do it like this:

(infix_operator) @prepend_space

This, on the other hand, will not work:

@append_space (infix_operator)

Configuration

At the top of a query file you can set some configuration options like this:

; Configuration
(#language! rust)
(#indent! "    ")

The #language! pragma must be included in any query file and dictates which language to parse. The queries themselves will refer to node types that are specific to this language.

The #indent! pragma is optional and controls what indentation to use to indent a block whenever @append_indent_start or @prepend_indent_start is processed. If it is omitted, it defaults to two spaces.

@allow_blank_line_before

The matched nodes will be allowed to have a blank line before them, if specified in the input. For any other nodes, blank lines will be removed.

Example

; Allow comments and type definitions to have a blank line above them
[
  (comment)
  (type_definition)
] @allow_blank_line_before

@append_delimiter / @prepend_delimiter

The matched nodes will have a delimiter appended to them. The delimiter must be specified using the predicate #delimiter!.

Example

; Put a semicolon delimiter after field declarations, unless they already have
; one, in which case we do nothing.
(
  (field_declaration) @append_delimiter
  .
  ";"* @do_nothing
  (#delimiter! ";")
)

If there is already a semicolon, the @do_nothing instruction will be activated and prevent the other instructions in the query (the @append_delimiter, here) from applying. Otherwise, the ";"* captures nothing and in this case the associated instruction (@do_nothing) does not activate.

Note that @append_delimiter is the same as @append_space when the delimiter is set to " " (i.e., a space).

@append_multiline_delimiter / @prepend_multiline_delimiter

The matched nodes will have a multi-line-only delimiter appended to them. It will be printed only in multi-line nodes, and omitted in single-line nodes. The delimiter must be specified using the predicate #delimiter!.

Example

; Add a semicolon at the end of lists only if they are multi-line, to avoid [1; 2; 3;].
(list_expression
  (#delimiter! ";")
  (_) @append_multiline_delimiter
  .
  ";"? @do_nothing
  .
  "]"
  .
)

If there is already a semicolon, the @do_nothing instruction will be activated and prevent the other instructions in the query (the @append_multiline_delimiter, here) from applying. Likewise, if the node is single-line, the delimiter will not be appended either.

@append_empty_softline / @prepend_empty_softline

The matched nodes will have an empty softline appended or prepended to them. This will be expanded to a newline for multi-line nodes and to nothing for single-line nodes.

Example

; Put an empty softline before dots, so that in multi-line constructs we start
; new lines for each dot.
(_
  "." @prepend_empty_softline
)

@append_hardline / @prepend_hardline

The matched nodes will have a newline appended or prepended to them.

Example

; Consecutive definitions must be separated by line breaks
(
  (value_definition) @append_hardline
  .
  (value_definition)
)

@single_line_no_indent

The matched node will be printed alone, on a single line, with no indentation.

Example

(#language! ocaml)
; line number directives must be alone on their line, and can't be indented
(line_number_directive) @single_line_no_indent

@append_indent_start / @prepend_indent_start

The matched nodes will trigger indentation before or after them. This will only apply to lines following, until an indentation end is signalled. If indentation is started and ended on the same line, nothing will happen. This is useful, because we get the correct behaviour whether a node is formatted as single-line or multi-line. It is important that all indentation starts and ends are balanced.

Example

; Start an indented block after these
[
  "begin"
  "else"
  "then"
  "{"
] @append_indent_start

@append_indent_end / @prepend_indent_end

The matched nodes will trigger that indentation ends before or after them.

Example

; End the indented block before these
[
  "end"
  "}"
] @prepend_indent_end

; End the indented block after these
[
  (else_clause)
  (then_clause)
] @append_indent_end

@append_input_softline / @prepend_input_softline

The matched nodes will have an input softline appended or prepended to them. An input softline is a newline if the node has a newline in front of it in the input document, otherwise it is a space.

Example

; Input softlines before and after all comments. This means that the input
; decides if a comment should have line breaks before or after. But don't put a
; softline directly in front of commas or semicolons.

