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

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 the --language command line flag, or language detection (based on file extension).

• Bash
• JSON
• Nickel
• OCaml (both implementations and interfaces)
• OCamllex
• TOML
• Tree Sitter Queries

Contributed

These languages' formatting styles have been generously provided by external contributors. They are built in, by default, so are exposed in the same way as supported languages.

• CSS by @lavigneer

Experimental

These languages' formatting styles are subject to change and/or not yet considered production-ready. They are not built by default and are gated behind a feature flag (either experimental, for all of them, or by their individual name). Once included, they can be accessed in Topiary in the usual way.

• Rust

Getting Started

Installing

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

cargo install --path topiary-cli

Topiary needs to find the language query files (.scm) to function properly. By default, topiary looks for a languages directory in the current working directory.

This won't work if you are running Topiary from another directory than this repository. In order to use Topiary without restriction, you must set the environment variable TOPIARY_LANGUAGE_DIR to point to the directory where Topiary's language query files (.scm) are located. By default, you should set it to <local path of the topiary repository>/topiary-queries/queries, for example:

export TOPIARY_LANGUAGE_DIR=/home/me/tools/topiary/topiary-queries/queries
topiary fmt ./projects/helloworld/hello.ml

TOPIARY_LANGUAGE_DIR can alternatively be set at build time. Topiary will pick the correspond path up and embed it into the topiary binary. In that case, you don't have to worry about making TOPIARY_LANGUAGE_DIR available at run-time anymore. When TOPIARY_LANGUAGE_DIR has been set at build time and is set at run-time as well, the run-time value takes precedence.

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

The Topiary CLI uses a number of subcommands to delineate functionality. These can be listed with topiary --help; each subcommand then has its own, dedicated help text.

CLI app for Topiary, the universal code formatter.

Usage: topiary [OPTIONS] <COMMAND>

Commands:
  format      Format inputs
  visualise   Visualise the input's Tree-sitter parse tree
  config      Print the current configuration
  prefetch    Prefetch all languages in the configuration
  coverage    Checks how much of the tree-sitter query is used
  completion  Generate shell completion script
  help        Print this message or the help of the given subcommand(s)

Options:
  -C, --configuration <CONFIGURATION>  Configuration file [env: TOPIARY_CONFIG_FILE]
  -M, --merge-configuration            Enable merging for configuration files
  -v, --verbose...                     Logging verbosity (increased per occurrence)
  -h, --help                           Print help
  -V, --version                        Print version

Format

Format inputs

Usage: topiary format [OPTIONS] <--language <LANGUAGE>|FILES>

Arguments:
  [FILES]...
          Input files and directories (omit to read from stdin)

          Language detection and query selection is automatic, mapped from file extensions defined
          in the Topiary configuration.

Options:
  -t, --tolerate-parsing-errors
          Consume as much as possible in the presence of parsing errors

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

  -l, --language <LANGUAGE>
          Topiary language identifier (for formatting stdin)

  -q, --query <QUERY>
          Topiary query file override (when formatting stdin)

  -C, --configuration <CONFIGURATION>
          Configuration file

          [env: TOPIARY_CONFIG_FILE]

  -M, --merge-configuration
          Enable merging for configuration files

  -v, --verbose...
          Logging verbosity (increased per occurrence)

  -h, --help
          Print help (see a summary with '-h')

When formatting inputs from disk, language selection is detected from the input files' extensions. To format standard input, you must specify the --language and, optionally, --query arguments, omitting any input files.

Note: fmt is a recognised alias of the format subcommand.

Visualise

Visualise the input's Tree-sitter parse tree

Usage: topiary visualise [OPTIONS] <--language <LANGUAGE>|FILE>

Arguments:
  [FILE]
          Input file (omit to read from stdin)

          Language detection and query selection is automatic, mapped from file extensions defined
          in the Topiary configuration.

