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+# Configuration Metadata API
+
+## Summary
+
+Define a common metadata API for all mods that have configuration that can be modified in-game. These metadata should be sufficient to automatically
+generate user-friendly configuration screens for mods. In order to achieve this, this RFC defines metadata necessary to display configuration to the
+user and to convert user input into configuration values.
+
+
+## Motivation
+
+Mod configuration is a common problem that mods must solve. Defining an API within QSL that Quilt mods use to define configuration metadata
+unifies the configuration UI model among mods and allows mods to avoid reinventing the wheel. Exposing a common API to query config metadata
+allows config screens to be generated rather than relying on mods to manually construct screens using general-purpose GUI libraries.
+
+
+## Explanation
+
+### Definitions
+- **Setting**: A single value and its associated metadata.
+- **Category**: A logical grouping of Settings. A Setting may be a member of multiple Categories.
+- **Validation Group**: A logical grouping of Settings used for validation. A Setting may be a member of only one Validation Group.
+- **Value Converter**: An object that handles conversion between the value and UI state.
+- **Formatter**: A function that maps a value to Minecraft `Text` suitable for display.
+- **Setting Validator**: A function that is invoked to accept or reject the value of a single Setting.
+- **Group Validator**: A function that is invoked to accept or reject the set of values in a single Validation Group.
+- **Environment Policy**: A value that defines how a Setting interacts with the client and/or server.
+
+A new QSL module, `quilt_config_metadata`, contains the implementation of this API. The API is built on top of the Loader config API, specialized
+with metadata for Minecraft mods.
+
+### Settings API Detail
+
+A Setting has the following metadata.
+- Widget Type
+- Value Calculator
+- Formatter
+- Label
+- Tooltip
+- Categories
+- Group Validator
+- Environment Policy
+
+#### Widget Type
+
+```java
+public enum WidgetType {
+  TOGGLE,
+  CYCLE,
+  SELECT,
+  FREE_TEXT,
+  SLIDER,
+  NUMERIC,
+  UNORDERED_COLLECTION
+  ORDERED_COLLECTION;
+}
+
+MetadataType<WidgetType, ?> WIDGET_TYPE;
+```
+
+A Setting's Widget Type describes the type of widget that best represents the Setting.
+- `TOGGLE` widgets are binary, typically displayed as check boxes or buttons.
+- `CYCLE` widgets are small ordered lists of options, typically displayed as cycle buttons or radio buttons.
+- `SELECT` widgets are large unordered lists of options, typically displayed as a drop-down or searchable menu.
+- `FREE_TEXT` widgets are strings of text, typically displayed as a text box.
+- `SLIDER` widgets are imprecise numeric values, typically displayed as a slider.
+- `NUMERIC` widgets are precise numeric values, typically displayed as a text box.
+- `UNORDERED_COLLECTION` widgets are made up of a collection of values with no defined ordering, all with the same Widget Type.
+- `ORDERED_COLLECTION` widgets are made up of a sequence of values with a defined ordering, all with the same Widget Type.
+
+The most appropriate Widget Type for a given Setting depends on the Setting's underlying data type and on any constraints a Setting's
+value must obey.
+
+If a Setting's Widget Type is `UNORDERED_COLLECTION` or `ORDERED_COLLECTION`, additional data is stored `WIDGET_ELEMENT_TYPE`.
+
+```java
+MetadataType<WidgetType, ?> WIDGET_ELEMENT_TYPE;
+```
+
+#### Value Converter
+
+```java
+public interface ToggleValueConverter<T> {
+  Optional<T> widgetStateToValue(boolean widgetState, ValidationDiagnostics diagnostics);
+  boolean valueToWidgetState(T value);
+}
+
+MetadataType<ToggleValueConverter<?>, ?> TOGGLE_VALUE_CONVERTER;
+
+public interface CycleValueConverter<T> {
+  Optional<T> widgetStateToValue(int widgetState, ValidationDiagnostics diagnostics);
+  int valueToWidgetState(T value);
+  int valueCount();
+  int hiddenValueCount();
+}
+
+MetadataType<CycleValueConverter<?>, ?> CYCLE_VALUE_CONVERTER;
+
+// etc...
+```
+
+A Value Converter handles conversion between the stored value in a Setting and the canonical widget state. It also defines properties necessary for the widget to
+properly handle user input. For example, toggle widgets' state is a boolean value (checked or unchecked), while the stored values in the Setting may be any two
+values of any type.
