Introduce FacetReader

FEATURE: The new `FacetReader` type provides a way to export a read-only handle
to a `Facet`.

See https://discuss.codemirror.net/t/read-only-facets/7175

src/README.md

@@ -104,6 +104,8 @@ extensions for the editor state.
 
 @Facet
 
+@FacetReader
+
 @Prec
 
 @Compartment

src/facet.ts

@@ -37,7 +37,10 @@ type FacetConfig<Input, Output> = {
 /// size](#state.EditorState^tabSize), [editor
 /// attributes](#view.EditorView^editorAttributes), and [update
 /// listeners](#view.EditorView^updateListener).
-export class Facet<Input, Output = readonly Input[]> {
+///
+/// Note that `Facet` instances can be used anywhere where
+/// [`FacetReader`](#state.FacetReader) is expected.
+export class Facet<Input, Output = readonly Input[]> implements FacetReader<Output> {
   /// @internal
   readonly id = nextID++
   /// @internal
@@ -59,6 +62,10 @@ export class Facet<Input, Output = readonly Input[]> {
     this.extensions = typeof enables == "function" ? enables(this) : enables
   }
 
+  /// Returns a facet reader for this facet, which can be used to
+  /// [read](#state.EditorState.facet) it but not to define values for it.
+  get reader(): FacetReader<Output> { return this }
+
   /// Define a new facet.
   static define<Input, Output = readonly Input[]>(config: FacetConfig<Input, Output> = {}) {
     return new Facet<Input, Output>(config.combine || ((a: any) => a) as any,
@@ -102,13 +109,31 @@ export class Facet<Input, Output = readonly Input[]> {
     if (!get) get = x => x as any
     return this.compute([field], state => get!(state.field(field)))
   }
+
+  tag!: typeof FacetTag
+}
+
+declare const FacetTag: unique symbol
+
+/// A facet reader can be used to fetch the value of a facet, though
+/// [`EditorState.facet`](#state.EditorState.facet) or as a dependency
+/// in [`Facet.compute`](#state.Facet.compute), but not to define new
+/// values for the facet.
+export type FacetReader<Output> = {
+  /// @internal
+  id: number
+  /// @internal
+  default: Output
+  /// Dummy tag that makes sure TypeScript doesn't consider all object
+  /// types as conforming to this type.
+  tag: typeof FacetTag
 }
 
 function sameArray<T>(a: readonly T[], b: readonly T[]) {
   return a == b || a.length == b.length && a.every((e, i) => e === b[i])
 }
 
-type Slot<T> = Facet<any, T> | StateField<T> | "doc" | "selection"
+type Slot<T> = FacetReader<T> | StateField<T> | "doc" | "selection"
 
 const enum Provider { Static, Single, Multi }
 

src/index.ts

@@ -1,6 +1,6 @@
 export {EditorStateConfig, EditorState} from "./state"
 export {StateCommand} from "./extension"
-export {Facet, StateField, Extension, Prec, Compartment} from "./facet"
+export {Facet, FacetReader, StateField, Extension, Prec, Compartment} from "./facet"
 export {EditorSelection, SelectionRange} from "./selection"
 export {Transaction, TransactionSpec, Annotation, AnnotationType, StateEffect, StateEffectType} from "./transaction"
 export {combineConfig} from "./config"

src/state.ts

@@ -5,7 +5,8 @@ import {EditorSelection, SelectionRange, checkSelection} from "./selection"
 import {Transaction, TransactionSpec, resolveTransaction, asArray, StateEffect} from "./transaction"
 import {allowMultipleSelections, changeFilter, transactionFilter, transactionExtender,
         lineSeparator, languageData, readOnly} from "./extension"
-import {Configuration, Facet, Extension, StateField, SlotStatus, ensureAddr, getAddr, Compartment, DynamicSlot} from "./facet"
+import {Configuration, Facet, FacetReader, Extension, StateField, SlotStatus, ensureAddr, getAddr,
+        Compartment, DynamicSlot} from "./facet"
 import {CharCategory, makeCategorizer} from "./charcategory"
 
 /// Options passed when [creating](#state.EditorState^create) an
@@ -189,7 +190,7 @@ export class EditorState {
   }
 
   /// Get the value of a state [facet](#state.Facet).
-  facet<Output>(facet: Facet<any, Output>): Output {
+  facet<Output>(facet: FacetReader<Output>): Output {
     let addr = this.config.address[facet.id]
     if (addr == null) return facet.default
     ensureAddr(this, addr)