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2026-04-08 | 🔐 Domain Types for Safety and Clarity 🏗️

🎯 The Mission

🧱 Today we continued the Haskell architecture improvement roadmap by introducing domain types that replace raw primitives with meaningful, safe abstractions.

🔑 The key insight driving this work is simple: when everything is Text, the compiler cannot help you distinguish a secret from a blog title from a URL slug. 🤷 Mistakes slip through silently. 💡 Domain types turn those silent mistakes into compile-time errors.

🔐 Secret: Sensitive Values That Cannot Leak

🛡️ The most impactful change was introducing a Secret newtype in a dedicated Automation.Secret module. 📦 Secret wraps Text with a custom Show instance that always outputs the fixed string “angle-bracket redacted angle-bracket” instead of the actual value.

🔍 Previously, every config type that held an API key, password, or access token used plain Text. 😱 If any logging statement accidentally printed a credential value, the raw secret would appear in the logs.

📦 Now every sensitive field across the codebase uses Secret: API keys in TwitterCredentials, GeminiConfig, and ImageProviderConfig; passwords in BlueskyCredentials; access tokens in MastodonCredentials and ObsidianCredentials; and all four OAuth fields in TwitterCredentials. ✅ The smart constructor mkSecret validates that the value is not empty or whitespace-only. 🧪 A property test guarantees that Show never reveals the underlying text regardless of input.

🏠 Placing Secret in its own Automation.Secret module follows Domain-Driven Design principles: each domain concept lives in its own focused module rather than a generic catch-all.

📊 PlatformLimits and SocialPost: Type-Safe Social Posting

📏 Previously, platform character limits were scattered as independent Int constants. 🏗️ Now they live in a proper PlatformLimits data type with two fields: platformMaxCharacters for the character cap, and platformUrlCountLength for how many characters each URL counts as. 🐦 Twitter counts every URL as 23 characters regardless of actual length, while Bluesky and Mastodon count URLs at face value, represented by Nothing in the type.

🎯 Three named constants, twitterLimits, blueskyLimits, and mastodonLimits, provide the per-platform values. 🧹 The old backward-compatible Int constants and wrapper functions were removed entirely because this is a single-user codebase with no external consumers.

📬 Going further, we introduced a SocialPost algebraic data type with three constructors: Tweet, BlueskyPost, and MastodonPost. 🔒 Each has a smart constructor, mkTweet, mkBlueskyPost, and mkMastodonPost, that validates the text fits within that platform’s character limits at construction time. 🎲 A dispatching constructor mkSocialPost accepts a Platform value and routes to the correct validator. 🧪 Property tests verify that text under the minimum platform limit always succeeds, and that round-tripping through construction preserves the original text.

📅 Standard Day Instead of Custom DateStr

🤔 The original DateStr newtype was a wrapper around formatted date text, essentially reinventing a standard library type. 🏛️ Data.Time already provides Day, the canonical Haskell type for calendar dates.

🧹 We removed DateStr entirely and replaced all usage with Day from Data.Time. 📐 A simple formatDay helper function converts Day to Text when the YYYY-MM-DD string form is needed for frontmatter or file paths. 🔌 The existing todayPacificDay function already returned Day, so many call sites became simpler: instead of pattern-matching on DateStr to extract text, code now works directly with the standard Day type and formats at the edges.

📈 The Numbers

🧪 The test suite grew from 837 to 873 tests. 🏗️ Every new type has both unit tests and property-based tests. 🔧 The build produces zero warnings under the strict Werror flag.

🗂️ Over twenty files were modified across the codebase, including a new Automation.Secret module for the Secret domain type.

🗺️ Module Dependency Graph

📊 We generated an SVG module dependency graph and embedded it in the README. 🎨 Modules are color-coded by domain: green for core infrastructure, blue for platform integrations, yellow for blog modules, pink for social posting, purple for automation and AI, and orange for the main entry point.

🔗 The graph reveals the architecture at a glance. 📦 Types.hs sits at the center as the most-depended-on module, while RunScheduled (Main) fans out to every feature module.

🗺️ What Remains

📋 Two domain types from the roadmap are still unchecked: Url and Title. 🌊 RelativePath also remains as a future candidate. 🔜 After the remaining domain types, the next major phase introduces an AppContext record to replace parameter threading, followed by explicit error types, ImageProviderConfig refactoring, and splitting RunScheduled into focused modules.

🧠 Lessons Learned

🎓 Generalizing from ApiKey to Secret was a design win. 🔑 The original ApiKey type only covered API keys, leaving passwords and access tokens unprotected. 🛡️ Secret covers all sensitive values uniformly with a single type.

🧹 Removing backward compatibility aliases was liberating. 🎯 In a single-maintainer codebase, the aliases were pure accidental complexity. 📐 Deleting them forced every call site to use the structured PlatformLimits type directly, making the code clearer.

🏛️ Replacing DateStr with the standard Day type reinforced the principle of checking for standard library types before creating custom ones. 🔧 The standard type is better tested, better understood, and integrates with the rest of the Data.Time ecosystem.

📚 Book Recommendations

📖 Similar

• Domain Modeling Made Functional by Scott Wlaschin is relevant because it demonstrates how to use types to encode business rules and make invalid states unrepresentable, which is exactly the philosophy behind Secret and SocialPost.
• Algebra-Driven Design by Sandy Maguire is relevant because it shows how algebraic thinking and property-based testing can guide the design of correct-by-construction abstractions.

↔️ Contrasting

• 🧑‍💻📈 The Pragmatic Programmer: Your Journey to Mastery by David Thomas and Andrew Hunt offers a more language-agnostic approach to software craft, sometimes favoring convention and discipline over type-level enforcement.

🔗 Related

• Haskell in Depth by Vitaly Bragilevsky explores advanced Haskell patterns for building real-world applications, including newtypes, smart constructors, and the functional core imperative shell architecture.
• Secure by Design by Dan Bergh Johnsson, Daniel Deogun, and Daniel Sawano is relevant because it advocates using domain primitives to prevent security vulnerabilities, which directly parallels the Secret redaction strategy.