2026-03-26 | ⏰ Porting the Scheduler to Haskell — Sum Types, Pacific Time, and Frontmatter Parsing
🎯 The Mission
🗓️ The scheduler is the beating heart of the automation pipeline, deciding which tasks run at which Pacific hours.
🔄 This session ports the TypeScript scheduler module into idiomatic Haskell, translating string union types into algebraic data types and converting timezone-aware logic into pure functions.
🧬 Translation Strategy
🏷️ The TypeScript TaskId is a string union of eight literal values like blog-series:chickie-loo and social-posting.
🎲 In Haskell, this becomes a proper sum type with eight constructors, deriving Show, Eq, Ord, Bounded, and Enum, which gives us exhaustive pattern matching, free ordering, and the ability to enumerate all values.
📝 Two conversion functions, taskIdToText and taskIdFromText, bridge the gap between the Haskell ADT and its text representation, with the reverse lookup powered by a Map built from the Bounded Enum instance.
📐 Design Decisions
🗺️ The blog series run configs use Data.Map.Strict keyed by series name, and the valid task ID set uses Data.Set, both matching the TypeScript Map and Set semantics while gaining Haskell’s persistent data structure benefits.
🕐 Pacific time conversion avoids external timezone libraries by computing DST transitions from first principles: the second Sunday of March at ten AM UTC for spring forward, the first Sunday of November at nine AM UTC for fall back.
📆 The nthSundayOf helper uses Data.Time’s dayOfWeek with a pattern-matched daysUntilSunday function, making the day-of-week arithmetic unambiguous regardless of GHC’s internal Enum encoding.
🔍 Rather than pulling in regex-tdfa for frontmatter parsing, the module uses pure Text operations: stripPrefix for the opening triple-dash delimiter, breakOn for finding the closing delimiter, and lines with stripPrefix for detecting the regenerate marker.
🧩 The isBlogSeries predicate uses exhaustive pattern matching on the TaskId constructors rather than checking a text prefix, ensuring that new blog series constructors trigger a compile-time incomplete pattern warning.
🔄 The scheduling predicate faithfully replicates the TypeScript semantics where blog series tasks and at-or-after tasks match any hour greater than or equal to their scheduled hour, while other tasks require an exact hour match.
🧮 What Got Ported
🏗️ Three data types: TaskId as a sum type, ScheduleEntry as a record with task, hours, and at-or-after flag, and BlogSeriesRunConfig as a record with series name, model chain, and priority user environment variable.
📋 The full schedule table with all eight tasks and their Pacific hour assignments.
🗂️ Three blog series run configs with their Gemini model fallback chains.
🔧 Six pure functions for task lookup, validation, series extraction, scheduling, and Pacific time conversion.
📁 Two IO functions for checking blog post existence and finding posts marked for regeneration, using System.Directory for filesystem access.
🔬 Pure Core, Thin IO Shell
⚗️ The pacificHour function is pure, taking a UTCTime and returning an Int, which makes it trivially testable.
📡 The nowPacificHour function is a one-line IO wrapper that feeds getCurrentTime into the pure core.
🧪 The findM helper implements monadic short-circuit search, reading files one at a time and stopping at the first match, matching the lazy evaluation semantics of the TypeScript find method.
🔭 Looking Ahead
🧪 Property-based tests can verify round-trip consistency between taskIdToText and taskIdFromText, schedule invariants like valid hour ranges, and DST boundary correctness.
🔗 With Types and Scheduler in place, the next modules can wire up environment parsing and the orchestration layer that dispatches tasks based on the schedule.
📚 Book Recommendations
🔗 Similar
- 📘 🐣🌱👨🏫💻 Haskell Programming from First Principles by Christopher Allen and Julie Moronuki
- 📘 Algebra-Driven Design by Sandy Maguire
- 📘 Thinking with Types by Sandy Maguire
🔀 Contrasting
- 📕 Programming TypeScript by Boris Cherny
- 📕 JavaScript: The Good Parts by Douglas Crockford
🎨 Creatively Related
- 📗 🧮➡️👩🏼💻 Category Theory for Programmers by Bartosz Milewski
- 📗 A Philosophy of Software Design by John Ousterhout
- 📗 Why Time Flies by Alan Burdick