+
+### Formatter
+
+```java
+@FunctionalInterface
+public interface Formatter<T> {
+  Text format(T value);
+}
+
+MetadataType<Formatter, ?> FORMATTER;
+```
+
+A Formatter is a function that converts a Setting's stored value into Text for displaying to players. This text is not meant to be mapped back to
+the value that produced it.
+
+#### Label
+
+```java
+MetadataType<Text, ?> LABEL;
+```
+
+A Setting's Label is the simple display name of a Setting.
+
+#### Tooltip
+
+```java
+@FunctionalInterface
+public interface TooltipSupplier<T> {
+  Text getTooltip(T value);
+}
+
+MetadataType<TooltipSupplier<?>, ?> TOOLTIP;
+```
+
+A Setting's tooltip is a brief description of the current state of the Setting, dependent on the Setting's value.
+
+#### Categories
+
+```java
+MetadataType<Collection<Identifier>, ?> CATEGORIES;
+```
+
+A Category is a logical grouping of Settings; for example, video, audio, accessibility, or gameplay. A Setting may be a member of any number
+of Categories. Categories are intended to be used to partition Settings into broad groupings for display purposes.
+
+This RFC defines a standard Category `c:accessibility`, to be used for Settings that are used to make a mod more accessible.
+
+#### Setting Validator
+
+Settings will use the existing `Constaint<T>` interface in Quilt Loader's configuration API to define Setting Validators.
+
+#### Group Validator
+
+```java
+@FunctionalInterface
+public interface GroupValidator {
+  void validate(ValidationDiagnostics diagnositics);
+  boolean equals(Object o);
+}
+
+MetadataType<GroupValidator, ?> GROUP_VALIDATOR;
+```
+
+A Group Validator is used to validate properties that pretain to the relationship among multiple Settings. For example, two Settings that
+represent the minimum and maximum value of a range may have a Group Validator that ensures the maximum is always greater than or equal to
+the minimum. It is intended that Group Validators that perform the same validation compare equal so that consumers of this API do not
+run validation logic multiple times.
+
+#### Environment Policy
+
+```java
+public enum EnvironmentPolicy {
+  CLIENT_ONLY,
+  SERVER_ONLY,
+  CLIENT_SYNCED,
+  SERVER_SYNCED
+}
+
+MetadataType<EnvironmentPolicy, ?> ENVIRONMENT_POLICY;
+```
+
+- `CLIENT_ONLY` Settings are specific to a player and are always editable.
+- `CLIENT_SYNCED` Settings are specific to a player and may be overridden by the server.
+  - If a `CLIENT_SYNCED` Setting is a member of the Category `c:accessibility`, then the API shall issue a warning.
+- `SERVER_ONLY` Settings are specific to a level and are not sent to the client from a dedicated server.
+- `SERVER_SYNCED` Settings are specific to a level and are sent to the client from a dedicated server
+
+## Drawbacks
+
+This API depends on (and is therefore limited by) Quilt Loader's configuration API.
+
+This API also uses Minecraft types. Therefore, mod configuration that uses this API cannot be constructed or queried before game initialization (for example,
+in mixin plugins or CHASM transformers). However, this type of configuration can't be changed during gameplay, so displaying it to users has limited
+benefits.
+
+## Rationale and Alternatives
+
+This RFC describes a common API between mods that define configuration and mods that consume configuration.
+This approach gives the most control to mods that wish to consume multiple mods' configuration, e.g. to construct a config screen,
+while avoiding including too many "batteries" that not all mods may use.
+
+### Alternatives
+
+Define an API for constructing config UI elements that is completely separate from the underlying config. This would give mods more freedom to choose
+specific UI representations.
+
+Do nothing. In the absence of a config API, mods will likely tend toward using fully featured GUI libraries for displaying configuration.
+
+## Prior Art
+
+- [ModConfig](https://github.com/kvverti/mod-config), a global config screen that extracts configuration metadata from mods that use Cloth UI.
+- [Fiber](https://github.com/FabLabsMC/fiber), a library that defines a rich config tree.
+
+
+## Unresolved Questions
+
+- How precisely should Value Converters define widget state?
+- Should Setting Validators be separate from Loader's `Constraint` type?
+
+### Implementation
+- Which other common formatter/validator/widget type instances should this API provide? E.g. enum, registry entry, `Identifier`, etc.
+- How strict should the API be on the well-formedness of metadata?
+
+### Out of Scope
+- Key binding configuration is considered out of scope for this API because Minecraft handle key bindings differently from other game options.
+- Creating a standard consumer of this API, e.g. a config screen generator may be specified in another RFC.