ABRIDGED VERSION - Joys & frustrations of putting 34,000 lines of Haskell into production (at Vacation Labs)
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The document discusses the challenges and triumphs of deploying 34,000 lines of Haskell code in a production environment at Vacation Labs, particularly emphasizing issues with the Haskell ecosystem such as library fragmentation, lack of best practices, and inadequate tooling. While the author appreciates certain features of Haskell, they express frustration with the broken library ecosystem and the difficulties of effective error handling, testing, and database management. Despite the challenges faced, the author highlights their commitment to raising awareness about these issues and contributing to improvements in the Haskell community.
![-- Created our own library `record-splicer` to deal with this...
createRecordSplice SpliceArgs
{
sourcePrefix = "user"
, source = ''User
, requiredFields =
[
'userName
, 'userEmail
, 'userAge
]
, targetPrefix = "newuser"
, targetName = "NewUser"
, deriveClasses = [''Eq, ''Show, ''Generic]
}
makeLensesWith abbreviatedFields ''NewUser](https://image.slidesharecdn.com/functionconf2017presentation-actualtalk-171120112427/85/ABRIDGED-VERSION-Joys-frustrations-of-putting-34-000-lines-of-Haskell-into-production-at-Vacation-Labs-49-320.jpg)
![DB Library.
Dare I say ORM?
Either you use an ORM or you write an ad-hoc ORM.
Tried to understand how to build apps without an ORM.
Not convinced. At all. [1] [2] [3] [4]
Lack of ORM](https://image.slidesharecdn.com/functionconf2017presentation-actualtalk-171120112427/85/ABRIDGED-VERSION-Joys-frustrations-of-putting-34-000-lines-of-Haskell-into-production-at-Vacation-Labs-51-320.jpg)
ABRIDGED VERSION - Joys & frustrations of putting 34,000 lines of Haskell into production (at Vacation Labs)
- 1. Joys & frustrations of putting 34,000 lines of Haskell into production NOTE: Another version of these slides (with more text) is available at https://www.slideshare.net/saurabhnanda/joys-frustrations-of-putting-34000-lines- of-haskell-into-production-at-vacation-labs Haskell
- 2. Source of all this gyaan 34,000 LoC of Haskell in production Touched (almost) all aspects of a typical web-app stack Except Haskell (or Purescript) in the frontend. Using Angular2 + TypeScript instead.
- 4. Why Haskell?Why Haskell? आ बैल मुझे मारआ बैल मुझे मार
- 5. Vacation Labs Travel-Commerce Platform Enabling travel companies with e-commerce technology. Built mostly with Rails & Angular-v1 (Javascript)
- 6. Remember those numbers “It's always good to remember where you come from and celebrate it. To remember where you come from is part of where you're going.” - Anthony Burgess
- 7. Problems with Rails/JS Code Unable to refactor core data-structures confidently Unable to remove dead-code without fear of breaking Unable to review code effectively
- 11. Building webapps in Haskell is harder than it ought to be What I felt a year ago, just starting out... http://tinyurl.com/haskell-is-needlessly-hard
- 12. Haskell. First, the GOOD parts.
- 14. { “Type-safe” : “JSON” } Aeson is brilliant. Auto-derives a lot of boilerplate. OTOH writing JSON codecs in TypeScript by hand.
- 15. HTTP / Web / App Server WAI/Warp has pretty good performance. One lightweight thread per-incoming request. Solid concurrency without an event-driven / callback model (unlike node).
- 16. Now, for the BAD parts.
- 17. Big problems faced first-hand Library eco-system is effectively broken Lack of established best-practices Editor tooling Records Lack of ORM
- 18. Library ecosystem is effectively broken Lack of libraries it NOT a problem Fragmentation, non-standardization Heaps of cruft on Hackage Broken library ecosystem
- 19. I can easily compare Haskell libraries to select the best one Strongly disagree Disagree Neutral Agree Strongly agree 100 200 300 400 500
- 20. I can easily compare Haskell libraries to select the best one Strongly disagree Disagree Neutral Agree Strongly agree 100 200 300 400 500 44% respondents replied negatively
- 21. Emails SES, Postmark, Mandrill, raw SMTP, etc. All there. Fiasco with `smtp-mail` library. Broken library ecosystem
- 22. Sendgrid library `haskell-sendgrid` is incomplete. We wrote our own. Ping us on Twitter if you want to help open-source our code. (Open bounty) http://www.vacationlabs.com/haskell-bounty-program/ Call for contribution
- 23. Testing 287 packages related to testing None of them runs tests in isolated DB txns out-of-the-box. Broken library ecosystem
- 24. DB libraries persistent, hedgehog, hasql, pg-simple, opaleye, mysql-simple, HaskellDB Which to use when? Why so many? Broken library ecosystem
- 25. Lack of established best-practices Community keeps arguing about core stuff stack vs cabal mtl vs free-monads error handling Is TH good or bad? No best practices
- 26. Installing Haskell FOUR different ways to install Haskell - seriously?! PS: Just use “stack” - tried & tested. LTS snapshots are awesome! No best practices
- 28. Error handling EIGHT different ways. Each library uses something else. http://www.randomhacks.net/2007/03/10/haskell-8-ways-to-report-errors/ `ExceptT` vs `Conrol.Monad.Catch` fiasco No best practices
- 29. do x <- runExceptT (liftIO $ throwIO DivideByZero) case x of Left e -> pure “exception caught” Right r -> pure “should never happen!”
