#6117: Cyclic Type Class Hierarchy Produces <<loop>>
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Reporter: jun0 | Owner: simonpj
Type: bug | Status: new
Priority: normal | Milestone: 7.8.1
Component: Compiler (Type checker) | Version: 7.4.1
Keywords: | Os: Linux
Architecture: Unknown/Multiple | Failure: Incorrect result at
runtime
Difficulty: Unknown | Testcase: T6117
Blockedby: | Blocking:
Related: |
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Description changed by simonpj:
Old description:
> 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:
>
> 1. be accepted by the compiler even in cases where B (Additive a) is
> not
> derivable.
> 2. 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.
New description:
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:
1. be accepted by the compiler even in cases where `B (Additive a)` is
not derivable.
2. 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.
--
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/6117#comment:4>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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