Skip to content

GitLab

  • Menu
Projects Groups Snippets
    • Loading...
  • Help
    • Help
    • Support
    • Community forum
    • Submit feedback
  • Sign in / Register
  • GHC GHC
  • Project information
    • Project information
    • Activity
    • Labels
    • Members
  • Repository
    • Repository
    • Files
    • Commits
    • Branches
    • Tags
    • Contributors
    • Graph
    • Compare
    • Locked Files
  • Issues 4,832
    • Issues 4,832
    • List
    • Boards
    • Service Desk
    • Milestones
    • Iterations
  • Merge requests 450
    • Merge requests 450
  • CI/CD
    • CI/CD
    • Pipelines
    • Jobs
    • Schedules
    • Test Cases
  • Deployments
    • Deployments
    • Releases
  • Analytics
    • Analytics
    • CI/CD
    • Code review
    • Insights
    • Issue
    • Repository
    • Value stream
  • Wiki
    • Wiki
  • Snippets
    • Snippets
  • Activity
  • Graph
  • Create a new issue
  • Jobs
  • Commits
  • Issue Boards
Collapse sidebar
  • Glasgow Haskell Compiler
  • GHCGHC
  • Issues
  • #4492

Closed
Open
Created Nov 11, 2010 by rl@cse.unsw.edu.au@trac-rl

Type families regression

Here is a small program:

type family F a b
type instance F (Maybe a) b = b -> F a b

class C a where
  go :: (forall a. Maybe a -> b -> a) -> a -> F a b

instance C a => C (Maybe a) where
  go f a b = go f (f a b)

The current head complains:

    Could not deduce (F a b ~ F a b) from the context (C a)
    NB: `F' is a type function, and may not be injective
    Expected type: b -> F a b
      Actual type: F (Maybe a) b
  1. 12.3 accepts it.
Trac metadata
Trac field Value
Version 7.1
Type Bug
TypeOfFailure OtherFailure
Priority normal
Resolution Unresolved
Component Compiler (Type checker)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture
To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information
Assignee
Assign to
Time tracking