--- title: A Couple Million Lines of Haskell: Production... description: An experienced engineer at Mercury, a fintech serving 300,000 businesses, shares hard-won lessons from running 2 million lines of Haskell in production. Key... canonical: https://daily.dev/posts/a-couple-million-lines-of-haskell-production-engineering-at-mercury-apjrivjvs twitter:card: summary_large_image twitter:site: @dailydotdev og:type: website og:site_name: daily.dev og:title: A Couple Million Lines of Haskell: Production Engineering at Mercury | daily.dev og:description: An experienced engineer at Mercury, a fintech serving 300,000 businesses, shares hard-won lessons from running 2 million lines of Haskell in production. Key... og:url: https://daily.dev/posts/a-couple-million-lines-of-haskell-production-engineering-at-mercury-apjrivjvs og:image: https://api.daily.dev/og/posts/ApjRIVjvs.png og:image:alt: A Couple Million Lines of Haskell: Production Engineering at Mercury og:image:width: 1200 og:image:height: 630 og:locale: en --- > ## Documentation Index > Fetch the complete documentation index at: https://daily.dev/llms.txt > Use this file to discover all available pages before exploring further. # A Couple Million Lines of Haskell: Production Engineering at Mercury **[Hacker News](https://daily.dev/sources/hn)** · 43 min read · 1 upvotes · 0 comments ## Summary An experienced engineer at Mercury, a fintech serving 300,000 businesses, shares hard-won lessons from running 2 million lines of Haskell in production. Key themes include treating purity as a boundary rather than a property, encoding operational invariants in types to preserve institutional knowledge, using Temporal for durable execution to replace fragile cron-based state machines, designing domain models independent of transport concerns (e.g., not leaking HTTP status codes into business logic), and using records-of-functions for observability and testability. The post also covers the tradeoffs of heavy type encoding, the importance of pragmatism over idealism in Haskell teams, and honest assessments of ecosystem gaps like limited library instrumentation and HTTP/2 support. ## Full article daily.dev links to this article rather than hosting it. Read it at the original source: ## Similar posts on daily.dev - [Haskell in Mercury: Interview with Max Tagher](https://daily.dev/posts/haskell-in-mercury-interview-with-max-tagher-ah0rkc4d9) · Serokell · 0 upvotes · 0 comments - [Enterprise Haskell at H-E-B](https://daily.dev/posts/enterprise-haskell-at-h-e-b-sjpe3tn1b) · Lobsters · 0 upvotes · 0 comments - [Haskell in Production: Scrive](https://daily.dev/posts/haskell-in-production-scrive-afcztblsi) · Serokell · 0 upvotes · 0 comments - [Issue 534](https://daily.dev/posts/issue-534-rpp60utgy) · Haskell Weekly · 0 upvotes · 0 comments - [Mitchell Vitez](https://daily.dev/posts/mitchell-vitez-k7xy6fvhi) · Lobsters · 0 upvotes · 0 comments --- Tags: [#fintech](https://daily.dev/tags/fintech), [#haskell](https://daily.dev/tags/haskell), [#type-systems](https://daily.dev/tags/type-systems) [View this post on daily.dev](https://daily.dev/posts/a-couple-million-lines-of-haskell-production-engineering-at-mercury-apjrivjvs) ```json {"@context":"https://schema.org","@graph":[{"@type":"Organization","@id":"https://daily.dev/#organization","name":"daily.dev","url":"https://daily.dev","logo":{"@type":"ImageObject","url":"https://daily.dev/apple-touch-icon.png","width":180,"height":180},"sameAs":["https://twitter.com/dailydotdev","https://github.com/dailydotdev","https://www.linkedin.com/company/daily-dev-ltd"]},{"@type":"WebSite","@id":"https://daily.dev/#website","url":"https://daily.dev","name":"daily.dev","publisher":{"@id":"https://daily.dev/#organization"},"potentialAction":{"@type":"SearchAction","target":{"@type":"EntryPoint","urlTemplate":"https://daily.dev/search?q={search_term_string}"},"query-input":"required name=search_term_string"}}]} {"@context":"https://schema.org","@type":"TechArticle","headline":"A Couple Million Lines of Haskell: Production Engineering at Mercury","url":"https://daily.dev/posts/a-couple-million-lines-of-haskell-production-engineering-at-mercury-apjrivjvs","mainEntityOfPage":{"@type":"WebPage","@id":"https://daily.dev/posts/a-couple-million-lines-of-haskell-production-engineering-at-mercury-apjrivjvs"},"datePublished":"2026-05-03T03:36:41.828Z","dateModified":"2026-05-03T03:37:05.639Z","description":"An experienced engineer at Mercury, a fintech serving 300,000 businesses, shares hard-won lessons from running 2 million lines of Haskell in production. 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