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2026-03-26 | 🧵 Porting Text, HTML, and Retry to Haskell — Pure Functions, Progressive Truncation, and Exponential Backoff

🎯 The Mission

📦 Three TypeScript utility modules form the foundation of the social media automation pipeline: text processing for platform-specific length constraints, HTML escaping for safe rendering, and retry logic for resilient HTTP calls.

🔄 This session ports all three into idiomatic Haskell, replacing mutable loops with recursive strategies, Map lookups with pattern-matched character functions, and Promise-based retries with IO exception handling.

📝 Text Processing — Five Strategies for Fitting Posts

🧮 The heart of the Text module is fitPostToLimit, a function that progressively removes content from a social media post until it fits within a platform’s grapheme limit.

📐 The TypeScript version uses a mutable while loop to pop topic tags and splice arrays in place. The Haskell version decomposes this into five named strategy functions, each returning a Maybe Text, chained together with a local Alternative-style operator.

🏷️ Strategy one removes pipe-separated topic tags from right to left using recursive list shortening with init. Strategy two removes the entire topic line and its preceding blank line. Strategy three strips the subtitle from the title by finding the first colon. Strategy four removes the title entirely. Strategy five truncates the remaining content and appends the URL.

🔗 The URL line is always preserved across all strategies because social platforms use it for link previews and facet detection.

🐦 Tweet length calculation accounts for Twitter’s t.co URL shortening rule, where every URL counts as exactly 23 characters regardless of actual length. The Haskell version extracts URLs by filtering whitespace-delimited words that start with http or https, then computes the length delta.

🖊️ HTML — Composing Character Escapes

🔡 The HTML module replaces the TypeScript Map-based character lookup with a direct pattern match in a Char-to-Text function passed to Data.Text.concatMap.

📆 Date formatting parses a YYYY-MM-DD string by splitting on hyphens, looking up the month name from a zero-indexed list, and stripping leading zeros from the day. No Date object or locale dependency needed, just pure text manipulation.

🔀 The textToHtml function composes escapeHtml with a newline-to-br replacement, matching the TypeScript pipeline in a single point-free expression.

🔁 Retry — Exponential Backoff with Type-Safe Exceptions

⚡ The Retry module ports the higher-order retry pattern using Control.Exception for error handling and Control.Concurrent.threadDelay for backoff delays.

🎯 A dedicated HttpCodeException type with a Typeable instance enables safe downcasting from SomeException using fromException, replacing the TypeScript duck-typed code property check.

📊 The transient HTTP codes set contains 429, 502, 503, and 504, stored in a Data.Set for efficient membership testing.

⏱️ The withRetry function takes a RetryOptions record with max retries, base delay in milliseconds, and an onRetry callback. The exponential backoff multiplies the base delay by two raised to the current attempt number, matching the TypeScript semantics exactly.

🛡️ Non-transient errors are immediately rethrown with throwIO, preserving the original exception for upstream handlers.

🧩 Design Principles

🔬 All three modules follow the pure core, thin IO shell pattern. Text and HTML are entirely pure, with no IO anywhere. Retry wraps IO actions but keeps the transient error detection logic pure.

📏 The custom Alternative-style operator in Text avoids importing Control.Applicative, keeping the module’s dependency surface minimal and making the Maybe-chaining intent explicit.

🏗️ Record syntax with named fields in RetryOptions provides self-documenting configuration, while defaultRetryOptions gives a sensible baseline that callers can selectively override.

🔭 Looking Ahead

🧪 Property-based tests can verify that fitPostToLimit never exceeds the grapheme limit, that escapeHtml is idempotent on already-escaped text, and that withRetry respects the maximum retry count.

🔗 With these foundational modules in place, the platform-specific posting modules for Twitter, Bluesky, and Mastodon can build on top of validated text fitting and resilient HTTP calls.

📚 Book Recommendations

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🔀 Contrasting

• 📕 Programming TypeScript by Boris Cherny
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