RE: evaluation fault in ghci.

> The interactive loop of ghci displays an interesting evaluation fault
> to do with derived equality.  In the attached source file, there is
> a simple guard which tests some equalities, and basically the same
> value is given on the left and right of the (==).  Yet, it evaluates
> to False in interactive mode, whereas it (correctly) evaluates to True
> when compiled.
> 
> 
> $ ghci
>    ___         ___ _
>   / _ \ /\  /\/ __(_)
>  / /_\// /_/ / /  | |      GHC Interactive, version 5.04.2, 
> for Haskell 98.
> / /_\\/ __  / /___| |      http://www.haskell.org/ghc/
> \____/\/ /_/\____/|_|      Type :? for help.
> 
> Loading package base ... linking ... done.
> Loading package haskell98 ... linking ... done.
> Prelude> :l Small
> Compiling Main             ( Small.hs, interpreted )
> Ok, modules loaded: Main.
> *Main> main
> "*** Exception: Four.bceFour: precondition fails:arrows not parallel
> f = E
> g = E

Nice bug.  We've fixed it in CVS, but I fear the changes are too large
to backport.  The workaround is to define your own instance of Eq.

Cheers,
        Simon