Options:
  -f, --format <FORMAT>
          Visualisation format

          [default: dot]

          Possible values:
          - dot:  GraphViz DOT serialisation
          - json: JSON serialisation

  -l, --language <LANGUAGE>
          Topiary language identifier (for formatting stdin)

  -q, --query <QUERY>
          Topiary query file override (when formatting stdin)

  -C, --configuration <CONFIGURATION>
          Configuration file

          [env: TOPIARY_CONFIG_FILE]

  -M, --merge-configuration
          Enable merging for configuration files

  -v, --verbose...
          Logging verbosity (increased per occurrence)

  -h, --help
          Print help (see a summary with '-h')

When visualising inputs from disk, language selection is detected from the input file's extension. To visualise standard input, you must specify the --language and, optionally, --query arguments, omitting the input file. The visualisation output is written to standard out.

Note: vis, visualize and view are recognised aliases of the visualise subcommand.

Configuration

Print the current configuration

Usage: topiary config [OPTIONS]

Options:
  -C, --configuration <CONFIGURATION>  Configuration file [env: TOPIARY_CONFIG_FILE]
  -M, --merge-configuration            Enable merging for configuration files
  -v, --verbose...                     Logging verbosity (increased per occurrence)
  -h, --help                           Print help

Note: cfg is a recognised alias of the config subcommand.

Shell Completion

Shell completion scripts for Topiary can be generated with the completion subcommand. The output of which can be sourced into your shell session or profile, as required.

Generate shell completion script

Usage: topiary completion [OPTIONS] [SHELL]

Arguments:
  [SHELL]  Shell (omit to detect from the environment) [possible values: bash, elvish, fish,
           powershell, zsh]

Options:
  -C, --configuration <CONFIGURATION>  Configuration file [env: TOPIARY_CONFIG_FILE]
  -M, --merge-configuration            Enable merging for configuration files
  -v, --verbose...                     Logging verbosity (increased per occurrence)
  -h, --help                           Print help

For example, in Bash:

source <(topiary completion)

Prefetching

Topiary dynamically downloads, builds, and loads the tree-sitter grammars. In order to ensure offline availability or speed up startup time, the grammars can be prefetched and compiled.

Prefetch all languages in the configuration

Usage: topiary prefetch [OPTIONS]

Options:
  -C, --configuration <CONFIGURATION>  Configuration file [env: TOPIARY_CONFIG_FILE]
  -M, --merge-configuration            Enable merging for configuration files
  -v, --verbose...                     Logging verbosity (increased per occurrence)
  -h, --help                           Print help

Coverage

This subcommand checks how much of the language query file is used to process the input. Specifically, it checks the percentage of queries in the query file that match the given input, And prints the queries that don't matched anything.

Checks how much of the tree-sitter query is used

Usage: topiary coverage [OPTIONS] <--language <LANGUAGE>|FILE>

Arguments:
  [FILE]
          Input file (omit to read from stdin)

          Language detection and query selection is automatic, mapped from file extensions defined
          in the Topiary configuration.

Options:
  -l, --language <LANGUAGE>
          Topiary language identifier (for formatting stdin)

  -q, --query <QUERY>
          Topiary query file override (when formatting stdin)

  -C, --configuration <CONFIGURATION>
          Configuration file

          [env: TOPIARY_CONFIG_FILE]

  -M, --merge-configuration
          Enable merging for configuration files

  -v, --verbose...
          Logging verbosity (increased per occurrence)

  -h, --help
          Print help (see a summary with '-h')

The coverage subcommand will exit with error code 1 if the coverage is less than 100%.

Logging

By default, the Topiary CLI will only output error messages. You can increase the logging verbosity with a respective number of -v/--verbose flags:

Verbosity Flag	Logging Level
None	Errors
-v	...and warnings
-vv	...and information
-vvv	...and debugging output
-vvvv	...and tracing output

Exit Codes

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

Reason	Code
Negative result	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
Multiple errors	9
Unspecified error	10

Negative results with error code 1 only happen when Topiary is called with the coverage sub-command, if the input does not cover 100% of the query.

