#7099: 'instance Show' results in stack overflow
----------------------------+-----------------------------------------------
Reporter: pl | Owner:
Type: bug | Status: closed
Priority: normal | Milestone:
Component: Compiler | Version: 7.4.2
Resolution: invalid | Keywords:
Os: Linux | Architecture: x86_64 (amd64)
Failure: Runtime crash | Difficulty: Unknown
Testcase: | Blockedby:
Blocking: | Related:
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Changes (by pcapriotti):
* status: new => closed
* difficulty: => Unknown
* resolution: => invalid
Old description:
> When I load the code below in ghci and run 'create :: C' the execution
> ends after a while with a stack overflow. However, when I delete the
> line 'instance Show C' and erase the '--' before 'deriving (Show)' the
> command 'create :: C' delivers a 'C' as I would expect it.
>
> > module Main where[[BR]]
> >[[BR]]
> > data C = C --deriving (Show)[[BR]]
> >[[BR]]
> > instance Show C[[BR]]
> >[[BR]]
> > class T m where[[BR]]
> > create :: m [[BR]]
> >
> > instance T C where[[BR]]
> > create = C[[BR]]
New description:
When I load the code below in ghci and run 'create :: C' the execution
ends after a while with a stack overflow. However, when I delete the line
'instance Show C' and erase the '--' before 'deriving (Show)' the command
'create :: C' delivers a 'C' as I would expect it.
{{{
module Main where
data C = C --deriving (Show)[[BR]]
instance Show C
class T m where
create :: m
instance T C where
create = C
}}}
--
Comment:
This is expected behavior. The `Show` class defines defaults for all its
methods, and the `instance Show C` line creates an empty instance where
all the defaults are used, resulting in a non-terminating definition of
`show`.
If for some reason you want a standalone `deriving` statement, use
`-XStandaloneDeriving` and
{{{
deriving instance Show C
}}}
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7099#comment:1>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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