Added AI skills
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Jose Selesan
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---
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id: solid-isp-interfaces
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title: SOLID - Interface Segregation (Small Interfaces)
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category: solid-principles
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priority: critical
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tags: [SOLID, ISP, interface-segregation, cohesion]
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related: [solid-isp-clients, solid-srp-class, core-separation-concerns]
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---
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# Interface Segregation Principle - Small Cohesive Interfaces
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Interfaces should be small and cohesive, grouping only closely related methods. Split large interfaces into smaller, more focused ones.
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## Bad Example
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```typescript
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// Anti-pattern: Large interface with unrelated methods grouped together
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interface Document {
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// Content operations
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getContent(): string;
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setContent(content: string): void;
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appendContent(content: string): void;
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// Persistence operations
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save(): Promise<void>;
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load(id: string): Promise<void>;
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delete(): Promise<void>;
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// Export operations
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exportToPdf(): Promise<Buffer>;
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exportToWord(): Promise<Buffer>;
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exportToHtml(): Promise<string>;
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// Collaboration operations
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share(userId: string): Promise<void>;
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getCollaborators(): Promise<User[]>;
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addComment(comment: Comment): Promise<void>;
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getComments(): Promise<Comment[]>;
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// Version control
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createVersion(): Promise<Version>;
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getVersionHistory(): Promise<Version[]>;
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revertToVersion(versionId: string): Promise<void>;
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// Permissions
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setPermissions(permissions: Permissions): Promise<void>;
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getPermissions(): Promise<Permissions>;
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checkPermission(userId: string, action: string): Promise<boolean>;
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}
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// Simple note-taking app forced to implement everything
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class SimpleNote implements Document {
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private content: string = '';
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getContent(): string { return this.content; }
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setContent(content: string): void { this.content = content; }
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appendContent(content: string): void { this.content += content; }
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// Forced to implement methods it doesn't need
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async save(): Promise<void> { throw new Error('Not supported'); }
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async load(id: string): Promise<void> { throw new Error('Not supported'); }
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async delete(): Promise<void> { throw new Error('Not supported'); }
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async exportToPdf(): Promise<Buffer> { throw new Error('Not supported'); }
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async exportToWord(): Promise<Buffer> { throw new Error('Not supported'); }
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async exportToHtml(): Promise<string> { throw new Error('Not supported'); }
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async share(userId: string): Promise<void> { throw new Error('Not supported'); }
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async getCollaborators(): Promise<User[]> { throw new Error('Not supported'); }
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async addComment(comment: Comment): Promise<void> { throw new Error('Not supported'); }
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async getComments(): Promise<Comment[]> { throw new Error('Not supported'); }
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async createVersion(): Promise<Version> { throw new Error('Not supported'); }
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async getVersionHistory(): Promise<Version[]> { throw new Error('Not supported'); }
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async revertToVersion(versionId: string): Promise<void> { throw new Error('Not supported'); }
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async setPermissions(permissions: Permissions): Promise<void> { throw new Error('Not supported'); }
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async getPermissions(): Promise<Permissions> { throw new Error('Not supported'); }
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async checkPermission(userId: string, action: string): Promise<boolean> { throw new Error('Not supported'); }
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}
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```
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## Good Example
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```typescript
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// Correct approach: Small, cohesive interfaces
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// Core content interface - the essence of a document
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interface DocumentContent {
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getContent(): string;
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setContent(content: string): void;
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}
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// Extended content operations
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interface EditableContent extends DocumentContent {
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appendContent(content: string): void;
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insertContent(position: number, content: string): void;
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deleteRange(start: number, end: number): void;
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}
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// Persistence operations
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interface Persistable {
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save(): Promise<void>;
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load(id: string): Promise<void>;
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}
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interface Deletable {
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delete(): Promise<void>;
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}
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// Export capabilities
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interface PdfExportable {
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exportToPdf(): Promise<Buffer>;
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}
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interface WordExportable {
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exportToWord(): Promise<Buffer>;
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}
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interface HtmlExportable {
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exportToHtml(): Promise<string>;
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}
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// Combine export interfaces when needed
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interface FullyExportable extends PdfExportable, WordExportable, HtmlExportable {}
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// Collaboration interfaces
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interface Shareable {
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share(userId: string, permission: Permission): Promise<void>;
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unshare(userId: string): Promise<void>;
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getCollaborators(): Promise<Collaborator[]>;
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}
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interface Commentable {
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addComment(comment: Comment): Promise<void>;
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removeComment(commentId: string): Promise<void>;
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getComments(): Promise<Comment[]>;
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}
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// Version control
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interface Versionable {
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createVersion(message?: string): Promise<Version>;
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getVersionHistory(): Promise<Version[]>;
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revertToVersion(versionId: string): Promise<void>;
