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2026-04-08 | 🏷️ Domain Types and Pure Extraction: Architecture Done Right 🧬

🎯 The Mission

🏗️ Yesterday we created a seven-phase architecture roadmap for improving the Haskell codebase, and completed the first extraction as proof of concept. 🔄 Today we finished the remaining pure function extractions and then learned a series of important lessons: extracting pure functions without introducing proper domain types is only half the work, and where you put a function matters as much as how you write it.

🧠 Why Pure Functions Matter

🔬 A pure function always returns the same output for the same input and has no side effects. 🎯 This makes them trivially testable: no mocks, no temp directories, no setup, no teardown. ⚡ Tests run in microseconds instead of milliseconds. 🛡️ Pure functions are also easier to reason about: you can understand them by reading their type signature and a few test cases, without tracing through IO dependencies.

🏗️ The Vertical Slice Lesson

🚨 Our original architecture plan separated pure function extraction from domain type introduction into two horizontal phases. 📐 This was a mistake. 🎯 When we extracted checkCandidateEligibility, it initially took four Text parameters: directory, today’s date, filename, and file content. 🤔 But three of those four parameters were just Text, even though each represents a fundamentally different concept. ⚠️ Nothing in the type system prevented accidentally swapping the directory with the date.

🔑 The key insight: extracting a pure function and introducing its domain types are one concern, not two. 🏗️ Separating them into phases encourages horizontally-sliced work that leaves functions in an intermediate state that Haskell’s type system could protect us from. ✅ The fix: always deliver vertical slices where types, logic, tests, and documentation arrive together.

🏠 The Module Organization Lesson

📦 We initially placed selectMostRecentReflection in SocialPosting since that’s where it was first used. 🔗 When InternalLinking needed the same function, we imported it from SocialPosting. 🚨 But this created a misleading dependency: InternalLinking appeared to depend on SocialPosting when it really only needed reflection file selection logic. 📐 The fix: create an Automation.Reflection module that owns reflection-related functions, and have both SocialPosting and InternalLinking import from it.

🔧 What We Built

🗂️ ContentDirectory: A Closed Set as an ADT

📋 The codebase has exactly 13 content directories used for image backfill. 🏷️ Previously these were raw Text strings compared against magic string literals like “reflections”. 🧬 Now they are a proper algebraic data type with one constructor per directory: Reflections, AiBlog, AutoBlogZero, ChickieLoo, and so on. 🔄 Round-trip functions convert between the ADT and Text for IO boundaries. 🧪 A round-trip property test verifies every constructor survives the conversion.

📅 parseDateFromFilename: Proper Day Values

🕰️ The original function returned Text and used empty string to signal failure. 🧬 Now it returns Maybe Day using the standard Data.Time library. 🎯 This makes it impossible to accidentally compare a date with a directory, since they are different types. 🧪 Tests use fromGregorian to construct expected dates rather than comparing strings.

🔧 CandidateEligibility: Result Types Over Booleans

🤔 The original function returned Maybe Bool, which is hard to interpret: does Nothing mean ineligible? 🧬 Now it returns a CandidateEligibility type with two constructors: Eligible (carrying a boolean for whether regeneration is needed) and Ineligible (carrying an IneligibilityReason). 📋 The IneligibilityReason ADT has three constructors: FutureReflection, AlreadyHasImage, and UntitledReflection. 🧪 Tests assert specific ineligibility reasons rather than checking for Nothing.

🕐 todayPacificDay: Dates in Pacific Time

📅 We added todayPacificDay, which returns a Day directly in Pacific time without going through a Text round-trip. 🎯 The old backfillImages function called todayPacific to get a DateStr, converted it to Text, then parsed the Text back into a Day. 🧹 Now it simply calls todayPacificDay, which returns the Day directly.

📅 yesterdayDate

🕰️ The original function called getCurrentTime internally and formatted the result as Text. 🧬 We extracted a pure core that accepts a UTCTime and returns a Day. 🧪 Property tests verify the result is always the predecessor of the UTC day, and unit tests cover year boundaries and leap years.

⏰ pacificHour

✅ This function was already pure, accepting a UTCTime and returning an Int for the Pacific hour. 🧪 It just lacked tests. 📊 We added a property test ensuring the result is always between 0 and 23, plus unit tests covering both Pacific Standard Time and Pacific Daylight Time conversions.

🗂️ selectMostRecentReflection

📂 The original function mixed directory listing with file selection logic. 🧬 We extracted a pure function that accepts a list of filenames and returns the most recent date-matching file. 🏠 This function now lives in its own Automation.Reflection module, since both SocialPosting and InternalLinking need it but neither owns the concept of reflection file selection.

📰 blogPostMatchesToday

📁 The blog post existence checker listed a directory and searched for date-prefixed files. 🧬 We extracted a pure function that checks whether any filename in a list starts with today’s date. 🧪 Tests cover matching, non-matching, empty lists, and non-date filenames.

📝 Process Improvements

🔄 We updated both AGENTS.md and the architecture spec to prevent recurring issues. 📐 Seven new guidelines were added covering: no redundant type name suffixes, no single-letter variable names, domain-specific module organization, Pacific time for dates, vertical slices, domain types at extraction, and closed sets as ADTs. 🗺️ The architecture roadmap was restructured to eliminate numbered horizontal phases in favor of a flat list of vertical improvements.

📊 Results

🧪 All 837 tests pass with zero warnings. 🏷️ The ContentDirectory ADT, CandidateEligibility, and IneligibilityReason types ensure the type system catches mistakes at compile time. 🏠 The new Automation.Reflection module keeps domain boundaries clean.

📚 Book Recommendations

📖 Similar

• Algebra of Programming by Richard Bird and Oege de Moor is relevant because it formalizes the idea that programs are algebraic objects that can be reasoned about equationally, which is exactly what pure function extraction enables.
• Thinking with Types by Sandy Maguire is relevant because it explores advanced Haskell type-level programming techniques that underpin the domain types we introduced.

↔️ Contrasting

• 🧱🛠️ Working Effectively with Legacy Code by Michael Feathers offers a contrasting approach where you add tests around impure code using seams and mocks, rather than extracting pure cores as we did here.

🔗 Related

• Domain Modeling Made Functional by Scott Wlaschin explores how functional programming and strong types naturally express domain concepts, which aligns with the vertical slice philosophy of always introducing types alongside extraction.