[Haskell-cafe] Derivation of Eq given Ord

Isaac Dupree isaacdupree at charter.net
Thu Aug 9 14:06:54 EDT 2007

Andreas Marth wrote:
> I would say that qualifies as a bug because it relays an error from compile
> time to run time.

It doesn't relay anything to run time - ghci has to _compile_ the 
expressions you give it too. If you _compile something_ successfully, 
you will know that _it_ will not fail in the stated way (which is all 
you need to know for a normal program with 'main').

However it does relay an error from deriving module to using 
[module/ghci], which may not be right.  HOWEVER, I can't reproduce it - 
ghc (6.6.1) always tells me
     No instance for (Eq Foo)
       arising from the superclasses of an instance declaration
whether data or newtype, deriving Ord, no Eq instance, and no using 
operations at all, and Hugs does too (in different words).


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