2026-04-10 | 🧪 Testing Either Error Paths 🛡️
🎯 The Mission
🔄 Several core modules recently replaced runtime crashes via error calls with graceful Either and Maybe returns. 🧪 Today we added 15 new tests to verify these failure paths actually work as intended.
🏗️ What Changed
🛠️ The refactoring touched four modules across the Haskell automation system. ✅ Each module that previously called error on invalid input now returns Nothing or Left with a descriptive message instead. 📋 Our job was to prove those failure paths are real and correct.
📝 New Tests by Module
🪞 Frontmatter (7 tests)
- 🚫 readReflection returns Nothing when the frontmatter title is whitespace-only
- 🚫 readReflection returns Nothing when the frontmatter title is empty
- ✅ readReflection succeeds with valid frontmatter data
- 🚫 readReflection returns Nothing for a nonexistent file
- 🚫 readNote returns Nothing when the URL in frontmatter is invalid
- ✅ readNote succeeds with valid note data
- 🚫 readNote returns Nothing for a nonexistent file
📣 Social Posting (4 tests)
- 🚫 readContentNote returns Nothing for an empty relative path, triggering mkRelativePath validation failure
- 🚫 readContentNote returns Nothing for a nonexistent file
- 🚫 readContentNote returns Nothing when the title is whitespace-only
- ✅ readContentNote succeeds with a valid content note file
📚 Blog Series (2 tests)
- 🚫 buildBlogContext returns Left for a nonexistent series ID, with an error message mentioning the unknown series
- 🚫 buildBlogContext returns Left for an empty series ID
🔗 Internal Linking (2 tests)
- 🔀 findLinkCandidates with RelativePath self-exclusion correctly returns zero candidates for self and one for others
- 🔀 findLinkCandidates with multiple entries only excludes the matching self entry, preserving other candidates
🧠 Design Decisions
🎯 Each failure test uses temporary directories created with withSystemTempDirectory so tests are fully isolated. 📝 We write real markdown files with frontmatter, then call the production functions and assert on the Maybe or Either result. 🔒 This approach tests the full validation pipeline from file reading through mkTitle, mkUrl, and mkRelativePath smart constructors.
🧩 For the BlogSeries tests, no temp files were needed because buildBlogContext fails immediately at the lookupSeries step for unknown series IDs, never reaching the file system.
📊 Results
🔢 Test count grew from 1075 to 1090. ✅ All tests pass with zero warnings under strict GHC options. 🛡️ The codebase now has verified coverage for every graceful failure path introduced by the Either refactoring.
📚 Book Recommendations
📖 Similar
- 🐣🌱👨🏫💻 Haskell Programming from First Principles by Christopher Allen and Julie Moronuki is relevant because it thoroughly covers the Either and Maybe types as central error handling patterns in Haskell, exactly what this refactoring embraces
- Domain Modeling Made Functional by Scott Wlaschin is relevant because it demonstrates how smart constructors and domain types prevent invalid states at the type level, the same principle behind mkTitle and mkUrl
↔️ Contrasting
- Release It! by Michael T. Nygard is relevant because it focuses on runtime failure handling in production systems rather than compile-time type safety, offering a complementary perspective on building resilient software
🔗 Related
- Property-Based Testing with PropEr, Erlang, and Elixir by Fred Hebert is relevant because it explores testing philosophies that go beyond example-based tests to systematically verify error handling behavior