(comment) @prepend_input_softline

(
  (comment) @append_input_softline
  .
  [ "," ";" ]* @do_nothing
)

@append_space / @prepend_space

The matched nodes will have a space appended or prepended to them. Note that this is the same as @append_delimiter / @prepend_delimiter, with space as delimiter.

Example

[
  (infix_operator)
  "if"
  ":"
] @append_space

@append_antispace / @prepend_antispace

It is often the case that tokens need to be juxtaposed with spaces, except in a few isolated contexts. Rather than writing complicated rules that enumerate every exception, an "antispace" can be inserted with @append_antispace / @prepend_antispace; this will destroy any spaces (not newlines) from that node, including those added by other formatting rules.

Example

[
  ","
  ";"
  ":"
  "."
] @prepend_antispace

@append_spaced_softline / @prepend_spaced_softline

The matched nodes will have a spaced softline appended or prepended to them. This will be expanded to a newline for multi-line nodes and to a space for single-line nodes.

Example

; Append spaced softlines, unless there is a comment following.
(
  [
    "begin"
    "else"
    "then"
    "->"
    "{"
    ";"
  ] @append_spaced_softline
  .
  (comment)* @do_nothing
)

@delete

Remove the matched node from the output.

Example

; Move semicolon after comments.
(
  ";" @delete
  .
  (comment)+ @append_delimiter
  (#delimiter! ";")
)

@do_nothing

If any of the captures in a query match are @do_nothing, then the match will be ignored.

Example

; Put a semicolon delimiter after field declarations, unless they already have
; one, in which case we do nothing.
(
  (field_declaration) @append_delimiter
  .
  ";"* @do_nothing
  (#delimiter! ";")
)

Understanding the different newline captures

Type	Single-Line Context	Multi-Line Context
Hardline	Newline	Newline
Empty Softline	Nothing	Newline
Spaced Softline	Space	Newline
Input Softline	Input-Dependent	Input-Dependent

"Input softlines" are rendered as newlines whenever the targeted node follows a newline in the input. Otherwise, they are rendered as spaces.

Example

Consider the following JSON, which has been hand-formatted to exhibit every context under which the different newline capture names operate:

{
  "single-line": [1, 2, 3, 4],
  "multi-line": [
    1, 2,
    3
    , 4
  ]
}

We'll apply a simplified set of JSON format queries that:

1. Opens (and closes) an indented block for objects;
2. Each key-value pair gets its own line, with the value split onto a second;
3. Applies the different newline capture name on array delimiters.

That is, iterating over each @NEWLINE type, we apply the following:

(#language! json)

(object . "{" @append_hardline @append_indent_start)
(object "}" @prepend_hardline @prepend_indent_end .)
(object (pair) @prepend_hardline)
(pair . _ ":" @append_hardline)

(array "," @NEWLINE)

The first two formatting rules are just for clarity's sake. The last rule is what's important; the results of which are demonstrated below:

@append_hardline

{
  "single-line":
  [1,
  2,
  3,
  4],
  "multi-line":
  [1,
  2,
  3,
  4]
}

@prepend_hardline

{
  "single-line":
  [1
  ,2
  ,3
  ,4],
  "multi-line":
  [1
  ,2
  ,3
  ,4]
}

@append_empty_softline

{
  "single-line":
  [1,2,3,4],
  "multi-line":
  [1,
  2,
  3,
  4]
}

@prepend_empty_softline

{
  "single-line":
  [1,2,3,4],
  "multi-line":
  [1
  ,2
  ,3
  ,4]
}

@append_spaced_softline

{
  "single-line":
  [1, 2, 3, 4],
  "multi-line":
  [1,
  2,
  3,
  4]
}

@prepend_spaced_softline

{
  "single-line":
  [1 ,2 ,3 ,4],
  "multi-line":
  [1
  ,2
  ,3
  ,4]
}

@append_input_softline

{
  "single-line":
  [1, 2, 3, 4],
  "multi-line":
  [1, 2,
  3, 4]
}

@prepend_input_softline

{
  "single-line":
  [1 ,2 ,3 ,4],
  "multi-line":
  [1 ,2 ,3
  ,4]
}

Custom scopes and softlines

So far, we've expanded softlines into line breaks depending on whether the CST node they are associated with is multi-line. Sometimes, CST nodes define scopes that are either too big or too small for our needs. For instance, consider this piece of OCaml code:

