Configuring conformetry and writing templates

A generator pairs a name with a template — an ordinary folder of ordinary files — and the inputs needed to render it. The configuration declares generators; the template folder is the whole definition of what gets produced. The same template is then what instances are measured against, so a template is a standard as much as a starting point.

The configuration

The configuration file default-exports an array, one entry per generator. Only two fields are required:

Unknown keys are rejected, so a misspelled field — or aliases, which was removed outright — fails the load rather than quietly doing nothing. Check your entry took effect by listing the generators. Every conformetry command requires --config <path> — there is no default configuration file:

conformetry templates --config projects/configuration/conformetry.config.ts

A run without a path fails with a ConfigurationPathRequiredError naming the flag or option that was left unset. In Nx that is the configurationPath option of the @conformetry/nx plugin in nx.json, or the configuration option of the executor and the sync generator.

A generator with neither inputs nor instances is legal: it renders a fixed template that nothing validates afterwards.

Four collisions the configuration refuses to load with

1. Two generators may not share a name. A host resolves the first match, leaving the other unreachable.
2. Two generators may not name the same templatePath. Validation could not tell which generator a matching instance belongs to.
3. A name may not be empty or contain a path separator. It becomes a filename in the emitted Nx plugin.
4. A name may not be all. validate --templates all already means every template, and a generator answering to that word would make the same token mean two things on one command line.

Instance groups: where output already lives

instances is a list of groups, each with optional patterns, exclude, tags, substitutions, and threshold. Groups exist so substitutions — and the conformance bar — can differ per set of paths.

exclude lists workspace-relative globs for paths that are never instances. A directory or file the patterns found is dropped when an exclude glob matches it, with or without tags. Use it for vendored or generated code that sits among real instances, such as shadcn's component output: exclude: ["projects/lexico/lexico-web/src/components/ui/**"].

Under the Nx plugin, tags is what changes a group's meaning:

• A group with tags is a project selector. It applies to any project carrying one of those tags, and its patterns are read inside each selected project — so src/modules/* means the same thing in every project.
• A group with no tags is a plain workspace glob, used verbatim.
• A group with tags and no patterns selects without locating. That is how a generator scopes which projects it may target while validating nothing.

Directory patterns and file patterns behave differently, and the difference decides which template can win a match. See references/languages.md for what each language then compares.

Inputs

Each input is a JSON Schema fragment. These keys are honored; anything else is ignored:

The two hosts disagree about whether inputs are required, and the configuration cannot change that. The Nx plugin marks every declared input required. The command-line host marks none required. Write templates so that a missing value is obvious in the output rather than assuming either behavior.

Four input names carry extra meaning under the Nx plugin and are worth using deliberately rather than by accident: project, module, type, and directory all steer where output lands. Declaring an input called project on a generator whose groups carry tags additionally turns it into a project picker.

config, help, instancePath, and template are reserved and never treated as inputs — --template is how the command-line host selects which template to render. name is deliberately not reserved.

Hooks

A generator may declare hooks with preGenerate and postGenerate: plain functions the host runs around generation. Use one to refuse bad input before anything is written, or to edit a file the template cannot own, such as an index or a helm values file.

• preGenerate runs before anything is written, with an empty generatedFilePaths. Throw to refuse the run.
• postGenerate runs after the template is rendered and before formatting, with the sorted generatedFilePaths.

A configuration cannot declare hooks in JSON, because JSON cannot hold a function; any JavaScript or TypeScript configuration can. Type them with GeneratorHookContext from @conformetry/configuration. Three rules keep a hook correct under both hosts:

• Write through context.filesystem, never node:fs. Under Nx the adapter writes to the virtual tree, so --dry-run and a failed run stay clean.
• Give context.filesystem absolute paths inside the workspace, built from context.configurationDirectoryPath. A path outside is re-rooted inside the workspace under Nx.
• Push a path you wrote onto context.generatedFilePaths so the host formats and reports it.

On the command line a throwing postGenerate leaves the rendered files on disk. The generation README holds the full context table and the error behavior per host, and the hooks example runs both hooks.

Writing a template

The folder is the definition. Nothing registers a template's files.

A template that should produce a folder contains that folder. Its instance path is then the folder's parent. A template that produces loose files holds them at its root. Get this wrong and output lands one level off.

Placeholders

Rendering is mustache, with HTML escaping switched off — so write {{field}} and never {{{field}}}. Paths use the same braces as content: a file named {{nameKebabCase}}.service.ts renders to orders.service.ts.