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}
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// Permissions
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interface PermissionControlled {
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setPermissions(permissions: Permissions): Promise<void>;
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getPermissions(): Promise<Permissions>;
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checkPermission(userId: string, action: string): Promise<boolean>;
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}
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// Simple note only implements what it needs
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class SimpleNote implements DocumentContent {
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private content: string = '';
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getContent(): string {
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return this.content;
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}
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setContent(content: string): void {
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this.content = content;
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}
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}
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// Persistent note adds storage
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class PersistentNote implements EditableContent, Persistable {
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private content: string = '';
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private id: string | null = null;
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constructor(private storage: StorageService) {}
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getContent(): string { return this.content; }
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setContent(content: string): void { this.content = content; }
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appendContent(content: string): void { this.content += content; }
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insertContent(position: number, content: string): void {
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this.content = this.content.slice(0, position) + content + this.content.slice(position);
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}
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deleteRange(start: number, end: number): void {
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this.content = this.content.slice(0, start) + this.content.slice(end);
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}
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async save(): Promise<void> {
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this.id = await this.storage.save(this.content);
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}
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async load(id: string): Promise<void> {
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this.content = await this.storage.load(id);
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this.id = id;
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}
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}
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// Full-featured collaborative document
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class CollaborativeDocument implements
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EditableContent,
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Persistable,
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Deletable,
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FullyExportable,
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Shareable,
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Commentable,
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Versionable,
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PermissionControlled
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{
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constructor(
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private storage: StorageService,
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private exportService: ExportService,
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private collaborationService: CollaborationService,
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private versionService: VersionService,
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private permissionService: PermissionService
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) {}
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// Implement all methods with actual functionality
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// Each service handles its domain
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getContent(): string { /* ... */ }
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setContent(content: string): void { /* ... */ }
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appendContent(content: string): void { /* ... */ }
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insertContent(position: number, content: string): void { /* ... */ }
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deleteRange(start: number, end: number): void { /* ... */ }
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async save(): Promise<void> { /* delegate to storage */ }
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async load(id: string): Promise<void> { /* delegate to storage */ }
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async delete(): Promise<void> { /* delegate to storage */ }
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async exportToPdf(): Promise<Buffer> { return this.exportService.toPdf(this); }
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async exportToWord(): Promise<Buffer> { return this.exportService.toWord(this); }
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async exportToHtml(): Promise<string> { return this.exportService.toHtml(this); }
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async share(userId: string, permission: Permission): Promise<void> { /* ... */ }
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async unshare(userId: string): Promise<void> { /* ... */ }
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async getCollaborators(): Promise<Collaborator[]> { /* ... */ }
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async addComment(comment: Comment): Promise<void> { /* ... */ }
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async removeComment(commentId: string): Promise<void> { /* ... */ }
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async getComments(): Promise<Comment[]> { /* ... */ }
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async createVersion(message?: string): Promise<Version> { /* ... */ }
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async getVersionHistory(): Promise<Version[]> { /* ... */ }
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async revertToVersion(versionId: string): Promise<void> { /* ... */ }
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async setPermissions(permissions: Permissions): Promise<void> { /* ... */ }
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async getPermissions(): Promise<Permissions> { /* ... */ }
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async checkPermission(userId: string, action: string): Promise<boolean> { /* ... */ }
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}
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// Functions can accept only the interfaces they need
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function renderDocument(doc: DocumentContent): void {
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console.log(doc.getContent());
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}
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async function exportToPdf(doc: PdfExportable): Promise<void> {
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const pdf = await doc.exportToPdf();
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// Send pdf...
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}
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function canUserEdit(doc: PermissionControlled, userId: string): Promise<boolean> {
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return doc.checkPermission(userId, 'edit');
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}
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// All document types work where their interfaces are expected
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const simpleNote = new SimpleNote();
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renderDocument(simpleNote); // Works!
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const collabDoc = new CollaborativeDocument(/* ... */);
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renderDocument(collabDoc); // Works!
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await exportToPdf(collabDoc); // Works!
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await canUserEdit(collabDoc, 'user123'); // Works!
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```
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## Why
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1. **Cohesion**: Each interface groups related methods. `Commentable` is about comments, `Versionable` about versions.
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2. **Flexibility**: Classes implement only the interfaces that match their capabilities.
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3. **Composability**: Complex types are built by combining simple interfaces: `EditableContent & Persistable & Shareable`.
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4. **No Empty Implementations**: No need for `throw new Error('Not supported')` - just don't implement the interface.
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5. **Clear Capabilities**: Looking at a class's implemented interfaces tells you exactly what it can do.
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6. **Easier Evolution**: Add new capabilities by creating new interfaces without modifying existing ones.
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7. **Better Typing**: Functions declare exactly what they need. `exportToPdf(doc: PdfExportable)` is self-documenting.
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