(1,2,
3)

Its CST is the following:

{Node parenthesized_expression (0, 0) - (1, 2)} - Named: true
  {Node ( (0, 0) - (0, 1)} - Named: false
  {Node product_expression (0, 1) - (1, 1)} - Named: true
    {Node product_expression (0, 1) - (0, 4)} - Named: true
      {Node number (0, 1) - (0, 2)} - Named: true
      {Node , (0, 2) - (0, 3)} - Named: false
      {Node number (0, 3) - (0, 4)} - Named: true
    {Node , (0, 4) - (0, 5)} - Named: false
    {Node number (1, 0) - (1, 1)} - Named: true
  {Node ) (1, 1) - (1, 2)} - Named: false

We would want to add a line break after the first comma, but because the CST structure is nested, the node containing this comma (product_expression (0, 1) - (0, 4)) is not multi-line Only the top-level node product_expression (0, 1) - (1, 1) is multi-line.

To solve this issue, we introduce user-defined scopes and softlines.

@begin_scope / @end_scope

@begin_scope and @end_scope tags are used to define custom scopes. In conjunction with the #scope_id! predicate, they define scopes that can span multiple CST nodes, or only part of one. For instance, this scope matches anything between parenthesis in a parenthesized_expression:

(parenthesized_expression
  "(" @begin_scope
  ")" @end_scope
  (#scope_id! "tuple")
)

Scoped softlines

We have four predicates that insert softlines in custom scopes, in conjunction with the #scope_id! predicate:

• @prepend_empty_scoped_softline
• @prepend_spaced_scoped_softline
• @append_empty_scoped_softline
• @append_spaced_scoped_softline

When one of these scoped softlines is used, their behaviour depends on the innermost encompassing scope with the corresponding scope_id. If that scope is multi-line, the softline expands into a line break. In any other regard, they behave as their non-scoped counterparts.

Example

This Tree-sitter query:

(#language! ocaml)

(parenthesized_expression
  "(" @begin_scope @append_empty_softline @append_indent_start
  ")" @end_scope @prepend_empty_softline @prepend_indent_end
  (#scope_id! "tuple")
)

(product_expression
  "," @append_spaced_scoped_softline
  (#scope_id! "tuple")
)

...formats this piece of code:

(1,2,
3)

...as:

(
  1,
  2,
  3
)

...while the single-lined (1, 2, 3) is kept as is.

Testing context with predicates

Sometimes, similarly to what happens with softlines, we want a query to match only if the context is single-line, or multi-line. Topiary has several predicates that achieve this result.

#single_line_only! / #multi_line_only!

These predicates allow the query to trigger only if the matched nodes are in a single-line (resp. multi-line) context.

Example

; Allow (and enforce) the optional "|" before the first match case
; in OCaml if and only if the context is multi-line
(
  "with"
  .
  "|" @delete
  .
  (match_case)
  (#single_line_only!)
)

(
  "with"
  .
  "|"? @do_nothing
  .
  (match_case) @prepend_delimiter
  (#delimiter! "| ")
  (#multi_line_only!)
)

#single_line_scope_only! / #multi_line_scope_only!

These predicates allow the query to trigger only if the associated custom scope containing the matched nodes are is single-line (resp. multi-line).

Example

; Allow (and enforce) the optional "|" before the first match case
; in function expressions in OCaml if and only if the scope is multi-line
(function_expression
  (match_case)? @do_nothing
  .
  "|" @delete
  .
  (match_case)
  (#single_line_scope_only! "function_definition")
)
(function_expression
  "|"? @do_nothing
  .
  (match_case) @prepend_delimiter
  (#multi_line_scope_only! "function_definition")
  (#delimiter! "| ") ; sic
)

Suggested workflow

In order to work productively on query files, the following is one suggested way to work:

1. Add a sample file to topiary/tests/samples/input.

2. Copy the same file to topiary/tests/samples/expected, and make any changes to how you want the output to be formatted.