Four naming-case variants are always available, derived from the name:

Explicit inputs and configured substitutions are applied last, so they always beat a derived variant of the same name.

Generation refuses a placeholder nobody supplied. It raises MissingSubstitutionError, naming the placeholder and the template file, so a typo fails loudly rather than leaving a silent hole or an empty path segment.

Validation reads a placeholder nobody supplied from the instance. It renders a random stand-in for the placeholder and compares as usual. Where a template node holding the stand-in aligns with an instance node, the instance's text at that spot is captured. The first capture wins, in file order and then document order. The template is rendered again with that value and compared again, so a later use holding different text is an ordinary difference. When nothing aligns, validation reports Could not infer {{path}}: no instance node aligned with the template text that uses it and the instance fails at any threshold.

• name and its variants are never inferred; they come from the instance path.
• A configured substitutions value always wins over inference.
• Capture follows each language's own matching: a TypeScript node key (a string literal, a declaration name such as class {{commandName}}Command, a call's first argument), a JSON value, a Markdown node's text such as a heading, a plain-text line. The literal text around the placeholder must match exactly. Python, Jupyter, comments, and file paths capture nothing, so a placeholder used only there must be supplied.

Two things are still permissive, and are how a template asks for something optional:

• Section tags. {{#owner}}Owner: {{owner}}{{/owner}} and {{^owner}}Unowned.{{/owner}} are conditionals, so an absent owner renders nothing rather than raising. Interpolate inside the section, never outside it.
• A supplied empty value. substitutions: { owner: "" } is an answer. Only an absent key is a hole.

Adding a placeholder to a template is therefore safe for validation only when every instance shows the value somewhere the comparison aligns. Otherwise add the substitution in the same change.

There is no conditional-file mechanism

No per-file predicate, no extension filtering, no skip list. Every file in the tree is rendered and written. Mustache sections work inside content, but a path rendered from an empty placeholder yields an empty segment rather than a skipped file. Do not go looking for a way to make a file optional; there isn't one.

Marking a value as free

A comment containing TODO is a placeholder: any comment in the instance satisfies it. Use it for prose that legitimately varies per instance.

There is no equivalent for string literals. A string in a template is required verbatim, so a placeholder description or message becomes a permanent requirement for every instance. Prefer leaving such a value out of the template to pinning wording nobody wants.

Thresholds

Conformance is scored rather than merely passed, and a threshold is the lowest score an instance may have. It resolves narrowest-first: an instance group's threshold, then the generator's, then a run-level --threshold, then 1.

Leave it unset unless you are migrating. The default of 1 means a perfect match, which is what makes a template a standard rather than a suggestion. Lower it deliberately when bringing existing instances onto a new template gradually, and raise it back as they catch up — a permanently lowered threshold is a template that no longer describes its instances.

A lowered threshold does not suppress reporting: differences still print for an instance that cleared its bar.

After changing the configuration

The Nx generator namespace is emitted from the configuration, and every conformetry command refuses to run while it is out of date rather than working from stale definitions:

nx sync

Two failures come from this area and both name their own fix:

• A file being out of date with the configuration — run nx sync.
• A generator naming a template folder that does not exist — fix templatePath, or create the folder.

A dependency install only warns rather than failing when emission does not work, so a mid-edit configuration never breaks everyone's install. That also means a warning there is real and worth reading.

Checking your work

Generate an instance, then check it conforms. A freshly generated instance conforms by construction, so any difference means the template and the configuration disagree — see the conformetry-validate skill for reading the result, and conformetry-generate for the generation step itself.

Seeing it rather than reading about it

conformetry-examples is twelve self-contained examples, each a complete configuration small enough to read in one sitting. The ones that answer configuration questions:

• hello-template — the minimum fields, and where a template is laid down.
• case-variants — the derived variants, and overriding one.
• scoring-thresholds — two instance groups at two thresholds, and which of the three levels wins.
• ambiguous-attribution — what two templates claiming one path reports, and why declaring the same glob twice is a mistake.
• nx-host — tag-scoped instance groups, and what a host without a project graph does with them.
• hooks — a preGenerate that refuses a bad name and a postGenerate that writes a receipt.
• failure-modes — a placeholder nobody supplied and no instance text reveals, refused by generation and reported by validation.

See its AGENTS.md for the full index.