Recipe: File-based symbol import/export (#239)

hugo/content/docs/recipes/scoping/_index.md

@@ -23,10 +23,11 @@ In general, the way we resolve references is split into three phases of the docu
 - [Scope computation](/docs/reference/document-lifecycle#computing-scopes) determines which elements are reachable from a given position in your document.
 - Finally, the [linking phase](/docs/reference/document-lifecycle#linking) eagerly links each reference within a document to its target using your language's scoping rules.
 
-In this guide, we'll look at different scoping kinds and styles and see how we can achieve them using Langium:
+In this recipe, we'll look at different scoping kinds and styles and see how we can achieve them using Langium:
 
 1. [Qualified Name Scoping](/docs/recipes/scoping/qualified-name)
 2. [Class Member Scoping](/docs/recipes/scoping/class-member)
+3. [File-based scoping](/docs/recipes/scoping/file-based)
 
 Note that these are just example implementations for commonly used scoping methods.
 The scoping API of Langium is designed to be flexible and extensible for any kind of use case.

hugo/content/docs/recipes/scoping/file-based.md

@@ -0,0 +1,294 @@
+---
+title: "File-based scoping"
+weight: 300
+---
+
+## Goal
+
+By default, Langium will always expose all top-level AST elements to the global scope. That means they are visible to all other documents in your workspace. However, a lot of languages are better served with a JavaScript-like `import`/`export` mechanism:
+
+* Using `export` makes a symbol from the current file available for referencing from another file.
+* Using `import` allows to reference symbols for a different file.
+
+To make things easier I will modify the "Hello World" example from the [learning section](/docs/learn/workflow).
+
+## Step 1: Change the grammar
+
+First off, we are changing the grammar to support the `export` and the `import` statements. Let's take a look at the modified grammar:
+
+```langium
+grammar HelloWorld
+
+entry Model:
+    (
+        fileImports+=FileImport  //NEW: imports per file
+        | persons+=Person
+        | greetings+=Greeting
+    )*;
+
+FileImport: //NEW: imports of the same file are gathered in a list
+    'import' '{' 
+        personImports+=PersonImport (',' personImports+=PersonImport)* 
+    '}' 'from' file=STRING; 
+
+PersonImport:
+    person=[Person:ID] ('as' name=ID)?;
+
+Person:
+    published?='export'? 'person' name=ID; //NEW: export keyword
+
+type Greetable = PersonImport | Person
+
+Greeting:
+    'Hello' person=[Greetable:ID] '!';
+
+hidden terminal WS: /\s+/;
+terminal ID: /[_a-zA-Z][\w_]*/;
+terminal STRING: /"(\\.|[^"\\])*"|'(\\.|[^'\\])*'/;
+
+hidden terminal ML_COMMENT: /\/\*[\s\S]*?\*\//;
+hidden terminal SL_COMMENT: /\/\/[^\n\r]*/;
+```
+
+After changing the grammar you need to regenerate the abstract syntax tree (AST) and the language infrastructure. You can do that by running the following command:
+
+```bash
+npm run langium:generate
+```
+
+## Step 2: Exporting persons to the global scope
+
+The index manager shall get all persons that are marked with the export keyword. In Langium this is done by overriding the `ScopeComputation.getExports(…)` function. Here is the implementation:
+
+```typescript
+export class HelloWorldScopeComputation extends DefaultScopeComputation {
+    override async computeExports(document: LangiumDocument<AstNode>): Promise<AstNodeDescription[]> {
+        const model = document.parseResult.value as Model;
+        return model.persons
+            .filter(p => p.published)
+            .map(p => this.descriptions.createDescription(p, p.name));
+    }
+}
+```
+
+After that, you need to register the `HelloWorldScopeComputation` in the `HelloWorldModule`:
+
+```typescript
+export const HelloWorldModule: Module<HelloWorldServices, PartialLangiumServices & HelloWorldAddedServices> = {
+    //...
+    references: {
+        ScopeComputation: (services) => new HelloWorldScopeComputation(services)
+    }
+};
+```
+
+Having done this, will make all persons that are marked with the `export` keyword available to the other files through the index manager.
+
+## Step 3: Importing from specific files
+
+The final step is to adjust the cross-reference resolution by overriding the `DefaultScopeProvider.getScope(…)` function:
+
+```typescript
+export class HelloWorldScopeProvider extends DefaultScopeProvider {
+    override getScope(context: ReferenceInfo): Scope {
+        switch(context.container.$type as keyof HelloWorldAstType) {
+            case 'PersonImport':
+                if(context.property === 'person') {
+                    return this.getExportedPersonsFromGlobalScope(context);
+                }
+                break;
+            case 'Greeting':
+                if(context.property === 'person') {
+                    return this.getImportedPersonsFromCurrentFile(context);
+                }
+                break;
+        }
+        return EMPTY_SCOPE;
+    }
+    //...
+}
+```
+
+Do not forget to add the new service to the `HelloWorldModule`:
+
+```typescript
+export const HelloWorldModule: Module<HelloWorldServices, PartialLangiumServices & HelloWorldAddedServices> = {
+    //...
+    references: {
+        ScopeComputation: (services) => new HelloWorldScopeComputation(services),
+        ScopeProvider: (services) => new HelloWorldScopeProvider(services) //NEW!
+    }
+};
+```
+
+You noticed the two missing functions? Here is what they have to do.
+
+The first function (`getExportedPersonsFromGlobalScope(context)`) will take a look at the global scope and return all exported persons respecting the files that were touched by the file imports. Note that we are outputting all persons that are marked with the `export` keyword. The actual name resolution is done internally later by the linker.
+
+```typescript
+private getExportedPersonsFromGlobalScope(context: ReferenceInfo): Scope {
+    //get document for current reference
+    const document = AstUtils.getDocument(context.container);
+    //get model of document
+    const model = document.parseResult.value as Model;
+    //get URI of current document