- 30. do x <- runExceptT (liftIO $ throwIO DivideByZero) case x of Left e -> pure “exception caught” Right r -> pure “should never happen!” -- OUTPUT *** Exception: divide by zero -- Reaction: Wtf ?!
- 31. do x <- Control.Monad.Catch.try (liftIO $ throwIO DivideByZero) case x of Left (e :: SomeException) -> pure “exception caught” Right r -> pure “should never happen!”
- 32. do x <- Control.Monad.Catch.try (liftIO $ throwIO DivideByZero) case x of Left (e :: SomeException) -> pure “exception caught” Right r -> pure “should never happen!” -- OUTPUT “exception caught”
- 33. Error reporting Haskell code throws runtime errors. Please report them properly. Hype - “if it compiles, it runs”. Reality - “after a refactor, if it compiles, then there is a high chance that you haven’t broken anything” No best practices
- 35. Which file? Turns out “fastLogger” was trying to open a log-file in a non-existent directory. (On LTS-9.0)
- 36. *** Exception: UnexpectedNull { errSQLType = “varchar” , errSQLTableOid = Just (Oid 603560) , errSQLField = “result22_2” , errHaskellType = “Text” , errMessage = “” }
- 37. *** Exception: UnexpectedNull { errSQLType = “varchar” , errSQLTableOid = Just (Oid 603560) , errSQLField = “result22_2” , errHaskellType = “Text” , errMessage = “” } Which row? What SQL was being run? Any other info that can help me debug this? Stacktrace?
- 39. Occurred during stress testing of JSON API. Wasted a lot of time! Which file descriptor? Which library was accessing it? Nothing! Turned out that DB pool size was larger than what DB could handle.
- 40. Absence of CallStacks can probably be fixed Call for contribution Imperceptible perf-penalty with HasCallStack in IO code Not being used by IO-heavy libraries. Raise some PRs! GHC discussion to infer HasCallStack automatically https://ghc.haskell.org/trac/ghc/ticket/13360
- 41. Testing We drank the QuickCheck kool-aid QuickCheck fiasco No best practices
- 42. Editor tooling SAD TRUTH - Nothing works as well as it should. Tried a lot of things Emacs + haskell-mode, Spacemacs + Intero, SublimeHaskell, Haskero, Haskelly, HIE, ghcid, etc. Underlying GHCi hogs/leaks memory. Editor tooling
- 44. Important caveat about memory usage Haskell DOES NOT leak memory in production. Only present in GHC-i -- used in development. Production Rails-app: 360 MB of RSS (x 3 processes) Production Haskell-app: 78 MB of RSS (single process).
- 45. Recommendation: Editor tooling Use spacemacs + intero OR VSCode + HIE (Haskell IDE Engine) Keep ghci open in a terminal window and make friends with :set -fobject-code & :load & :reload
- 46. Shout-out to HIE (Haskell IDE Engine) Call for contribution Please use it, if it works for you. Make it work for you! Contribute if you can. https://github.com/haskell/haskell-ide-engine
- 47. Broken record Records. Records. Records. Records in Haskell are beyond horrible. Some say, Haskell doesn’t even have records. http://www.parsonsmatt.org/overcoming-records/
- 48. -- Your `users` table would probably map to the following… data User = User { userId :: UserId , userCreatedAt :: UTCTime , userUpdatedAt :: UTCTime, , userStatus :: UserStatus, , userName :: Maybe Text , userEmail :: Text , userAge :: Maybe Int } -- But, you can’t use this in your `createUser` endpoint. -- UI shouldn’t set `id, createdAt, updatedAt, status` -- No easy way to create a “sub-record” from a master record.
