Styles

Create your own rules to enforce custom style guides.

Overview #

Vale has a powerful extension system that doesn’t require knowledge of any programming language. Instead, it uses collections of individual YAML files (or “rules”) to enforce particular writing constructs.

# An example rule from the "Microsoft" style.
extends: existence
message: "Don't use end punctuation in headings."
link: https://docs.microsoft.com/en-us/style-guide/punctuation/periods
nonword: true
level: warning
scope: heading
action:
  name: edit
  params:
    - remove
    - ".?!"
tokens:
  - '[a-z0-9][.?!](?:\s|$)'

These collections are referred to as styles and are organized in a nested folder structure at a user-specified location (see Configuration). For example,

styles/
├── base/
│   ├── ComplexWords.yml
│   ├── SentenceLength.yml
│   ...
├── blog/
│   ├── TechTerms.yml
│   ...
└── docs/
    ├── Branding.yml
    ...

where base, blog, and docs are your styles that each contain certain rules.

Extension points #

The building blocks of styles are rules (YAML files ending in .yml), which utilize extension points to perform specific tasks.

The basic structure of a rule consists of a small header (shown below) followed by extension-specific arguments.

# All rules should define the following header keys:
#
# `extends` indicates the extension point being used (see below for information
# on the possible values).
extends: existence
# `message` is shown to the user when the rule is broken.
#
# Many extension points accept format specifiers (%s), which are replaced by
# extracted values. See the extension-specific sections below for more details.
message: "Consider removing '%s'"
# `level` assigns the rule's severity.
#
# The accepted values are suggestion, warning, and error.
level: warning
# `scope` specifies where this rule should apply -- e.g., headings, sentences, etc.
#
# See the Markup section for more information on scoping.
scope: heading
# `link` gives the source for this rule.
link: "https://errata.ai/"
# The number of times this rule should raise an alert.
#
# By default, there is no limit.
limit: 1

The available extension points are discussed below.

existence #

The most general extension point is existence. As its name implies, it looks for the “existence” of particular tokens.

extends: existence
message: Consider removing '%s'
level: warning
ignorecase: true
tokens:
  - appears to be
  - arguably

These tokens can be anything from simple phrases (as in the above example) to complex regular expressions—e.g., the number of spaces between sentences or the position of punctuation after quotes.

You may define the tokens as elements of lists named either tokens (shown above) or raw. The former converts its elements into a word-bounded, non-capturing group. For instance,

tokens:
  - appears to be
  - arguably

becomes \b(?:appears to be|arguably)\b.

raw, on the other hand, simply concatenates its elements—so, something like

raw:
  - '(?:foo)\sbar'
  - "(baz)"

becomes (?:foo)\sbar(baz).

substitution #

substitution associates a string with a preferred form.

extends: substitution
message: Consider using '%s' instead of '%s'
level: warning
ignorecase: false
# swap maps tokens in form of bad: good
swap:
  abundance: plenty
  accelerate: speed up
  # You can suggest multiple alternatives for a single token
  # by separating them with a pipe ("|").
  masterful: skilled|authoritative|commanding

If we want to suggest the use of “plenty” instead of “abundance,” for example, we’d write:

swap:
  abundance: plenty

The keys may be regular expressions, but they can’t include nested capture groups:

swap:
  "(?:give|gave) rise to": lead to # this is okay
  "(give|gave) rise to": lead to # this is bad!

substitution can have one or two %s format specifiers in its message. This allows us to do either of the following:

message: "Consider using '%s' instead of '%s'"
# or
message: "Consider using '%s'"

occurrence #

occurrence enforces the maximum or minimum number of times a particular token can appear in a given scope.

extends: occurrence
message: "More than 3 commas!"
level: error
# Here, we're counting the number of times a comma appears
# in a sentence.
#
# If it occurs more than 3 times, we'll flag it.
scope: sentence
ignorecase: false
max: 3
token: ","

In the example above, we’re limiting the number of commas per sentence. This is the only extension point that doesn’t accept a format specifier in its message.

repetition #

repetition looks for repeated occurrences of its tokens.

extends: repetition
message: "'%s' is repeated!"
level: error
alpha: true
tokens:
  - '[^\s.!?]+'

consistency #

consistency will ensure that a key and its value (e.g., “advisor” and “adviser”) don’t both occur in its scope.

extends: consistency
message: "Inconsistent spelling of '%s'"
level: error
scope: text
ignorecase: true
nonword: false
# We only want one of these to appear.
either:
  advisor: adviser
  centre: center

conditional #

conditional ensures that the existence of first implies the existence of second.

extends: conditional
message: "'%s' has no definition"
level: error
scope: text
ignorecase: false
# Ensures that the existence of 'first' implies the existence of 'second'.
first: '\b([A-Z]{3,5})\b'
second: '(?:\b[A-Z][a-z]+ )+\(([A-Z]{3,5})\)'
# ... with the exception of these:
exceptions:
  - ABC
  - ADD

For example, consider the following text:

According to Wikipedia, the World Health Organization (WHO) is a specialized agency of the United Nations that is concerned with international public health. We can now use WHO because it has been defined, but we can’t use DAFB because people may not know what it represents. We can use DAFB when it’s presented as code, though.

