confusing error message

[Simon Peyton-Jones]

Good point.   Error message improved, regression test added.

Thanks for the suggestion

Simon

| -----Original Message-----
| From: Dean Herington [mailto:heringto at cs.unc.edu]
| Sent: 05 February 2003 19:14
| To: glasgow-haskell-bugs at haskell.org
| Subject: confusing error message
| 
| buzzard(118)% cat Bug5.hs
| import Control.Monad.State
| data S = S Int
| newtype M a = M (StateT S IO a)
|   deriving (Monad)
| 
| main = return ()
| buzzard(119)% ghc -c Bug5.hs
| 
| Bug5.hs:3:
|     Can't make a derived instance of `Monad M'
|     (too hard for cunning newtype deriving)
|     When deriving instances for type `M'
| buzzard(120)% ghc --version
| The Glorious Glasgow Haskell Compilation System, version 5.04.2
| 
| 
| The real problem above is that I forgot to enable extensions.  It
would
| be helpful if the error message indicated that as the problem.
| 
| 
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