+    const currentUri = document.uri;
+    //get folder of current document
+    const currentDir = dirname(currentUri.path);
+    const uris = new Set<string>();
+    //for all file imports of the current file
+    for (const fileImport of model.fileImports) {
+        //resolve the file name relatively to the current file
+        const filePath = join(currentDir, fileImport.file);
+        //create back an URI
+        const uri = currentUri.with({ path: filePath });
+        //add the URI to URI list
+        uris.add(uri.toString());
+    }
+    //get all possible persons from these files
+    const astNodeDescriptions = this.indexManager.allElements(Person, uris).toArray();
+    //convert them to descriptions inside of a scope
+    return this.createScope(astNodeDescriptions);
+}
+```
+
+The second function (`getImportedPersonsFromCurrentFile(context)`) will take a look at the current file and return all persons that are imported from other files.
+
+```typescript
+private getImportedPersonsFromCurrentFile(context: ReferenceInfo) {
+    //get current document of reference
+    const document = AstUtils.getDocument(context.container);
+    //get current model
+    const model = document.parseResult.value as Model;
+    //go through all imports
+    const descriptions = model.fileImports.flatMap(fi => fi.personImports.map(pi => {
+        //if the import is name, return the import
+        if (pi.name) {
+            return this.descriptions.createDescription(pi, pi.name);
+        }
+        //if import references to a person, return that person
+        if (pi.person.ref) {
+            return this.descriptions.createDescription(pi.person.ref, pi.person.ref.name);
+        }
+        //otherwise return nothing
+        return undefined;
+    }).filter(d => d != undefined)).map(d => d!);
+    return this.createScope(descriptions);
+}
+```
+
+## Result
+
+Now, let's test the editor by `npm run build` and starting the extension.
+Try using these two files. The first file contains the Simpsons family.
+
+```plain
+export person Homer
+export person Marge
+person Bart
+person Lisa
+export person Maggy
+```
+
+The second file tries to import and greet them.
+
+```plain
+import { 
+    Marge,
+    Homer,
+    Lisa, //reference error, because not exported
+    Maggy as Baby
+} from "persons.hello"
+
+Hello Lisa! //reference error, because no valid import
+Hello Maggy! //reference error, because name was overwritten with 'Baby'
+Hello Homer!
+Hello Marge!
+Hello Baby!
+```
+
+<details>
+<summary>Full Implementation</summary>
+
+```ts
+import { AstNode, AstNodeDescription, AstUtils, DefaultScopeComputation, DefaultScopeProvider, EMPTY_SCOPE, LangiumDocument, ReferenceInfo, Scope } from "langium";
+import { CancellationToken } from "vscode-languageclient";
+import { HelloWorldAstType, Model, Person } from "./generated/ast.js";
+import { dirname, join } from "node:path";
+
+export class HelloWorldScopeComputation extends DefaultScopeComputation {
+    override async computeExports(document: LangiumDocument<AstNode>): Promise<AstNodeDescription[]> {
+        const model = document.parseResult.value as Model;
+        return model.persons
+            .filter(p => p.published)
+            .map(p => this.descriptions.createDescription(p, p.name))
+        ;
+    }
+}
+
+export class HelloWorldScopeProvider extends DefaultScopeProvider {
+    override getScope(context: ReferenceInfo): Scope {
+        switch(context.container.$type as keyof HelloWorldAstType) {
+            case 'PersonImport':
+                if(context.property === 'person') {
+                    return this.getExportedPersonsFromGlobalScope(context);
+                }
+                break;
+            case 'Greeting':
+                if(context.property === 'person') {
+                    return this.getImportedPersonsFromCurrentFile(context);
+                }
+                break;
+        }
+        return EMPTY_SCOPE;
+    }
+
+    protected getExportedPersonsFromGlobalScope(context: ReferenceInfo): Scope {
+        //get document for current reference
+        const document = AstUtils.getDocument(context.container);
+        //get model of document
+        const model = document.parseResult.value as Model;
+        //get URI of current document
+        const currentUri = document.uri;
+        //get folder of current document
+        const currentDir = dirname(currentUri.path);
+        const uris = new Set<string>();
+        //for all file imports of the current file
+        for (const fileImport of model.fileImports) {
+            //resolve the file name relatively to the current file
+            const filePath = join(currentDir, fileImport.file);
+            //create back an URI
+            const uri = currentUri.with({ path: filePath });
+            //add the URI to URI list
+            uris.add(uri.toString());
+        }
+        //get all possible persons from these files
+        const astNodeDescriptions = this.indexManager.allElements(Person, uris).toArray();
+        //convert them to descriptions inside of a scope
+        return this.createScope(astNodeDescriptions);
+    }
+
+    private getImportedPersonsFromCurrentFile(context: ReferenceInfo) {
+        //get current document of reference
+        const document = AstUtils.getDocument(context.container);
+        //get current model
+        const model = document.parseResult.value as Model;
+        //go through all imports
+        const descriptions = model.fileImports.flatMap(fi => fi.personImports.map(pi => {
+            //if the import is name, return the import
+            if (pi.name) {
+                return this.descriptions.createDescription(pi, pi.name);
+            }
+            //if import references to a person, return that person
+            if (pi.person.ref) {
+                return this.descriptions.createDescription(pi.person.ref, pi.person.ref.name);
+            }
+            //otherwise return nothing
+            return undefined;
+        }).filter(d => d != undefined)).map(d => d!);
+        return this.createScope(descriptions);
+    }
+}
+```
+
+</details>