Duncan Coutts wrote: | If the Eq instance equates all the invalid errnos then the Ord instance | should do so too because it is important that | compare a b == Eq <=> a == b Unfortunatly the default is not equal:
instance Eq Errno where errno1@(Errno no1) == errno2@(Errno no2) | isValidErrno errno1 && isValidErrno errno2 = no1 == no2 | otherwise
So if the errnos are invalid compare could return: A) {GT or LT} -- thus (a < b && b < a), no good B) EQ -- Contradicting the current definition of Eq C) Option 'B' and change the definition of Eq (allowing invalid == invalid) I encourage option C, which basically means deriving all the instances (the only invalid Errno is -1). I'm not sure how this affects existing code using Errno, but if its all static checks (unkErr == ePERM) then things should still be fine. Anyone know of the reason for or a use of the current behavior? Thomas