#2176 closed bug (fixed)
H98 module Array exports non-H98 instance Functor ((->) a)
Reported by: | duncan | Owned by: | |
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Priority: | normal | Milestone: | 6.10.1 |
Component: | libraries/haskell98 | Version: | 6.8.2 |
Keywords: | Cc: | ||
Operating System: | Unknown/Multiple | Architecture: | Unknown/Multiple |
Type of failure: | Test Case: | ||
Blocked By: | Blocking: | ||
Related Tickets: | Differential Rev(s): | ||
Wiki Page: |
Description
Here's a subtle problem for which we have global class instances to thank.
Take a look at how many modules in base and the other core libraries re-export the class instances defined in Control.Monad.Instances. A lot do. Indeed the Data.Array and thus the H98 Array module does. That's very bad.
It makes it easy to accidentally write non-portable non-H98 code. This bit us in Cabal. We try to keep Cabal working with ghc, nhc98, hugs etc. Malcolm discovered that we were relying on the instance Functor ((->) a) but were not importing it for nhc98 at least. Nothing in Cabal imports Control.Monad.Instances so I was wondering how we were coming to end up with it. Turns out we're getting it at least via Data.Array.
Tracking down the source of instances is quite tricky. I wonder if there is anything we can do to make it easier? I was using ghc --show-iface on all the imports to try and find it. In this case it was easier because all I had to look for was Control.Monad.Instances as an orphan module.
We should audit which modules are importing Control.Monad.Instances and see if they're essential or just convenience. The point of Control.Monad.Instances being in a separate module was that it'd not be in scope by default. That's defeated if other standard modules use it. For example does Control.Applicative really need it? In fact Control.Applicative is probably the root offender here, it's the one that causes Data.Array to re-export the unwanted instances.
Do we need a Control.Applicative.Instances perhaps for the Applicative ((->) a) instance?
Attachments (1)
Change History (10)
comment:1 Changed 8 years ago by ross
comment:2 Changed 8 years ago by ross
To clarify, it's the Functor and Monad instances in Control.Monad.Instances that are the Good Thing.
comment:3 in reply to: ↑ description Changed 8 years ago by claus
Replying to duncan:
Tracking down the source of instances is quite tricky. I wonder if there is anything we can do to make it easier? I was using ghc --show-iface on all the imports to try and find it.
use ghci and :info?
GHCi, version 6.9.20080217: http://www.haskell.org/ghc/ :? for help Loading package base ... linking ... done. Prelude> :i Functor class Functor f where fmap :: (a -> b) -> f a -> f b -- Defined in GHC.Base instance Functor Maybe -- Defined in Data.Maybe instance Functor [] -- Defined in GHC.Base instance Functor IO -- Defined in GHC.IOBase Prelude> :m +Data.Array Prelude Data.Array> :i Functor class Functor f where fmap :: (a -> b) -> f a -> f b -- Defined in GHC.Base instance (Ix i) => Functor (Array i) -- Defined in GHC.Arr instance Functor ((->) r) -- Defined in Control.Monad.Instances instance Functor ((,) a) -- Defined in Control.Monad.Instances instance Functor (Either a) -- Defined in Control.Monad.Instances instance Functor Maybe -- Defined in Data.Maybe instance Functor [] -- Defined in GHC.Base instance Functor IO -- Defined in GHC.IOBase Prelude Data.Array>
one problem with this is that instances aren't managed properly in ghci sessions:
Prelude Data.Array> :m -Data.Array Prelude> :i Functor class Functor f where fmap :: (a -> b) -> f a -> f b -- Defined in GHC.Base instance Functor ((->) r) -- Defined in Control.Monad.Instances instance Functor ((,) a) -- Defined in Control.Monad.Instances instance Functor (Either a) -- Defined in Control.Monad.Instances instance Functor Maybe -- Defined in Data.Maybe instance Functor [] -- Defined in GHC.Base instance Functor IO -- Defined in GHC.IOBase Prelude>
i thought the latter was a known bug, but i can't seem to find the ticket..
comment:4 Changed 8 years ago by ross
No good: Data.Foldable and Data.Traversable also import Control.Applicative.
comment:5 Changed 8 years ago by igloo
- difficulty set to Unknown
- Milestone set to 6.8.3
comment:6 Changed 8 years ago by igloo
- Milestone changed from 6.8.3 to 6.10.1
comment:7 Changed 8 years ago by ross
- Resolution set to fixed
- Status changed from new to closed
Fixed by elimination of orphan instances in Data.Array and then the patch
Sun Aug 17 01:23:48 BST 2008 Ross Paterson <ross@soi.city.ac.uk> * delete imports of Data.Foldable and Data.Traversable (fixes #2176)
comment:8 Changed 8 years ago by simonmar
- Architecture changed from Unknown to Unknown/Multiple
comment:9 Changed 8 years ago by simonmar
- Operating System changed from Unknown to Unknown/Multiple
Here's a test for this bug:
This can be used to show that Array is the only H98 module where this bites. The problem is that it defines Foldable and Traversable instances for the Array type, and thus needs those modules and Control.Applicative, which they import. (Another vector is Control.Monad.Fix, imported by Control.Arrow, but that doesn't seem to lead to H98 modules.)
I believe that these instances are a Good Thing, and we want them everywhere, except in H98 modules.
I think the fix is to have an extra version of Data.Array, without the Foldable and Traversable instances (which are orphans anyway), and have Array import that.