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  • #7800

Closed
Open
Opened Mar 29, 2013 by Krzysztof Gogolewski@monoidalDeveloper

Error message when deriving Typeable without PolyKinds

Module M:

{-# LANGUAGE PolyKinds #-}
module M where

data A a

Module N:

{-# LANGUAGE StandaloneDeriving, DeriveDataTypeable #-}
module N where

import M
import Data.Typeable

deriving instance Typeable A

We get a confusing error message:

N.hs:7:1:
    Derived typeable instance must be of form (Typeable A)
    In the stand-alone deriving instance for ‛Typeable A’

The error disappears when N is compiled with -XPolyKinds. The reason is: without -XPolyKinds, A defaults to A *. I think it's simplest to suggest the flag in the error message.

Trac metadata
Trac field Value
Version 7.7
Type Bug
TypeOfFailure OtherFailure
Priority normal
Resolution Unresolved
Component Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture
Assignee
Assign to
7.8.1
Milestone
7.8.1 (Past due)
Assign milestone
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None
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Reference: ghc/ghc#7800