Re: [GHC] #2247: GHC accepts FD violations, unless the conflicing instances are used
#2247: GHC accepts FD violations, unless the conflicing instances are used -+-- Reporter: claus| Owner: Type: bug | Status: reopened Priority: normal | Milestone: _|_ Component: Compiler (Type checker) |Version: 6.9 Severity: normal | Resolution: Keywords: TF vs FD | Difficulty: Unknown Testcase: | Architecture: Unknown Os: Unknown | -+-- Changes (by igloo): * milestone: = _|_ Comment: #1241 is _|_, so I guess this should be too -- Ticket URL: http://hackage.haskell.org/trac/ghc/ticket/2247#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler___ Glasgow-haskell-bugs mailing list Glasgow-haskell-bugs@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-bugs
Re: [GHC] #2247: GHC accepts FD violations, unless the conflicing instances are used
#2247: GHC accepts FD violations, unless the conflicing instances are used
-+--
Reporter: claus| Owner:
Type: bug | Status: closed
Priority: normal | Milestone:
Component: Compiler (Type checker) |Version: 6.9
Severity: normal | Resolution: duplicate
Keywords: TF vs FD | Difficulty: Unknown
Testcase: | Architecture: Unknown
Os: Unknown |
-+--
Changes (by simonpj):
* status: new = closed
* difficulty: = Unknown
* resolution: = duplicate
Comment:
I assume you are using some flags?
{{{
ghc -c -XFunctionalDependencies Foo.hs -XMultiParamTypeClasses
Foo.hs:5:1:
Illegal instance declaration for `FD a b'
(All instance types must be of the form (T a1 ... an)
where a1 ... an are distinct type *variables*
Use -XFlexibleInstances if you want to disable this.)
In the instance declaration for `FD a b'
}}}
Adding `-XFlexibleInstances` gives
{{{
bash-3.1$ ghc -c -XFunctionalDependencies Foo.hs -XMultiParamTypeClasses
-XFlexibleInstances
Foo.hs:5:1:
Constraint is no smaller than the instance head
in the constraint: CFD a b
(Use -fallow-undecidable-instances to permit this)
In the instance declaration for `FD a b'
Foo.hs:5:1:
Illegal instance declaration for `FD a b'
(the Coverage Condition fails for one of the functional
dependencies;
Use -fallow-undecidable-instances to permit this)
In the instance declaration for `FD a b'
}}}
In fact to get `Improve` to compile you need
{{{
$gpj -c -XFunctionalDependencies
-XMultiParamTypeClasses
-XFlexibleInstances
-fallow-undecidable-instances
-XFlexibleContexts
Foo.hs
}}}
The offending one is `-fallow-undecidable-instances`, and this ticket is
an excellent example of #1241. Currently saying `-fallow-undecidable-
instances` lifts the coverage condition. I acknowledge that this isn't the
Right Thing in #1241, but it doesn't threaten type soundness (ie programs
will not seg-fault).
So I'm going to close this bug and link to it from #1241. Re-open if I
have misunderstood.
Simon
--
Ticket URL: http://hackage.haskell.org/trac/ghc/ticket/2247#comment:2
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Re: [GHC] #2247: GHC accepts FD violations, unless the conflicing instances are used
#2247: GHC accepts FD violations, unless the conflicing instances are used
+---
Reporter: claus|Owner:
Type: bug | Status: new
Priority: normal |Milestone:
Component: Compiler (Type checker) | Version: 6.9
Severity: normal | Resolution:
Keywords: TF vs FD | Testcase:
Architecture: Unknown | Os: Unknown
+---
Comment (by claus):
here is a slight variation that does not rely on a GHCi session to raise
the issue
{{{
module Improve where
class FD a b | a - b
instance CFD a b = FD a b
class {- FD a b = -} CFD a b
instance CFD Bool Char
instance CFD Bool Bool
f' :: FD Bool Bool = Bool
f' = True
g' :: FD Bool Char = Bool
g' = False
x = f'
}}}
{{{
module Main where
import Improve
y = g'
main = print (x,y)
}}}
GHC (6.9.20080217) accepts this, and the executable outputs
`(True,False)`, even though the first component depends on `instance FD
Bool Bool` while the second depends on `instance FD Bool Char`.
Inlining `x` as `f'` is sufficient to raise the expected error message.
--
Ticket URL: http://hackage.haskell.org/trac/ghc/ticket/2247#comment:1
GHC http://www.haskell.org/ghc/
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