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  • Glasgow Haskell CompilerGlasgow Haskell Compiler
  • GHCGHC
  • Issues
  • #4246

"Conflicting family instance declarations" error mentions code I did not write

I get an error message mentioning code that I did not write but results from desugaring when compiling the following program:

{-# LANGUAGE TypeFamilies, FlexibleInstances, OverlappingInstances #-}

class Stupid a where
  type F a

instance Stupid a where
  type F a = a

instance Stupid Int where
  type F Int = Bool

The error I get is:

families.hs:7:7:
    Conflicting family instance declarations:
      type instance F a -- Defined at families.hs:7:7
      type instance F Int -- Defined at families.hs:10:7

The error mentions "type instance F a", but it should say "type F a", because that is what I wrote in the source file. Or maybe "type F a in instance Stupid a" to make it clear that it's an instance declaration.

Trac metadata
Trac field Value
Version 6.12.1
Type Bug
TypeOfFailure OtherFailure
Priority normal
Resolution Unresolved
Component Compiler
Test case
Differential revisions
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Blocking
CC rendel@informatik.uni-marburg.de
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