#9590: AMP breaks `haskell2010` package -------------------------------------+------------------------------------- Reporter: hvr | Owner: Type: bug | Status: new Priority: high | Milestone: 7.10.1 Component: | Version: 7.9 libraries/haskell2010 | Keywords: AMP Resolution: | Architecture: Unknown/Multiple Operating System: | Difficulty: Unknown Unknown/Multiple | Blocked By: Type of failure: GHC | Related Tickets: rejects valid program | Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Old description:
(and probably also `haskell98`)
I hate to be the one pointing this out, but the AMP has one ugly side- effect.
Consider the following session:
{{{ $ inplace/bin/ghc-stage2 --interactive -XHaskell2010 -hide-all-packages -package haskell2010 GHCi, version 7.9.20140914: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim-0.3.1.0 ... linking ... done. Loading package integer-gmp-0.5.1.0 ... linking ... done. Loading package base-4.8.0.0 ... linking ... done. Loading package array-0.5.0.1 ... linking ... done. Loading package haskell2010-1.1.2.1 ... linking ... done.
λ:2> data Identity a = Identity a data Identity a = Identity a
λ:3> instance Monad Identity
<interactive>:3:10: No instance for (base-4.8.0.0:GHC.Base.Applicative Identity) arising from the superclasses of an instance declaration In the instance declaration for ‘Monad Identity’
λ:4> :info Monad class base-4.8.0.0:GHC.Base.Applicative m => Monad (m :: * -> *) where (>>=) :: m a -> (a -> m b) -> m b (>>) :: m a -> m b -> m b return :: a -> m a fail :: String -> m a -- Defined in ‘base-4.8.0.0:GHC.Base’ instance Monad (Either e) -- Defined in ‘base-4.8.0.0:Data.Either’ instance Monad Maybe -- Defined in ‘base-4.8.0.0:Data.Maybe’ instance Monad [] -- Defined in ‘base-4.8.0.0:GHC.Base’ instance Monad IO -- Defined in ‘base-4.8.0.0:GHC.Base’ instance Monad ((->) r) -- Defined in ‘base-4.8.0.0:GHC.Base’ λ:5>
}}}
New description: (and probably also `haskell98`) I hate to be the one pointing this out, but the AMP has one ugly side- effect: As of now, GHC HEAD rejects Haskell2010 programs (which know nothing about the `Applicative` class). Moreover, there's no simple way to access the `Applicative` class in order to write an instance to satisfy non-standard superclass requirement, so it's impossible to define custom `Monad` instances. For instance, consider the following session which worked in GHC 7.6.3 (albeit with an AMP warning): {{{ $ inplace/bin/ghc-stage2 --interactive -XHaskell2010 -hide-all-packages -package haskell2010 GHCi, version 7.9.20140914: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim-0.3.1.0 ... linking ... done. Loading package integer-gmp-0.5.1.0 ... linking ... done. Loading package base-4.8.0.0 ... linking ... done. Loading package array-0.5.0.1 ... linking ... done. Loading package haskell2010-1.1.2.1 ... linking ... done. λ:2> data Identity a = Identity a data Identity a = Identity a λ:3> instance Monad Identity <interactive>:3:10: No instance for (base-4.8.0.0:GHC.Base.Applicative Identity) arising from the superclasses of an instance declaration In the instance declaration for ‘Monad Identity’ λ:4> :info Monad class base-4.8.0.0:GHC.Base.Applicative m => Monad (m :: * -> *) where (>>=) :: m a -> (a -> m b) -> m b (>>) :: m a -> m b -> m b return :: a -> m a fail :: String -> m a -- Defined in ‘base-4.8.0.0:GHC.Base’ instance Monad (Either e) -- Defined in ‘base-4.8.0.0:Data.Either’ instance Monad Maybe -- Defined in ‘base-4.8.0.0:Data.Maybe’ instance Monad [] -- Defined in ‘base-4.8.0.0:GHC.Base’ instance Monad IO -- Defined in ‘base-4.8.0.0:GHC.Base’ instance Monad ((->) r) -- Defined in ‘base-4.8.0.0:GHC.Base’ λ:5> }}} -- Comment (by hvr): (point out the problem in the description more explicitly) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9590#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler