Thanks for this - I’ll work through it.
Yeah, apparently I accidentaly wrote a correct instance there. You can also use what Gesh wrote, and try to derive an instance from something like: tabulate :: (Bool -> a) -> Pair a tabulate f = Pair (f True) (f False) index :: Pair a -> Bool -> a index (Pair a _) True = a index (Pair _ b) False = b instance Applicative Pair where pure = tabulate . pure fp <*> fa = tabulate $ index fp <*> index fa instance Monad Pair where m >>= f = tabulate $ index m >>= index . f The instances for a function are (in pseudo-Haskell, I don't think GHC accepts (a ->) instead of ((->) a), but it looks cleaner that way) instance Applicative (a ->) where pure = const f <*> g = \x -> f x (g x) instance Monad (a ->) where f >>= k = \ r -> k (f r) r
My wife once dreamt she was a C compiler - go figure… C !!??
C isn't exactly known for type safety, so things can indeed get Kafkaesque sometimes. Best regards, Marcin Mrotek