OK once I bit the bullet and looked at the code the bug was obvious.
When you pattern match
Ru{rrsrt = AlwaysExpr} -> error "blah"
GHC uses, well, pattern-matching to see if rrsrt is AlwaysExpr.
But when you say
Ru{} -> if (rrsrt r == AlwaysExpr) then error "blah" else ...
then GHC uses the (==) operation for the data type RuleType (of which
AlwaysExpr) is a data constructor.
Sadly you have not defined it. You just say
instance Eq RuleType
That uses the default methods for equality. Its equivalent to
instance Eq RuleType where
(==) a b = not (a /= b)
(/=) a b = not (a == b)
So it's not surprising that you get a loop. You probably wanted to use
"deriving Eq" on your data type declaration for RuleType, or
deriving instance Eq RuleType
So, clearly not a bug in GHC; but it would be more felicitous if it gave you a
warning about the instance declaration for Eq RuleType. The difficulty is that
it's not clear when to warn; it's ok to use default methods, but you must
define *either* (==) *or* (/=).
Simon
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