Hi
As an example, I'll use the Maybe monad. Suppose I want to write code to handle experimental data, in which there might be missing values. I might then decide to represent measurements by data of type "Maybe Double", with missing values represented by "Nothing". I could then go on to define functions on missing values, which would return "Nothing" when their argument is "Nothing", and I could string these functions together via the monad mechanism. Fine. But how would I handle e.g. addition of two such values? The result should be "Nothing" when either of its arguments is "Nothing". Is there any mechanism to handle that?
Yes, liftM2. Defined in module Monad (or Data.Monad resp.).
Konrad.
Wolfgang
Or, more generally, infixl 9 <$> (<$>) :: Monad m => m (s -> t) -> m s -> m t mf <$> ms = do f <- mf s <- ms return (f s) Now your lifted sum is return (+) <$> mx <$> my Being a sick type class hacker (a symptom of the Haskell guilt caused by working with dependent types) I've constructed an overloaded operator fun :: Monad m => (t0 -> ... -> tn) -> (m t0 -> ... -> m tn) where tn is of ground type. Effectively fun f x0 ... xn-1 = return f <$> x0 <$> ... <$> xn-1 In fact, it's good to weaken the requirement (on fun and <^>) from `being a Monad' to being Fun, where class Fun f where eta :: x -> f x (<$>) :: f (s -> t) -> f s -> f t `supporting return and <$>', as there are plenty of such structures which are not monadic (eg. lists wrt repeat and zipWith ($)). It's even more fun to work with lifted functors class LFunctor f where (<^>) :: Fun m => (s -> m t) -> f s -> m (f t) with, for example instance LFunctor [] where f <^> [] = fun [] f <^> (x : xs) = fun (:) (f x) (f <^> xs) You can use <^> to define mapping, flattening and all sorts of other goodies. What it comes down to, I suppose, is that sometimes we want to use the functional idiom to write programs modulo some modality, eg Maybe-ness statefulness, non-determinism, etc. I guess that Arrows generalize all this stuff still further, but the Fun class above is cheap and remarkably cheerful. I use it all the time... Cheers Conor