do {x <- (Just 3); y <- (Just 5); return (x:y:[])}
desugars to
Just 3 >>= \x -> Just 5 >>= \y -> return $ x:y:[]
Since the type >>= is equal to Monad m => ma -> (a -> mb) -> mb and for each argument Just 3 (alternatively Just 5 ), we have m ~ Maybe , the return type of the expression must be some type Maybe .
It is possible to make this return [[3, 5]] using natural transformations from . Since there is a natural transformation from Maybe a to [a] , namely
alpha :: Maybe a -> [a] alpha Nothing = [] alpha (Just a) = [a]
we have that your desired function is just a natural transformation applied to the result:
alpha (Just 3 >>= \x -> Just 5 >>= \y -> return $ x:y:[]) -- returns [[3, 5]]
Since this is a natural transformation, you can also apply alpha first and second:
alpha (Just 3) >>= \x -> alpha (Just 5) >>= \y -> return $ x:y:[] -- returns [[3, 5]]
As @duplode pointed out, you can find alpha in the Data.Maybe package as maybeToList .