Examples don't compile with GHC-7.8.3. about acid-state HOT 3 CLOSED

There was a bug with an early version of ghc 7.8.1 or 2 that was fixed, can you point out which files aren't compiling for you and confirm that you are on 7.8.3? Examples should all work.

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On Nov 8, 2014, at 12:00 AM, dschalk [email protected] wrote:

The acid-state example in The Happstack Book and some of the acid-state repository examples don't compile with GHC-7.8.3. The error messages say coercion is impossible because Data.SafeCopy.SafeCopy.Kind is nominal while two arguments differ. Simon Peyton-Jones explains what changed with the GHC-7.8.x in a video presentation at "https://skillsmatter.com/skillscasts/5296-safe-zero-cost-coercions-in-haskell".
The compiler messages says"Possible fix: use a standalone 'deriving instance' declaration . . ." Has anyone had any success implementing this suggestion? I wanted to use acid-state but the example code doesn't compile with the new GHC and I don't want to use GHC 7.x because the problem is not a bug in GHC-7.8.3; the problem stems from a bug having been fixed (see the Jones presentation). How are people dealing with this situation? Is the solution so trivial that nobody has bothered to mention it here? Are people removing "newtype" from their code, or using "unsafeCoerce", or what?

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from acid-state.

I apologize. The repo examples run fine, even with cereal-0.4.0.1 (the Stackage version). But here is what happens when I try to run IxSet.hs from The HappstackBook:

e@e:/acid-state/examples$ ghc --version
The Glorious Glasgow Haskell Compilation System, version 7.8.3
e@e:/acid-state/examples$ runghc IxSet
IxSet.hs:34:46:
Could not coerce from ‘safecopy-0.8.3:Data.SafeCopy.SafeCopy.Kind
Integer’ to ‘safecopy-0.8.3:Data.SafeCopy.SafeCopy.Kind PostId’
because the first type argument
of ‘safecopy-0.8.3:Data.SafeCopy.SafeCopy.Kind’ has role Nominal,
but the arguments ‘Integer’ and ‘PostId’ differ
arising from the coercion of the method
‘safecopy-0.8.3:Data.SafeCopy.SafeCopy.kind’ from type
‘safecopy-0.8.3:Data.SafeCopy.SafeCopy.Kind Integer’ to type
‘safecopy-0.8.3:Data.SafeCopy.SafeCopy.Kind PostId’
Possible fix:
use a standalone 'deriving instance' declaration,
so you can specify the instance context yourself
When deriving the instance for (SafeCopy PostId)
e@e:~/acid-state/examples$

from acid-state.

This is the code that failed:

newtype PostId = PostId { unPostId :: Integer }
deriving (Eq, Ord, Data, Enum, Typeable, SafeCopy)

This works:

newtype PostId = PostId { unPostId :: Integer }
deriving (Eq, Ord, Data, Enum, Typeable)
$(deriveSafeCopy 0 'base ''PostId)

So, it looks like the issue can be closed.

from acid-state.