## Newtype deriving should only work if superclasses are newtype-derived

Twan van Laarhoven writes: I just noticed some unexpected consequences of the way newtype deriving is implemented in GHC. Because the dictionary of the underlying type is reused, so are base classes. This message is a literate Haskell program illustrating the problem.

` > {-# OPTIONS_GHC -fglasgow-exts #-}`

This problem comes up when an instance method calls a method of a base class. Consider the following useless example:

```
> class Show a => Show2 a where
> show2 :: a -> String
> show2 = show
>
> instance Show2 Int
```

Now consider a type deriving Show2, but having a different implementation of Show (also derived).

```
> newtype Meter = Meter Int
> deriving (Eq, Show, Show2)
```

Now, for show2 the derived Show instance (which also shows the constructor name) is *not* used, instead the Show Int instance is used:

```
] > show2 (Meter 1)
] "1"
] > show (Meter 1)
] "Meter 1"
```

This is quite unexpected, unless you consider what is going on behind the scenes. Even more confusingly, GHC requires that there is a Show instance for Meters, even if it will not be used!

```
] No instance for (Show Meter)
] arising from the superclasses of an instance declaration at ...
] Probable fix: add an instance declaration for (Show Meter)
] In the instance declaration for `Show2 Meter'
```

This problem can come up whenever a class instance uses a function from a base class. Right now this not likely happen, but it will become more common if the standard classes are split up:

```
> class Additive a where
> add :: a -> a -> a
> class Additive a => Subtractive a where
> neg :: a -> a
> sub :: a -> a -> a
> sub x y = add x (neg y) -- calls base class function add
> class Functor m => Monad' m where
> return' :: a -> m a
> join' :: m (m a) -> m a
> join' x = bind' x id
> bind' :: m a -> (a -> m b) -> m b
> bind' ma k = join' (fmap k ma) -- calls base class function fmap
```

As a solution I would suggest that newtype deriving a class instance is only allowed if all base classes instances are also derived using newtype deriving. This presents problems for Show and Read, because they cannot be derived in that way. It will, however, catch problems with most other classes.

## Trac metadata

Trac field | Value |
---|---|

Version | 6.6 |

Type | Bug |

TypeOfFailure | OtherFailure |

Priority | normal |

Resolution | Unresolved |

Component | Compiler (Type checker) |

Test case | |

Differential revisions | |

BlockedBy | |

Related | |

Blocking | |

CC | Twan van Laarhoven [twanvl@gmail.com] |

Operating system | Unknown |

Architecture | Unknown |