Lexicon Interoperability in ATProto

Yes, absolutely! This is one of the most powerful features of ATProto - lexicons are open standards that any app can implement. This enables true interoperability between different applications.

How Lexicon Sharing Works

1. Anyone Can Use Any Lexicon

// Your app can write records using BlueSky's lexicons
await agent.com.atproto.repo.createRecord({
  repo: userDid,
  collection: 'app.bsky.feed.post', // BlueSky's lexicon
  record: {
    text: 'Hello from my custom app!',
    createdAt: new Date().toISOString()
  }
})

// Or someone else's custom lexicon
await agent.com.atproto.repo.createRecord({
  repo: userDid,
  collection: 'com.otherapp.recipe', // Another app's lexicon
  record: {
    title: 'Chocolate Cake',
    ingredients: ['flour', 'sugar', 'cocoa'],
    instructions: 'Mix and bake...'
  }
})

2. Cross-App Compatibility Example

// App A defines a "book review" lexicon
const bookReviewLexicon = {
  "lexicon": 1,
  "id": "com.bookapp.review",
  "defs": {
    "main": {
      "type": "record",
      "record": {
        "type": "object",
        "properties": {
          "book": {"type": "string"},
          "rating": {"type": "integer"},
          "review": {"type": "string"}
        }
      }
    }
  }
}

// App B can read and write the same lexicon
class BookCompatibleApp {
  async publishReview(book: string, rating: number, review: string) {
    // Uses App A's lexicon
    await this.agent.com.atproto.repo.createRecord({
      repo: this.userDid,
      collection: 'com.bookapp.review',
      record: { book, rating, review }
    })
  }
  
  async showReviews() {
    // Reads reviews from ALL apps using this lexicon
    const reviews = await this.agent.com.atproto.repo.listRecords({
      repo: this.userDid,
      collection: 'com.bookapp.review'
    })
    return reviews
  }
}

Real-World Interoperability Scenarios

3. Social Media Compatibility

// Your specialized app can still participate in the broader social graph
class NicheApp {
  async publishPost(content: string) {
    // Publish using standard BlueSky lexicon
    await this.agent.com.atproto.repo.createRecord({
      repo: this.userDid,
      collection: 'app.bsky.feed.post',
      record: {
        text: content,
        createdAt: new Date().toISOString()
      }
    })
    
    // Also publish using your specialized lexicon
    await this.agent.com.atproto.repo.createRecord({
      repo: this.userDid,
      collection: 'com.nicheapp.specialpost',
      record: {
        content: content,
        category: 'technical',
        tags: ['programming', 'atproto']
      }
    })
  }
}

4. Building on Existing Ecosystems

// Photography app that's compatible with general social media
class PhotoApp {
  async sharePhoto(imageBlob: Blob, caption: string) {
    // Upload image first
    const imageRef = await this.uploadImage(imageBlob)
    
    // Create standard social media post
    await this.agent.com.atproto.repo.createRecord({
      repo: this.userDid,
      collection: 'app.bsky.feed.post',
      record: {
        text: caption,
        embed: {
          $type: 'app.bsky.embed.images',
          images: [{ image: imageRef, alt: caption }]
        },
        createdAt: new Date().toISOString()
      }
    })
    
    // Also create detailed photo metadata
    await this.agent.com.atproto.repo.createRecord({
      repo: this.userDid,
      collection: 'com.photoapp.photo',
      record: {
        image: imageRef,
        caption,
        camera: 'Canon EOS R5',
        settings: { iso: 100, aperture: 'f/2.8' },
        location: { lat: 40.7128, lng: -74.0060 }
      }
    })
  }
}

Cross-App Discovery

5. Multi-Lexicon App Views

class UniversalFeedApp {
  async buildUniversalFeed(userDid: string) {
    const follows = await this.getFollows(userDid)
    const allContent = []
    
    // Collect content from multiple lexicons
    for (const followDid of follows) {
      // Standard posts
      const posts = await this.getRecords(followDid, 'app.bsky.feed.post')
      
      // Music posts
      const musicPosts = await this.getRecords(followDid, 'com.musicapp.track')
      
      // Recipe posts
      const recipes = await this.getRecords(followDid, 'com.recipeapp.recipe')
      
      allContent.push(...posts, ...musicPosts, ...recipes)
    }
    
    return allContent.sort((a, b) => 
      new Date(b.createdAt) - new Date(a.createdAt)
    )
  }
}

Best Practices for Lexicon Compatibility

6. Multi-Lexicon Publishing Strategy

class CompatibleApp {
  async publishContent(content: any) {
    const records = []
    
    // Always publish to standard lexicons for broad compatibility
    if (content.type === 'text') {
      records.push({
        collection: 'app.bsky.feed.post',
        record: {
          text: content.text,
          createdAt: new Date().toISOString()
        }
      })
    }
    
    // Also publish to your specialized lexicon
    records.push({
      collection: 'com.yourapp.content',
      record: {
        ...content,
        appVersion: '1.0.0',
        metadata: this.generateMetadata(content)
      }
    })
    
    // Batch create all records
    await Promise.all(records.map(r => 
      this.agent.com.atproto.repo.createRecord({
        repo: this.userDid,
        ...r
      })
    ))
  }
}

7. Lexicon Discovery

// Discover what lexicons a user's repository supports
async function discoverUserCapabilities(userDid: string) {
  const collections = await agent.com.atproto.repo.listRecords({
    repo: userDid,
    limit: 1000
  })
  
  const supportedLexicons = new Set()
  collections.records.forEach(record => {
    supportedLexicons.add(record.uri.split('/')[4]) // Extract collection
  })
  
  return Array.from(supportedLexicons)
}

The Power of Open Standards

This interoperability means:

• Users own their data across all apps
• Network effects benefit everyone
• Specialized apps can still participate in the broader ecosystem
• Innovation happens at the application layer, not the protocol layer

So yes, you can absolutely build an app that reads and writes to other apps' lexicons, creating a truly interoperable social ecosystem!