Using the above text with our example rule yields the following:

test.md:1:224:style.UnexpandedAcronyms:'DAFB' has no definition

conditional also takes an optional exceptions list. Any token listed as an exception won’t be flagged.

capitalization #

capitalization checks that the text in the specified scope matches the case of match.

extends: capitalization
message: "'%s' should be in title case"
level: warning
scope: heading
# $title, $sentence, $lower, $upper, or a pattern.
match: $title
style: AP # AP or Chicago; only applies when match is set to $title.
exceptions:
  - ABC
  - add

capitalization can have one or two %s format specifiers in its message. This allows us to do either of the following:

message: "Found: '%s'; expected: '%s'."
# or
message: "'%s' should use title-style capitalization."

metric #

metric enforces arbitrary formulas based on pre-defined, built-in variables.

extends: metric
message: "Try to keep the Flesch-Kincaid grade level (%s) below 8."
link: https://en.wikipedia.org/wiki/Flesch%E2%80%93Kincaid_readability_tests

formula: |
    (0.39 * (words / sentences)) + (11.8 * (syllables / words)) - 15.59

condition: "> 8.0"

The table below summarizes all available variables:

Since the pre-defined variables are calculated using the entire document, all metric-based rules are summary-scoped.

In addition to using the variables listed above, a formula may also use the following operators:

A condition may use one of >, <, ==, >=, and <=.

The result of a formula will be compared to its condition and inserted into its message format specifier (%s).

spelling #

spelling implements spell checking based on Hunspell-compatible dictionaries.

# Uses the built-in dictionary and filters.
extends: spelling
message: "Did you really mean '%s'?"
level: error

By default, spelling includes a custom, open-source dictionary for American English. You may instead use the dictionaries key to list multiple custom dictionaries:

extends: spelling
message: "'%s' is a typo!"
dicpath: ../../fixtures/spelling/dics
dictionaries:
  - en_US
  - en_medical

The spelling extension point will look for en_US.{dic,aff} and en_medical.{dic,aff} files in $DICPATH, which you can set through an environment variable or the dicpath key.

Ignoring non-dictionary words #

spelling offers two different ways of ignoring non-dictionary words:

1. Using ignore files: Ignore files are plain-text files that list words to be ignored during spell check (one case-insensitive entry per line) . For example,

   destructuring
   transpiler
   

   Here, we’re instructing spelling to ignore both [Dd]estructuring and [Tt]ranspiler.

   You can name these files anything you’d like and reference them relative to the active StylesPath/config/ignore directory.

   extends: spelling
   message: "Did you really mean '%s'?"
   level: error
   ignore:
     # Located at StylesPath/ignore1.txt
     - ignore1.txt
     - ignore2.txt
   

2. Using filters: You can also customize the spell-checking experience by defining filters, which are Go-compatible regular expressions to applied to individual words:

   extends: spelling
   message: "Did you really mean '%s'?"
   level: error
   # This disables the built-in filters. If you omit this
   # key or set it to false, custom filters (see below) are
   # added on top of the built-in ones.
   #
   # By default, filters for acronyms, abbreviations, and
   # numbers are included.
   custom: true
   # A "filter" is a regular expression specifying words
   # to ignore during spell checking.
   filters:
     # Ignore all words starting with 'py'.
     #
     # e.g., 'PyYAML'.
     - '[pP]y.*\b'
   

sequence #

While most extension points focus on writing style, sequence aims to support grammar-focused rules.

extends: sequence
# `%[4]s` is like `%s`, but specifically refers to the 4th token in our
# sequence.
message: "The infinitive '%[4]s' after 'be' requires 'to'. Did you mean '%[2]s %[3]s *to* %[4]s'?"
tokens:
  - tag: MD
  - pattern: be
  - tag: JJ
  # The `|` notation means that we'll accept `VB` or `VBN` in position 4.
  - tag: VB|VBN

Every sequence-based rule is required to have at least one pattern (such as pattern: be, shown above). This becomes the “anchor” of the sequence: we find all instances of the first pattern and then check that the left- and right-hand sides of the sequence match.

Each entry in a sequence is known as an NLPToken and has the following structure:

# [optional]: A regular expression (required
# if `tag` isn't given).
pattern: "..."

# [optional]: If true, indicates that we
# *shouldn't* match this token.
negate: true # or false

# [optional]: A part-of-speech tag (required
# if `pattern` isn't given).
tag: "..."

# [optional]: An integer meaning that there may
# be up to `n` (3, in this case) tokens between
# this token and the next one.
skip: 3

sequence-based are sentence-scoped. See prose/tagging for a full list of supported part-of-speech tags.

script #

Use script to create arbitrary logic-based rules using Tengo, a Go-like scripting language.

In each script:

1. Use Tengo’s text module, which provides a number of string- and regex-related utility functions.

2. Process the content in the scope variable. The scope string contains text based on the scope: <scope> setting for the rule. scope: raw provides the entire unprocessed contents of the file. For more information, see Scoping.

3. Populate the matches array with rule matches. Each match must be a map with the keys:

   • begin: where the match begins in the content provided by the scope variable.
   • end: where the match ends in the content provided by the scope variable.

   For more information about map values, refer to Tengo Language Syntax | Map values.

extends: script
message: "Consider inserting a new section heading at this point."
link: https://tengolang.com/
# The unprocessed file contents.
#
# We need this to access heading markup.
scope: raw
# NOTE: You may also reference a file containing the script that is stored in
# `<StylesPath>/config/scripts`.
#
# For example: `script: MyScript.tengo`.
script: |
  text := import("text")
  matches := []
  p_limit := 3 // at most 3 paragraphs per section
  // Remove all instances of code blocks since we don't want to count
  // inter-block newlines as a new paragraph.
  document := text.re_replace("(?s) *(\n```.*?```\n)", scope, "")
  count := 0
  for line in text.split(document, "\n") {
    if text.has_prefix(line, "#") {
      count = 0 // New section; reset count
    } else if count > p_limit {
      start := text.index(scope, line)
      matches = append(matches, {begin: start, end: start + len(line)})
      count = 0
    } else if text.trim_space(line) == "" {
      count += 1
    }
  }  

Built-in style #

Vale comes with a single built-in style named Vale that implements three rules, as described in the table below.