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2026-04-09 | 📦 Vertical Module Design: Think Like a Library Developer 🧩

🎯 The Mission

🧱 Our Haskell automation project had a monolithic Types module exporting over 40 symbols. 🔪 In a first attempt at breaking it up, we created three new modules: Credentials, Embed, and Platform. 🚨 But code review revealed these were still horizontal slices, grouping things by artifact kind rather than by feature.

💡 The Key Insight

🤔 The reviewer asked a powerful question: if an arbitrary project wants to post to Bluesky, does it also necessarily need Mastodon credentials, Gemini model constants, and environment variables for our application? 📚 The answer, obviously, is no. 🏗️ Each module should be designed as if it could become its own package.

🔀 Horizontal vs Vertical Slicing

🚫 Horizontal slicing groups code by artifact kind. 📂 A Credentials module puts all credential types together regardless of which feature they belong to. 📂 An Embed module puts all embed-related types together even though they serve different platforms.

✅ Vertical slicing groups code by feature. 🐦 The Twitter module owns TwitterCredentials, twitterLimits, tweetSectionHeader, TweetResult, and the posting API. 🦋 The Bluesky module owns BlueskyCredentials, blueskyLimits, EmbedResult, LinkCard, and the posting API. 🐘 The Mastodon module owns everything Mastodon.

🏗️ What Changed

🐦 Automation.Platforms.Twitter

🔑 TwitterCredentials moved from Credentials to Twitter. 📊 twitterLimits, twitterHandle, twitterDisplayName, and tweetSectionHeader moved from Platform to Twitter. 🎯 Now the Twitter module is completely self-contained: credential types, platform constants, result types, and API interaction code all in one place.

🦋 Automation.Platforms.Bluesky

🔑 BlueskyCredentials moved from Credentials to Bluesky. 📊 blueskyLimits, blueskyDisplayName, blueskySectionHeader, and the oEmbed delay constants moved from Platform to Bluesky. 🎨 EmbedResult and LinkCard moved from Embed to Bluesky because those types are only used for Bluesky link card embeds.

🐘 Automation.Platforms.Mastodon

🔑 MastodonCredentials moved from Credentials to Mastodon. 📊 mastodonLimits, mastodonDisplayName, and mastodonSectionHeader moved from Platform to Mastodon.

🖼️ Automation.Platforms.OgMetadata

🏷️ The OgMetadata type moved from Embed to OgMetadata, right next to the functions that construct it.

🤖 Automation.Gemini

🔑 GeminiConfig and all five Gemini model constants moved from Credentials to Gemini. 🏠 The Gemini module already owned the API interaction code, so the config type naturally belongs there.

🔧 Automation.Env

📋 EnvironmentConfig moved to Env, since it is application-level config that assembles platform-specific types for the running application. 🏗️ It references credential types from each platform module.

🗑️ Deleted Modules

🚫 Automation.Credentials is deleted entirely. 🚫 Automation.Embed is deleted entirely. 🔨 Both were horizontal slices that grouped unrelated types by artifact kind. 📐 Automation.Platform is slimmed to just PlatformLimits (the shared type definition) and updatesSectionHeader (which is not platform-specific).

✨ The Payoff

🧹 If we ever remove Twitter support, we delete the Twitter module and update a few imports in files that reference Twitter. 🚫 No need to edit a Credentials module, a Platform module, or an Embed module. 📦 If we ever extract Bluesky posting into its own library, the Bluesky module is already self-contained with all its types and constants.

📏 AGENTS.md Updates

📝 Two new rules were added to the Haskell Architecture Best Practices section. 📦 The first is library-developer module design: design every module as if it could become its own package. 🔀 The second is vertical over horizontal slicing: organize modules by feature, not by code artifact kind.

🧪 Tests

🏁 All 924 tests pass with zero warnings. 🔄 Test files updated to import from their owning feature modules instead of from deleted horizontal-slice modules.

📚 Book Recommendations

📖 Similar

• 🧩🧱⚙️❤️ Domain-Driven Design: Tackling Complexity in the Heart of Software by Eric Evans is relevant because placing types in the modules that own their domain concepts is a core DDD principle, and this refactoring is a direct application of that principle.
• A Philosophy of Software Design by John Ousterhout is relevant because it argues for deep modules with cohesive interfaces, which is exactly what vertical slicing achieves.

↔️ Contrasting

• Design Patterns: Elements of Reusable Object-Oriented Software by Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides offers a view where horizontal layers and abstract factories are primary organizational tools, contrasting with the vertical feature slicing preferred in functional programming.

🔗 Related

• 🐣🌱👨‍🏫💻 Haskell Programming from First Principles by Christopher Allen and Julie Moronuki explores the module system and type design patterns that make this kind of architectural cleanup possible in Haskell.