Ticket #6117 (closed bug: fixed)
Cyclic Type Class Hierarchy Produces <<loop>>
|Reported by:||jun0||Owned by:||simonpj|
|Component:||Compiler (Type checker)||Version:||7.4.1|
|Type of failure:||Incorrect result at runtime||Difficulty:||Unknown|
|Test Case:||T6117||Blocked By:|
Description (last modified by simonpj) (diff)
If there is a cyclic class hierarchy like
class B a => Semigroup a where ...[[BR]] class Semigroup (Additive a) => Ring a where ...[[BR]] instance Ring a => Semigroup (Additive a) where ...[[BR]]
then uses of B's methods on (Additive a) in the method implementations of the third declaration instance Ring a => Semigroup (Additive a) will:
- be accepted by the compiler even in cases where B (Additive a) is not derivable.
- result in <<loop>>.
The attached program prints <<loop>> when compiled with GHC-7.2.1 or newer but prints 1234567890 when compiled with GHC-7.0.4 or older. I haven't had time to try out any revisions in between those two, but I did check that HEAD produces <<loop>> as well.