On 20/03/2012, at 2:27 PM, Jerzy Karczmarczuk wrote:
Richard O'Keefe:
class (Eq a, Show a) => Num a where (+) (-) (*) negate abs signum fromInteger
where functions are for good reason not members of Eq or Show.
This is an old song, changed several times. I have no intention to discuss, but please, Richard O'Keefe: WHICH GOOD REASONS??
It is still there in the Haskell 2010 report. The UHC user manual at http://www.cs.uu.nl/groups/ST/Projects/ehc/ehc-user-doc.pdf lists differences between UHC and both Haskell 98 and Haskell 2010, but is completely silent about any change to the interface of class Num, and in fact compiling a test program that does 'instance Num Foo' where Foo is *not* an instance of Eq or Show gives me this response: [1/1] Compiling Haskell mynum (mynum.hs) EH analyses: Type checking mynum.hs:3-11: Predicates remain unproven: preds: UHC.Base.Eq mynum.Foo: This is with ehc-1.1.3, Revision 2422:2426M, the latest binary release, downloaded and installed today. The release date was the 31st of January this year. GHC 7.0.3 doesn't like it either. I know ghc 7.4.1 is out, but I use the Haskell Platform, and the currently shipping version says plainly at http://hackage.haskell.org/platform/contents.html that it provides GHC 7.0.4. You may have no intention of discussing the issue, but it seems to *me* that "this will not work in 2012 Haskell compiler mostly conforming to Haskell 2010 because Haskell 2010 says it shouldn't work" is a pretty sound position to take.