Hal I have fixed this bug in the HEAD. The fix is more than a tiny one, so I'm inclined to leave it as a know bug in 5.02.2. It'd cost me another few hours to patch it into the 5.02 branch. I don't think this is a widely-used feature, yet. Even John Hughes didn't notice the bug. Simon | -----Original Message----- | From: Hal Daume III [mailto:hdaume@ISI.EDU] | Sent: 02 October 2002 18:58 | To: GHC Users Mailing List | Subject: deriving weirdness on newtypes | | So I love the fact that I can derive anything I want on | newtypes. However, there seem to be problems with it. If I write: | | newtype Foo = Foo Int deriving (Show) | x = show (Foo 5) | | Then x is "Foo 5" | | However, if I do | | newtype Foo = Foo Int deriving (Num) | x = show (Foo 5) | | Then the definition of 'x' claims that there is no Show instance of Foo. | | However (this is where it gets weird). If I do: | | newtype Foo = Foo Int deriving (Num) | x = show ((Foo 5) + 4) | | then x is *valid* and has value "9", not "Foo 9". Moreoever, I can even | do: | | x = show ((5::Foo) + 4) | | and this x has value "9". | | Futhermore, if I do: | | newtype Foo = Foo Int deriving (Show,Num) | | then "show (Foo 5)" has value "5", not "Foo 5". | | IMO, the correct value of "show (Foo 5)" no matter which of the above we | have is "Foo 5". I think the deriving mechanism should be changed in two | ways. | | (1) If you derive X on a newtype and X is a subclass of Y, | you must have an instance of Y on that newtype. This is | how it works on data types, and I think makes sense. | For instance, | newtype Foo = Foo Int deriving (Ord) | should be just as illegal as | data Foo = Foo Int deriving (Ord) | | (2) The derived Show instances for newtypes in the presense of | derived Num instances should be fixed so that the | constructor is also printed. Rational: adding something | to the list of derived classes should *not* change | previously derived instances. | | There is yet another thing. If we have: | | newtype Foo = Foo Int deriving (Num) | instance Show Foo where { show = undefined } | | then, the value of 'show (Foo 5)' is undefined, but the value of 'show | (5::Foo)' is "5". definately *WRONG*. | | - Hal | | -- | Hal Daume III | | "Computer science is no more about computers | hdaume@isi.edu | than astronomy is about telescopes." -Dijkstra | www.isi.edu/~hdaume | | _______________________________________________ | Glasgow-haskell-users mailing list | Glasgow-haskell-users@haskell.org | http://www.haskell.org/mailman/listinfo/glasgow-haskell-users