- 49. -- Created our own library `record-splicer` to deal with this... createRecordSplice SpliceArgs { sourcePrefix = "user" , source = ''User , requiredFields = [ 'userName , 'userEmail , 'userAge ] , targetPrefix = "newuser" , targetName = "NewUser" , deriveClasses = [''Eq, ''Show, ''Generic] } makeLensesWith abbreviatedFields ''NewUser
- 50. // Same thing in TypeScript type interface User { readonly id: number , readonly createdAt: DateTime , readonly updatedAt: DateTime , readonly status: ‘unconfirmed’ | ‘confirmed’ | ‘blocked’ , readonly name: string | null , readonly email: string , readonly age: string | null } // Making a typesafe sub-record type NewUser = Pick <User, ‘name’ | ‘email’ | ‘age’>; // Making a typesafe super-record type Admin extends User { readonly permissions: Array<Permission> }
- 51. DB Library. Dare I say ORM? Either you use an ORM or you write an ad-hoc ORM. Tried to understand how to build apps without an ORM. Not convinced. At all. [1] [2] [3] [4] Lack of ORM
- 52. Associations? No DB library deals with associations. Except two new discoveries - beam & postgresql-orm Lack of ORM
- 53. Validations? No DB library deals with validations. Rails’ convention of putting strict invariants/validations in the DB layer works ridiculously well. Lack of ORM
- 54. Validations in DB-layer are an anti-pattern? Let’s look state-of-the-art: “digestive-functors” Lack of ORM
- 56. DB Library Problem as well. Incomplete / half-baked DB libraries mysql-simple - (had?) concurrency issues postgresql-simple doesn’t use PREPARE/EXECUTE Opaleye generates slow queries Lack of ORM
- 57. Applicative parsers? Or “Hodor” parsers. Special hate for applicative parsers! Negate all benefits of static typing. Especially true for postgresql-simple. Lack of ORM
- 58. query_ = ([qc| SELECT c.id as client_id , coalesce(c.custom_domain, c.domain) as domain , c.home_page as home_page , c.support_phone as support_phone , c.support_email as support_email , c.logo_photo_id as loto_photo_id , logo_photo.image_file_name as logo_file_name , logo_photo.image_fingerprint as logo_fingerprint , c.favicon_image_id as favicon_image_id , favicon.image_file_name as favicon_file_name …
- 59. query_ = ([qc| SELECT c.id as client_id , coalesce(c.custom_domain, c.domain) as domain , c.home_page as home_page , c.support_phone as support_phone , c.support_email as support_email , c.logo_photo_id as loto_photo_id , logo_photo.image_file_name as logo_file_name , logo_photo.image_fingerprint as logo_fingerprint , c.favicon_image_id as favicon_image_id , favicon.image_file_name as favicon_file_name … rowParser assetHost_ = Storefront <$> field <*> field <*> field <*> field <*> field <*> (photoEssentialsParser assetHost_ Photo.WebsiteLogo) <*> (photoEssentialsParser assetHost_ Photo.Favicon) <*> field <*> field <*> field <*> field <*> field <*> field <*> field <*> field <*> field <*> field <*> field <*> field <*> field <*> field <*> hstoreBoolParser <*> field <*> field <*> field <*> field <*> field <*> field <*> field
- 60. query_ = ([qc| SELECT c.id as hodor , coalesce(c.custom_domain, c.domain) as hodor , c.home_page as hodor , c.support_phone as hodor , c.support_email as hodor , c.logo_photo_id as hodor , logo_photo.image_file_name as hodor , logo_photo.image_fingerprint as hodor , c.favicon_image_id as hodor , favicon.image_file_name as hodor … rowParser assetHost_ = Storefront <$> hodor <*> hodor <*> hodor <*> hodor <*> hodor <*> (photoEssentialsParser assetHost_ Photo.WebsiteLogo) <*> (photoEssentialsParser assetHost_ Photo.Favicon) <*> hodor <*> hodor <*> hodor <*> hodor <*> hodor <*> hodor <*> hodor <*> hodor <*> hodor <*> hodor <*> hodor <*> hodor <*> hodor <*> hodor <*> hstoreBoolParser <*> hodor <*> hodor <*> hodor <*> hodor <*> hodor <*> hodor <*> hodor
- 61. PG-Simple’s “hodor” parser can be fixed There is an open bounty. https://github.com/lpsmith/postgresql-simple/issues/43 http://www.vacationlabs.com/haskell-bounty-program Call for contribution
- 62. What did we really build in 38,000 lines of Haskell? Flexi-payment plugin DelayedJob clone (but using LISTEN/NOTIFY) along with UI Sendgrid library Opaleye boilerplate code-generator Deep instrumentation wrappers over core libs
- 64. Logging & Instrumentation “Deep instrumentation” in Haskell seems to be non-existent Eg. kind of stuff provided by Rails out of box, or Skylight, or New Relic Very hard to build drop-in instrumentation libraries for Haskell. Probably because of the type-system itself. http://tinyurl.com/haskell-instrumentation Built our own ad-hoc logging & instrumentation layer by wrapping important libs
- 65. Job Queue Rails has DelayedJob (with an RDBMS backend). Works really well for moderate workloads. ACID compliant and doesn’t add more moving parts to production. Couldn’t find anything similar in Haskell world. Built our own. Will open-source. Except `yesod-job-queue` but we aren’t using Yesod. Redis & SQS job-queues available, but we didn’t use first-hand. Can’t comment.
- 66. JobAdmin UI built by intern in 4 months (included learning Haskell). Built using Servant, Lucid, Aeson, Opaleye.
- 67. Initial (excruciating) pain for long-term gain We have already gone through the pain Now, we are reaping the gains Haskell sucks. Why continue with it?
- 68. What’s next? Raising awareness about the issues we faced. The core language is not as confusing as the ecosystem is. Contributing in any way to fix the ecosystem.
- 69. Thank you @saurabhnanda on twitter @vacationlabs on instagram Haskell Bounty Program Haskell Internship Program We’re hiring Haskellers (and Haskell enthusiasts)