#3734: overlapping orphan instances behave like incoherent without warning/error
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Reporter: Liskni_si | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler (Type checker)
Version: 6.10.4 | Keywords:
Os: Unknown/Multiple | Testcase:
Architecture: Unknown/Multiple | Failure: Incorrect result at runtime
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Consider these three modules:
{{{
module A where
class (Show a) => A a
data A' = A' deriving (Show)
instance A A'
data A'' = A'' deriving (Show)
instance A A''
print_a :: (A a) => a -> IO ()
print_a a = print a
}}}
{{{
{-# LANGUAGE FlexibleInstances, OverlappingInstances #-}
module B where
import A
data B a = B a deriving (Show)
instance (A a) => A (B a)
}}}
{{{
{-# LANGUAGE FlexibleInstances, OverlappingInstances #-}
module Main where
import A
import B
instance Show (B A') where
show _ = "kokodak"
instance Show (B A'') where
show _ = "brekeke"
instance A (B A'')
main :: IO ()
main = do
print (B A')
print_a (B A')
putStrLn ""
print (B A'')
print_a (B A'')
}}}
Without understanding a thing about dictionaries, I would expect that if
this actually compiles (which I now understand it should not), I'd get
`"kokodak kokodak brekeke brekeke"` as output, but I got `"kokodak B A'
brekeke brekeke"` instead.
I figured that even though I redefined `Show (B A')`, the `A (B a)`
instance was defined in module `B` and consisted of the original `Show`
dictionary. If I move the `Show (B A')` instance to module B, ghc
complains that the definition of `A (B a)` depends on the instatiation of
`a` and refuses to compile it, unless I enable `IncoherentInstances`.
The problem here is that if the `Show (B A')` instance is orphan, I get
the `IncoherentInstances` behaviour for free without any warning or error,
giving me the false feeling that the code is actually OK. Is it possible
that ghc gives an error in this case, and may the documentation mention
that Overlapping + Orphan => Incoherent?
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3734>
GHC <http://www.haskell.org/ghc/>
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