3. If this is a new language, add its Tree-sitter grammar, extend crate::language::Language and process it everywhere, then make a mostly empty query file with just the (#language!) configuration.

4. Run RUST_LOG=debug cargo test.

   Provided it works, it should output a lot of log messages. Copy that output to a text editor. You are particularly interested in the CST output that starts with a line like this: CST node: {Node compilation_unit (0, 0) - (5942, 0)} - Named: true.

   💡 As an alternative to using the debugging output, the --visualise command line option exists to output the Tree-sitter syntax tree in a variety of formats.

5. The test run will output all the differences between the actual output and the expected output, e.g. missing spaces between tokens. Pick a difference you would like to fix, and find the line number and column in the input file.

   💡 Keep in mind that the CST output uses 0-based line and column numbers, so if your editor reports line 40, column 37, you probably want line 39, column 36.

6. In the CST debug or visualisation output, find the nodes in this region, such as the following:

   [DEBUG atom_collection] CST node:   {Node constructed_type (39, 15) - (39, 42)} - Named: true
   [DEBUG atom_collection] CST node:     {Node type_constructor_path (39, 15) - (39, 35)} - Named: true
   [DEBUG atom_collection] CST node:       {Node type_constructor (39, 15) - (39, 35)} - Named: true
   [DEBUG atom_collection] CST node:     {Node type_constructor_path (39, 36) - (39, 42)} - Named: true
   [DEBUG atom_collection] CST node:       {Node type_constructor (39, 36) - (39, 42)} - Named: true
   

7. This may indicate that you would like spaces after all type_constructor_path nodes:

   (type_constructor_path) @append_space

   Or, more likely, you just want spaces between pairs of them:

   (
     (type_constructor_path) @append_space
     .
     (type_constructor_path)
   )

   Or maybe you want spaces between all children of constructed_type:

   (constructed_type
     (_) @append_space
     .
     (_)
   )

8. Run cargo test again, to see if the output is better now, and then return to step 5.

Syntax Tree Visualisation

To support the development of formatting queries, the Tree-sitter syntax tree for a given input can be produced using the --visualise CLI option.

This currently supports JSON output, covering the same information as the debugging output, as well as GraphViz DOT output, which is useful for generating syntax diagrams. (Note that the text position serialisation in the visualisation output is 1-based, unlike the debugging output's 0-based position.)

Terminal-Based Playground

Nix users may also find the playground.sh script to be helpful in aiding the interactive development of query files. When run in a terminal, it will format the given source input with the requested query file, updating the output on any inotify event against those files.

Usage: playground.sh (LANGUAGE | QUERY_FILE) [INPUT_SOURCE]

LANGUAGE can be one of the supported languages (e.g., "ocaml", "rust",
etc.); alternatively, give the path to the query file itself, as
QUERY_FILE.

The INPUT_SOURCE is optional. If not specified, it defaults to trying
to find the bundled integration test input file for the given language.

For example, the playground can be run in a tmux pane, with your editor of choice open in another.

Related Tools

Tree-Sitter Specific

• Syntax Tree Playground: An interactive, online playground for experimenting with Tree-sitter and its query language.
• Neovim Treesitter Playground: A Tree-sitter playground plugin for Neovim.
• Difftastic: A tool that utilises Tree-sitter to perform syntactic diffing.

Meta and Multi-Language Formatters

• format-all: A formatter orchestrator for Emacs.
• null-ls.nvim: An LSP framework for Neovim that facilitates formatter orchestration.
• prettier: A formatter with support for multiple (web-development related) languages.
• treefmt: A general formatter orchestrator, which unifies formatters under a common interface.

Related Formatters

• gofmt: The de facto standard formatter for Go, and major source of inspiration for the style of our formatters.
• ocamlformat: A formatter for OCaml.
• ocp-indent: A tool to indent OCaml code.
• Ormolu: Our formatter for Haskell, which follows similar design principles as Topiary.
• rustfmt: The de facto standard formatter for Rust.
• shfmt: A parser, formatter and interpreter for Bash et al.