When given multiple inputs, Topiary will do its best to process them all, even in the presence of errors. Should any errors occur, Topiary will return a non-zero exit code. For more details on the nature of these errors, run Topiary at the warn logging level (with -v).

Example

Once built, the program can be run like this:

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

It will output the following formatted code:

{ "foo": "bar" }

Configuration

Topiary is configured using languages.ncl files. The .ncl extension relates to Nickel, a configuration language created by Tweag. There are up to four sources where Topiary checks for such a file.

Configuration Sources

At build time the languages.ncl in the root of this repository is embedded into Topiary. This file is parsed at runtime. The purpose of this languages.ncl file is to provide sane defaults for users of Topiary (both the library and the binary).

The next two are read by the Topiary binary at runtime and allow the user to configure Topiary to their needs. The first is intended to be user specific, and can thus be found in the configuration directory of the OS:

OS	Typical Configuration Path
Unix	/home/alice/.config/topiary/languages.ncl
Windows	C:\Users\Alice\AppData\Roaming\Topiary\config\languages.ncl
macOS	/Users/Alice/Library/Application Support/Topiary/languages.ncl

This file is not automatically created by Topiary.

The next source is intended to be a project-specific settings file for Topiary. When running Topiary in some directory, it will ascend the file tree until it finds a .topiary directory. It will then read any languages.ncl file present in that directory.

Finally, an explicit configuration file may be specified using the -C/--configuration command line argument (or the TOPIARY_CONFIG_FILE environment variable). This is intended for driving Topiary under very specific use-cases.

The Topiary binary parses these sources in the following order.

1. The builtin configuration file.
2. The user configuration file in the OS's configuration directory.
3. The project specific Topiary configuration.
4. The explicit configuration file specified as a CLI argument.

Configuration Options

The configuration file contains a record of languages. For instance, the one for Nickel is defined as such:

nickel = {
  extensions = ["ncl"],
},

The name field is used by Topiary to associate the language entry with the query file and Tree-sitter grammar. This value should be written in lowercase.

The list of extensions is mandatory for every language, but does not necessarily need to exist in every configuration file. It is sufficient if, for every language, there is a single configuration file that defines the list of extensions for that language.

A final optional field, called indent, exists to define the indentation method for that language. Topiary defaults to two spaces " " if it cannot find the indent field in any configuration file for a specific language.

Specifying the grammar

Topiary can fetch and build the grammar for you, or a grammar can be provided by some other method. To have Topiary fetch the grammar for you, specify the grammar.source.git attribute of a language:

nickel = {
  extensions = ["ncl"],
  grammar.source.git = {
    git = "https://github.com/nickel-lang/tree-sitter-nickel",
    rev = "43433d8477b24cd13acaac20a66deda49b7e2547",
  },
},

To specify a prebuilt grammar, specify the grammar.source.path attribute, which must point to a compiled grammar file:

nickel = {
  extensions = ["ncl"],
  grammar.source.path = "/path/to/compiled/grammar/file.so",
},

:bulb: If you want to link to a grammar file that has already been compiled by Topiary itself, those look like <GIT_HASH>.so.

[!WARNING] As of Topiary version 0.5.1, the option to directly specify a compiled grammar isn't available on Windows.

For usage in Nix, a languages_nix.ncl file is provided that specifies the paths of every language using the @nickel@ syntax. These can easily be replaced with nixpkgs' substituteAll.

Overriding with --merge-configuration

By default, Topiary only considers the configuration file with the highest priority. However, if the -M or --merge-configuration option is provided to the CLI, then all available configurations are merged together, as per the Nickel specification.

In that case, if one of the sources listed above attempts to define a language configuration already present in the builtin configuration, or if two configuration files have conflicting values, then Topiary will display a Nickel error.

To understand why, one can read the Nickel documentation on Merging. The short answer is that a priority must be defined. The builtin configuration has everything defined with priority 0. Any priority above that will replace any other priority. For example, to override the entire Bash configuration, use the following Nickel file.

