#1999: panic with GADT etc.
----------------------------+-----------------------------------------------
Reporter: jeltsch | Owner:
Type: bug | Status: new
Priority: normal | Milestone: 6.10 branch
Component: Compiler | Version: 6.9
Resolution: | Keywords:
Testcase: T1999, T1999a | Blockedby:
Difficulty: Unknown | Os: Linux
Blocking: | Architecture: x86
Failure: None/Unknown |
----------------------------+-----------------------------------------------
Changes (by simonpj):
* cc: sweirich@…, jbapple@…, byorgey@… (added)
* owner: chak =>
* status: closed => new
* resolution: fixed =>
Old description:
> The following code causes a panic with GHC installed from
> ghc-6.9.20071218-i386-unknown-linux.tar.bz2:
> {{{
> {-# LANGUAGE FlexibleContexts, FlexibleInstances, GADTs #-}
> module Bug where
>
> class C a where
>
> f :: G a -> ()
>
> instance (C ()) => C (b c) where
>
> f (G x) = f x where
>
> data G a where
>
> G :: G () -> G (b c)
> }}}
> The output of ghci is:
> {{{
> GHCi, version 6.9.20071218: http://www.haskell.org/ghc/ :? for help
> Loading package base ... linking ... done.
> [1 of 1] Compiling Bug ( Bug.hs, interpreted )
> ghc-6.9.20071218: panic! (the 'impossible' happened)
> (GHC version 6.9.20071218 for i386-unknown-linux):
> Coercion.splitCoercionKindOf
> base:GHC.Prim.left{(w) tc 34B} $co${tc aog} [tv]
> <pred>b{tv anB} [sk] ~ b{tv aoc} [sk]
>
> Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug
> }}}
> The output of ghc -c is:
> {{{
> ghc-6.9.20071218: panic! (the 'impossible' happened)
> (GHC version 6.9.20071218 for i386-unknown-linux):
> Coercion.splitCoercionKindOf
> base:GHC.Prim.left{(w) tc 34B} $co${tc a6t} [tv]
> <pred>b{tv a5O} [sk] ~ b{tv a6p} [sk]
>
> Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug
> }}}
New description:
The following code causes a panic with GHC installed from
ghc-6.9.20071218-i386-unknown-linux.tar.bz2:
{{{
{-# LANGUAGE FlexibleContexts, FlexibleInstances, GADTs #-}
module Bug where
class C a where
f :: G a -> ()
instance (C ()) => C (b c) where
f (G x) = f x where
data G a where
G :: G () -> G (b c)
}}}
The output of ghci is:
{{{
GHCi, version 6.9.20071218: http://www.haskell.org/ghc/ :? for help
Loading package base ... linking ... done.
[1 of 1] Compiling Bug ( Bug.hs, interpreted )
ghc-6.9.20071218: panic! (the 'impossible' happened)
(GHC version 6.9.20071218 for i386-unknown-linux):
Coercion.splitCoercionKindOf
base:GHC.Prim.left{(w) tc 34B} $co${tc aog} [tv]
<pred>b{tv anB} [sk] ~ b{tv aoc} [sk]
Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug
}}}
The output of ghc -c is:
{{{
ghc-6.9.20071218: panic! (the 'impossible' happened)
(GHC version 6.9.20071218 for i386-unknown-linux):
Coercion.splitCoercionKindOf
base:GHC.Prim.left{(w) tc 34B} $co${tc a6t} [tv]
<pred>b{tv a5O} [sk] ~ b{tv a6p} [sk]
Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug
}}}
--
Comment:
In our POPL paper [http://research.microsoft.com/en-
us/um/people/simonpj/papers/ext-f/popl163af-weirich.pdf "Generative type
abstraction and type-level computation"] we proposed to '''remove the
ability to decomopose type applications'''. So Ganesh's example above
would not work any more:
{{{
data EqTypes a b where
EqConstr :: EqTypes a b -> EqTypes (s a) (s b)
eqUnConstr :: EqTypes (s a) (s b) -> EqTypes a b
eqUnConstr (EqConstr eq) = eq
}}}
Neither would Jim Apple's example.
This is an un-forced change, and it does reduce expressiveness slightly,
with no workaround. On the other hand, it does undoubtedly make things a
bit simpler and, as Section 6.2 of the paper says, it allows FC to have
un-saturated type functions. BUT for type ''inference'' we still need
saturated type functions, so allowing un-saturated type functions in the
intermediate language only a theoretical improvement -- programmers don't
see the benefit.
Dimitrios and I think that it'd be possible to re-add `left` and `right`
coercions, without too much hassle (and re-add the saturated-type-function
reqt to FC). Stephanie, Brent: do you agree?
Meanwhile, Gasnesh, Jim, how much do you care?
All this relates to the `ghc-new-co` branch, which will shortly become the
master, so it's of more than theoretical interest.
Simon
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/1999#comment:12>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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