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.

Key summary
NameTypeDescription
appendboolAdds raw to the end of tokens, assuming both are defined.
ignorecaseboolMakes all matches case-insensitive.
nonwordboolRemoves the default word boundaries (\b).
rawarrayA list of tokens to be concatenated into a pattern.
tokensarrayA list of tokens to be transformed into a non-capturing group.
exceptionsarrayAn array of strings to be ignored.
extends: existence
message: Consider removing '%s'
level: warning
code: false
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.

Key summary
NameTypeDescription
appendboolAdds raw to the end of tokens, assuming both are defined.
ignorecaseboolMakes all matches case-insensitive.
nonwordboolRemoves the default word boundaries (\b).
swapmapA sequence of observed: expected pairs.
exceptionsarrayAn array of strings to be ignored.
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

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.

Key summary
NameTypeDescription
maxintThe maximum amount of times token may appear in a given scope.
minintThe minimum amount of times token has to appear in a given scope.
tokenstringThe token of interest.
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.

Key summary
NameTypeDescription
ignorecaseboolMakes all matches case-insensitive.
alphaboolLimits all matches to alphanumeric tokens.
tokensarrayA list of tokens to be transformed into a non-capturing group.
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.

Key summary
NameTypeDescription
nonwordboolRemoves the default word boundaries (\b).
ignorecaseboolMakes all matches case-insensitive.
eitherarrayA map of option 1: option 2 pairs of which only one may appear.
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.

Key summary
NameTypeDescription
ignorecaseboolMakes all matches case-insensitive.
firststringThe antecedent of the statement.
secondstringThe consequent of the statement.
exceptionsarrayAn array of strings to be ignored.
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.

Key summary
NameTypeDescription
matchstring$title, $sentence, $lower, $upper, or a pattern.
stylestringAP or Chicago; only applies when match is set to $title.
exceptionsarrayAn array of strings to be ignored.
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

There are a few pre-defined variables that can be passed as matches:

  • $title: “The Quick Brown Fox Jumps Over the Lazy Dog.”
  • $sentence: “The quick brown fox jumps over the lazy dog.”
  • $lower: “the quick brown fox jumps over the lazy dog.”
  • $upper: “THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG.”

Additionally, when using match: $title, you can specify a style of either “AP” or “Chicago”.

metric

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

Key summary
NameTypeDescription
formulastringA formula of pre-defined variables to be evaluated.
conditionstringA binary condition upon which formula will trigger an alert.
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"

The table below summarizes all available variables:

VariableDescription
blockquoteThe number of blockquote tags.
charactersThe number of characters.
complex_wordsThe number of polysyllabic words without common suffixes (es, ed, ing, …).
heading.h{n}The number of headings at the specified level (for example, heading.h1).
listThe number of ol and ul tags.
long_wordsThe number of words with more than 6 characters.
paragraphsThe number of paragraphs.
polysyllabic_wordsThe number of words with more than 2 syllables.
preThe number of pre tags.
sentencesThe number of sentences.
syllablesThe number of syllables.
wordsThe number of words.

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:

OperatorDescription
+Addition
-Subtraction
*Multiplication
/Division
math.sqrt(x)Square root of x
math.abs(x)Absolute value of x

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.

Key summary
NameTypeDescription
customboolTurn off the default filters for acronyms, abbreviations, and numbers.
filtersarrayAn array of patterns to ignore during spell checking.
ignorestringA relative path (from StylesPath) to a file consisting of one word per line to ignore.
dicpathstringThe location to look for .dic and .aff files.
dictionariesarrayAn array of dictionaries to load.
# 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:

    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.

Key summary
NameTypeDescription
tokens[]NLPTokenA list of tokens with associated NLP metadata.
ignorecaseboolMakes all matches case-insensitive.
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.

script

script allows for the creation of arbitrary logic-based rules using Tengo, a Go-like scripting language.

Key summary
NameTypeDescription
scriptstringThe Tengo script to execute.
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
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
    }
  }  

All scripts have access to Tengo’s text module, which provides a number of string- and regex-related utility functions.

Additionally, all scripts are passed a global variable, scope, that contains the text of value of the rule’s scope:. In our example definition, for instance, scope will be the entire, unprocessed document since the rule used scope: raw.

See Scoping for more information.

Built-in style

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

RuleScopeLevelDescription
Vale.SpellingtexterrorSpell checks text while respecting the active project’s vocabulary.
Vale.TermstexterrorEnforces the current project’s Preferred vocabulary terms.
Vale.AvoidtexterrorEnforces the current project’s Do not use vocabulary terms.