Suggest -XDeriveDataTypeable with "newtype" too
I'm forwarding wishlist bug #499216 from Debian's BTS.
If I try to compile or load this program, ghc complains that I need to pass -XDeriveDataTypeable to enable the derivation of Typeable instances:
------ begin Test.hs --------
module Test where
import Data.Typeable
data List a = List [a] deriving(Typeable)
-----------------------------
Test.hs:5:0:
Can't make a derived instance of `Typeable (List a)'
(You need -XDeriveDataTypeable to derive an instance for this class)
In the data type declaration for `List'
If I use "newtype" and I have GeneralizedNewtypeDeriving turned on, however, I get this error:
/tmp/Test.hs:5:0:
Can't make a derived instance of `Typeable (List a)'
(even with cunning newtype deriving:)
In the newtype declaration for `List'
It would be nice if ghc suggested DeriveDataTypeable here too. JFTR, it does suggest this if newtype deriving is off:
/tmp/Test.hs:5:0:
Can't make a derived instance of `Typeable (List a)'
(You need -XDeriveDataTypeable to derive an instance for this class
Try -XGeneralizedNewtypeDeriving for GHC's newtype-deriving extension)
In the newtype declaration for `List'
Trac metadata
Trac field | Value |
---|---|
Version | 6.8.2 |
Type | FeatureRequest |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | Unknown |
Architecture | Unknown |