type error causes type checker stack overflow

[chiral]

The following code causes

*** Exception: stack overflow

during type checking. I've tried to whittle the test case down, this may not be minimal. The (Ord a) instances and the unused vertexID member of the datatype seem to be necessary, otherwise the infinite type is correctly caught. Does not occur under 6.6.1

module GHCBug(
  MeshVertex(..)
  ) where

data (Ord a) => MeshVertex a b = MeshVertex {
  vertexID         :: a,
  vertexChildren   :: [a]
  } deriving (Show)

split :: (Ord a)   =>
  a                ->
  [MeshVertex a b] ->
  [MeshVertex a b]
split a m = let
    av      = head m
    newVTs  = preSplit a m
    av'     = av { vertexChildren = newVTs }
    newMVs  = av' : m
    newMesh = newMVs
  in newMesh

preSplit :: (Ord a) =>
  a                 ->
  [MeshVertex a b]  ->
  [MeshVertex a b]
preSplit a m = preSplit a $ split a m

[simonpj]

I believe this bug from Bryan Donlan is closely related. (He sent the report by email; I don't think there's a ticket for it.) I propose not to chase it down until Manuel has fixed this ticket (he is on the job). Then let's re-investigate.

Simon

I've found a case where GHC will hang (consuming more and more memory) when it should report an occurs check error. The test case is at  http://fushizen.net/~bd/kaos-ghc-loop.tgz - the error in question is that the arguments in the StateT instance for HOLift are reversed like so:

diff --git a/src/Kaos/KaosM.hs b/src/Kaos/KaosM.hs
index ec4e69a..268250a 100644
--- a/src/Kaos/KaosM.hs
+++ b/src/Kaos/KaosM.hs
@@ -88,7 +88,7 @@ genlift' unbox box f m = box $ \s -> f (unbox m s)
 class HOLift m b | m -> b where
     genlift :: (forall r'. b r' -> b r') -> (forall r. m r -> m r)

-instance HOLift m (StateT s m) where
+instance HOLift (StateT s m) m where
     genlift = genlift' runStateT StateT

 instance (HOLift m b, KaosDiagM r b) => KaosDiagM r m where

I'm using GHC 6.8.2 on ubuntu hardy.

Bryan Donlan

[chak]

All fine with the new solver.