{
  languages = {
    bash | priority 1 = {
      extensions = [ "sh" ],
      indent = "    ",
    },
  },
}

To override only the indentation, use the following Nickel file:

{
  languages = {
    bash = {
      indent | priority 1 = "    ",
    },
  },
}

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.

A note on anchors

The behaviour of "anchors" can be counterintuitive. Consider, for instance, the following query:

(
  (list_entry) @append_space
  .
)

One might assume that this query only matches the final element in the list but this is not true. Since we did not explicitly march a parent node, the engine will match on every list_entry. After all, the when looking only at the nodes in the query, the list_entry is indeed the last node.

To resolve this issue, match explicitly on the parent node:

(list
  (list_entry) @append_space
  .
)

Or even implicitly:

(_
  (list_entry) @append_space
  .
)

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)

@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)
)

@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! ";")
)

@multi_line_indent_all

To be used on comments or other leaf nodes, to indicate that we should indent all its lines, not just the first.

Example

(#language! ocaml)
(comment) @multi_line_indent_all

@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

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.

@prepend_begin_scope / @append_begin_scope / @prepend_end_scope / @append_end_scope

These 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
  "(" @append_begin_scope
  ")" @prepend_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.

If we used @append_spaced_softline rather than @append_spaced_scoped_softline, the 1, would be followed by a space rather than a newline, because it's inside a single-line product_expression.

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
)

@prepend_begin_measuring_scope / @append_begin_measuring_scope / @prepend_end_measuring_scope / @append_end_measuring_scope

Sometimes, custom scopes are not enough: we may want to format a node depending on the multi-line-ness of a piece of code that does not include the node in question. For instance, consider this function application in OCaml:

foo bar (fun x -> qux)

We may also want to format it as any of the following two, depending on the actual length of foo, bar, and qux:

foo bar (fun x ->
  qux
)

foo
  bar
  (fun x ->
    qux
  )

Consider the indentation of (fun x -> qux): if foo bar is single-line, we don't want to indent it. But if foo bar is multi-line, we do want to indent it.

Because custom scopes can only impact the behaviour of nodes inside the scope, we can't use them to solve this issue. This is why we need measuring_scope.

Measuring scopes are opened/closed with a similar syntax as "regular" custom scopes, with any of the following tags, in conjunction with the #scope_id! predicate:

• @prepend_begin_measuring_scope
• @append_begin_measuring_scope
• @prepend_end_measuring_scope
• @prepend_begin_measuring_scope

Measuring scopes behave as follows:

• A measuring scope must always be contained in a regular custom scope with the same #scope_id!. There can't be two measuring scopes with the same #scope_id! inside the same regular custom scope.
• If a regular custom scope contains a measuring scope, then all tags contained in the regular scope that depend on its multi-line-ness will instead depend on the multi-line-ness of the measuring scope (hence the name: the inner, measuring scope measures the multi-line-ness of the outer, regular scope).

Example

The example below solves the problem of indenting function application in OCaml stated above, using measuring scopes.

(application_expression
  .
  (_) @append_indent_start @prepend_begin_scope @prepend_begin_measuring_scope
  (#scope_id! "function_application")
  (_) @append_end_scope
  .
)
; The end of the measuring scope depends on the last argument: if it's a function,
; end it before the function, otherwise end it after the last argument. In that case,
; it's the same as the regular scope.
(application_expression
  (#scope_id! "function_application")
  (_
    [
      (fun_expression)
      (function_expression)
    ]? @do_nothing
  ) @append_end_measuring_scope
  .
)
(application_expression
  (#scope_id! "function_application")
  (_
    [
      (fun_expression)
      (function_expression)
    ] @prepend_end_measuring_scope
  )
  .
)
; If the measuring scope is single-line, end indentation _before_ the last node.
; Otherwise, end the indentation after the last node.
(application_expression
  (#multi_line_scope_only! "function_application")
  (_) @append_indent_end
  .
)
(application_expression
  (#single_line_scope_only! "function_application")
  (_) @prepend_indent_end
  .
)

#query_name!

