RE: incoherent instance selection when it should be still coherent

What I really find odd about it is that GHC has a notion of coherences
that it tries to adhere to (and this is great), unless you ask it to
violate coherence. However this specific situation is the only case that
I am aware of where GHC is even incoherent when not positively
instructed to may be so ...

Is there some well-argued example where someone would explain why this
could be at all useful?

Thanks,
Ralf

> -----Original Message-----
> From: Simon Peyton-Jones
> Sent: Tuesday, July 25, 2006 5:45 AM
> To: Ralf Lammel; glasgow-haskell-bugs@xxxxxxxxxxx
> Subject: RE: incoherent instance selection when it should be still
> coherent
> 
> It's clear that the type checker cannot do better, because in
>       uuh (Wrap x) (Wrap y) = bar x y
> the call to 'bar' can use *only* evidence stored in the Wrap
constructor.
> Previously GHC gave an error message at this point, but some
collection of
> users persuaded me that the behaviour below is the "right" one.  As
you
> say, it's not altogether clear!
> 
> Simon
> 
> | -----Original Message-----
> | From: glasgow-haskell-bugs-bounces@xxxxxxxxxxx
[mailto:glasgow-haskell-
> bugs-bounces@xxxxxxxxxxx]
> | On Behalf Of Ralf Lammel
> | Sent: 24 July 2006 09:53
> | To: glasgow-haskell-bugs@xxxxxxxxxxx
> | Subject: FW: incoherent instance selection when it should be still
> coherent
> |
> | Repro contained in forwarded email.
> |
> | Here is a bit more reasoning. I guess *always* selecting the generic
> | instance is really incoherent, and then best called a bug. Since
more
> | specific instance selection does not make sense w/o violating the
> | contract of existential types, I also feel like concluding that the
> | program must not typecheck in the first place. I do agree that
> | "-fallow-incoherent-instances" could admit this program, if the
option
> | was chosen (even though I generally fail to understand the use of
this
> | option anyway :-).
> |
> | Thanks,
> | Ralf
> |
> | -----Original Message-----
> | From: Ralf Lammel
> | Sent: Friday, July 21, 2006 5:37 PM
> | To: 'haskell-cafe@xxxxxxxxxxx'
> | Subject: incoherent instance selection when it should be still
coherent
> |
> | The following pain is sort of ghc (6.4.2) specific.
> | (The same behavior is achievable with hugs -98 +O which is Ok in so
far
> | that +O (as opposed to +o) is not strongly supposed to be coherent.)
> |
> | Note the following GHC options preceding the code.
> | They do not contain -fallow-incoherent-instances.
> |
> | ---------------------------------------------------------
> |
> | {-# OPTIONS -fglasgow-exts #-}
> |
> | {-# OPTIONS -fallow-undecidable-instances #-}
> |
> | {-# OPTIONS -fallow-overlapping-instances #-}
> |
> |
> | -- A single parameter class with two silly instances
> |
> | class Foo x where foo :: x -> (); foo = const ()
> |
> | instance Foo ()
> |
> | instance Foo Bool
> |
> |
> | -- A two-parameter class with a generic and a non-generic instance
> |
> | class (Foo x, Foo y) => Bar x y where bar :: x -> y -> String
> |
> | instance (Foo x, Foo y) => Bar x y where bar _ _ = "generic
instance"
> |
> | instance Foo z => Bar () z where bar _ _ = "non-generic instance"
> |
> |
> | -- An existential wrapper around foos
> |
> | data Wrap = forall x. Foo x => Wrap x
> |
> |
> | -- A wrapper-based variation on the type-class member bar
> |
> | uuh :: Wrap -> Wrap -> String
> |
> | uuh (Wrap x) (Wrap y) = bar x y
> |
> |
> | -- Let's try all unwrapped and wrapped combinations of bar and uuh
> |
> | t1 = ()
> |
> | t2 = True
> |
> | w1 = Wrap t1
> |
> | w2 = Wrap t2
> |
> | main = do
> |           print $ bar t1 t1
> |           print $ uuh w1 w1 -- uuh!
> |           print $ bar t1 t2
> |           print $ uuh w1 w2 -- uuh!
> |           print $ bar t2 t1
> |           print $ uuh w2 w1
> |           print $ bar t2 t2
> |           print $ uuh w2 w2
> |
> | --------------------------------------------------------
> |
> | We get:
> |
> | {-
> |
> | "non-generic instance"
> | "generic instance"
> | "non-generic instance"
> | "generic instance"
> | "generic instance"
> | "generic instance"
> | "generic instance"
> | "generic instance"
> |
> | -}
> |
> | This means that the generic instance is consistently chosen by uuh.
> | This is clearly incoherent.
> | I would also complain that uuh type-checks in the first place.
> | Opinions?
> |
> | Regards,
> | Ralf
> |
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