#2356: GHC accepts multiple instances for the same type in different modules
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Reporter: claus | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler
Version: 6.8.2 | Severity: normal
Keywords: | Testcase:
Architecture: Unknown | Os: Unknown
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as mentioned by Simon PJ in this thread:
http://www.haskell.org/pipermail/haskell/2008-June/020436.html
here is the example, spelled out:
{{{
module B0 where
class C a where c :: a -> String
data T = T deriving Show
module B1 where
import B0
instance C T where c _ = "B1"
b = c T
module B2 where
import B0
instance C T where c _ = "B2"
b = c T
module Main where
import B1
import B2
main = print (B1.b,B2.b)
}}}
this is accepted without flags or errors and prints `("B1","B2")`.
the [http://haskell.org/onlinereport/decls.html#sect4.3.2 language report,
section 4.3.2] clearly states:
A type may not be declared as an instance of a particular class more
than once in the program.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2356>
GHC <http://www.haskell.org/ghc/>
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