When the logging verbosity is set to -vv or higher, Topiary outputs information about which queries are matched, for instance:

[2024-10-08T15:48:13Z INFO  topiary_core::tree_sitter] Processing match: LocalQueryMatch { pattern_index: 17, captures: [ {Node "," (1,3) - (1,4)} ] } at location (286,1)

The predicate #query_name! takes a string argument, is optional, and can be added to any query. It will modify the log line to display its argument.

Example

Considering the log line above, and let us assume that the query at location (286,1) is:

(
  "," @append_space
  .
  (_)
)

If we add a query_name predicate:

(
  (#query_name! "comma spacing")
  "," @append_space
  .
  (_)
)

Then the log line will become:

[2024-10-08T15:48:13Z INFO  topiary_core::tree_sitter] Processing match of query "comma spacing": LocalQueryMatch { pattern_index: 17, captures: [ {Node "," (1,3) - (1,4)} ] } at location (286,1)

Add a new language

This section illustrates how to add a supported language to Topiary, provided it already has a tree-sitter grammar.

We will use C as the running example in this section.

Minimal steps

The two following steps are enough to jumpstart the formatting of a new language:

Register the grammar in topiary-config/languages.ncl:

    c = {
      extensions = ["c", "h"],
      grammar = {
        git = "https://github.com/tree-sitter/tree-sitter-c.git",
        rev = "6c7f459ddc0bcf78b615d3a3f4e8fed87b8b3b1b",
      },
    },

Create the query file

touch topiary-queries/queries/c.scm

Testing

You can now check that Topiary is able to "format" your new language with:

$ echo 'void main();' | cargo run -- format -s --language c
voidmain();

$ echo 'void main();' > foo.c && cargo run -- format -s foo.c && cat foo.c
voidmain();

Add the new language to the test suite

Create input/expected files

echo 'void main ();' > topiary-cli/tests/samples/input/c.c
echo 'voidmain();' > topiary-cli/tests/samples/expected/c.c

Add the Cargo feature flags

In topiary-cli/Cargo.toml

experimental = [
  "clang",
]

clang = ["topiary-config/clang", "topiary-queries/clang"]

In topiary-config/Cargo.toml

clang = []

all = [
  "clang",
]

In topiary-queries/Cargo.toml

clang = []

Add tests in topiary-cli/tests/sample-tester.rs

fn input_output_tester() {

[...]

    #[cfg(feature = "clang")]
    io_test("c.c");

[...]

fn coverage_tester() {

[...]

    #[cfg(feature = "clang")]
    coverage_test("c.c");

Testing

You should be able to successfully run the new tests with

cargo test --no-default-features -F clang -p topiary-cli --test sample-tester

Include the query file in Topiary at compile time

In topiary-queries/src/lib.rs

/// Returns the Topiary-compatible query file for C.
#[cfg(feature = "clang")]
pub fn c() -> &'static str {
    include_str!("../queries/c.scm")
}

In topiary-cli/src/io.rs

fn to_query<T>(name: T) -> CLIResult<QuerySource>

[...]

        #[cfg(feature = "clang")]
        "c" => Ok(topiary_queries::c().into()),

This will allow your query file to by considered as the default fallback query, when no other file can be found at runtime for your language.

Suggested workflow

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

1. If you're working on a new language, follow the steps in the previous section.

2. Add a snippet of code you want to format to topiary-cli/tests/samples/input/mylanguage.mlg.

3. Add the properly formatted version of the code to topiary-cli/tests/samples/expected/mylanguage.mlg.

4. Run:

   cargo test \
    --no-default-features \
    -F mylanguage \
    -p topiary-cli \
    input_output_tester \
    -- --nocapture

   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.

   :bulb: As an alternative to using the debugging output, the vis visualisation subcommand 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.

   :bulb: 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: ${PROGNAME} LANGUAGE [QUERY_FILE] [INPUT_SOURCE]

LANGUAGE can be one of the supported languages (e.g., "ocaml", "rust",
etc.). The packaged formatting queries for this language can be
overridden by specifying a 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.