The culprit seems to be that instance Foldable [] where elem = List.elem foldl = List.foldl foldl' = List.foldl' foldl1 = List.foldl1 foldr = List.foldr foldr1 = List.foldr1 length = List.length maximum = List.maximum minimum = List.minimum null = List.null product = List.product sum = List.sum toList = id delegates to monomorphic methods rather than implementing them directly. The early testing where we showed that it didn't break RULES was done with the final definitions in the class rather than delegating to GHC.OldList. Somewhere when reorganizing of the code we switched to this style and introduced this problem, making RULES much more brittle. =/ -Edward On Wed, Apr 1, 2015 at 7:14 AM, Joachim Breitner wrote:

Hi,

I just stumbled over a FTP breakage: A RULES with "foldr" on the left hand side would no longer fire, because it would refer to Foldable.foldr, and not the monomorphic foldr.

Adding #if MIN_VERSION_base(4,8,0) import Prelude hiding (foldr) import GHC.OldList (foldr) #endif to the file solved the problem for now, but GHC.OldList is supposed to go away eventually, so I believe we need a permanent location for the monomorphic foldr.

Or will we simply not get rid of GHC.OldList at all?

(One could argue that "foldr" on the left-hand-side of a RULE should work and that this is a GHC bug, though.)

Greetings, Joachim

-- Joachim “nomeata” Breitner mail@joachim-breitner.de • http://www.joachim-breitner.de/ Jabber: nomeata@joachim-breitner.de • GPG-Key: 0xF0FBF51F Debian Developer: nomeata@debian.org

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