#4259: Relax restrictions on type family instance overlap
----------------------------------------+-----------------------------------
Reporter: lilac | Owner:
Type: feature request | Status: new
Priority: normal | Milestone: 7.4.1
Component: Compiler (Type checker) | Version: 6.12.1
Keywords: | Testcase:
Blockedby: | Difficulty:
Os: Unknown/Multiple | Blocking:
Architecture: Unknown/Multiple | Failure: None/Unknown
----------------------------------------+-----------------------------------
Comment(by basvandijk):
I would like to give another use-case for overlapping type family
instances:
As explained in ticket #5595 I'm giving `monad-control`
[https://github.com/basvandijk/monad-
control/blob/master/Control/Monad/Trans/Control.hs a new design].
My [http://hackage.haskell.org/package/regions regions] package uses
`monad-control` so I'm now adapting that package to the new design.
However I'm stuck because I can't use an overlapped instance of an
associated data type. First some background:
For [https://github.com/basvandijk/regions/wiki/unsafeLiftControlIO
safety-reasons] I don't want to make `RegionT` an instance of
`MonadControlIO`. However I do need the ability to lift control operators
like `bracket` or `alloca` into regions.
So what I did is, I created a class which is isomorphic to
`MonadControlIO` called
[http://hackage.haskell.org/packages/archive/regions/0.11/doc/html
/Control-Monad-Trans-Region.html#t:RegionControlIO RegionControlIO]. Just
like `MonadControlIO` has the method `liftControlIO`, this class has the
method [http://hackage.haskell.org/packages/archive/regions/0.11/doc/html
/Control-Monad-Trans-Region-Unsafe.html#v:unsafeLiftControlIO
unsafeLiftControlIO]. This method however is only exported from the
`Unsafe` module. So users are warned when using it. (I plan to use
`SafeHaskell` to formalize this)
I made `RegionT` an instance of `RegionControlIO`:
{{{
instance RegionControlIO pr ⇒ RegionControlIO (RegionT s pr) where
unsafeLiftControlIO f = ...
}}}
Besides that I added a catch-all instance:
{{{
{-# LANGUAGE OverlappingInstances #-}
instance MonadControlIO m ⇒ RegionControlIO m where
unsafeLiftControlIO = liftControlIO
}}}
These two instances allows you to lift control operators into a stack of
regions with an arbitrary stack of monad transformers at its base. For
example: `RegionT s1 (RegionT s2 (RegionT s3 (StateT Int (WriterT String
IO))))`
Now back to the problem. As said, I'm adapting `regions` to use the new
design of `monad-control`. The `RegionControlIO` type class now looks like
this:
{{{
class MonadIO m ⇒ RegionControlIO m where
data RegionStIO m ∷ * → *
unsafeLiftControlIO ∷ (RegionRunInIO m → IO α) → m α
unsafeRestore ∷ RegionStIO m α → m α
type RegionRunInIO m = ∀ β. m β → IO (RegionStIO m β)
}}}
regions are again made an instance of this type class:
{{{
instance RegionControlIO pr ⇒ RegionControlIO (RegionT s pr) where
newtype RegionStIO (RegionT s pr) α = StIOR (RegionStIO pr ... α))
unsafeLiftControlIO f = ...
unsafeRestore (StIOR stIO) = ...
}}}
As before, I also want to add a catch-all instance:
{{{
instance MonadControlIO m ⇒ RegionControlIO m where
newtype RegionStIO m α = StIOG (StIO m α)
unsafeLiftControlIO f = liftControlIO $ \runInIO → f $ liftM StIOG ∘
runInIO
unsafeRestore (StIOG st) = restore st
}}}
However this is not allowed because type families may not overlap:
{{{
Conflicting family instance declarations:
newtype RegionStIO (RegionT s pr) α
newtype RegionStIO m α
}}}
This is currently blocking me to use the new design of `monad-control` in
my `regions` library.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4259#comment:11>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
_______________________________________________
Glasgow-haskell-bugs mailing list
[email protected]
http://www.haskell.org/mailman/listinfo/glasgow-haskell-bugs
