haskell-servant/servant

Generic records causes very slow compile times

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#1038 opened on Sep 18, 2018

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 (6 comments) (0 reactions) (0 assignees)Haskell (1,953 stars) (422 forks)batch import
help wantedperformance

Description

apparently, large APIs using the Servant.API.Generic record mechanism are orders of magnitude slower to compile than their :<|> equivalent

https://twitter.com/mattoflambda/status/1041622124576661504

I'm not sure how much we can do about this, but we could at least benchmark, create a minimal example that triggers this behaviour, and submit it to GHC as a bug

Contributor guide

Tech stack
None
Domain
backendperformance
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
5
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-2 days
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
mostly clear
Prerequisites
Haskell knowledgeServant library familiarityGHC compiler internals
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
10
Research direction
The issue describes slow compile times when using Servant.API.Generic record mechanism compared to :<|> style. To address this, first create a benchmark that reproduces the performance difference using a minimal example. Then analyze the generated Core or GHC's output to identify the cause. If the issue lies in GHC, submit a bug report to the GHC tracker. Relevant files include the Servant.Generic module.