data Either' a b = Left' a | Right' b

case' :: (a->g, b->g) -> Either' a b -> g case' (f, g) (Left' x) = f x case' (f, g) (Right' y) = g y

plus' :: (a->b , g->d) -> Either' a g -> Either' b d plus' (f,g) = case' (Left' f, Right' g)

case' takes as first argument a tuple of functions as described by the type "(a->g,b->g)": * a->g is a function taking something of type a and returning something of type g. * b->g is a function taking something of type b and returning something of type g. * (x,y) is a tuple taking something of type x as first element, and something of type y as second element. * Substitute x = a->g and y = b->g and you get a tuple of functions returning the same type g but taking as input types a and b respectively. case' takes as second argument something of type (Either' a b) which has 2 possible forms: an object (Left' x) or an object (Right' y). The following line states that the object x must be of type a, and the object y must be of type b:

data Either' a b = Left' a | Right' b

Take your function plus':

plus' :: (a->b , g->d) -> Either' a g -> Either' b d plus' (f,g) = case' (Left' f, Right' g)

Uncurry it (convert it such that all its arguments are explicit): plus' (f,g) = case' (Left' f, Right' g) => plus' (f,g) = \x -> case' (Left' f, Right' g) x => plus' (f,g) x = case' (Left' f, Right' g) x Notice that you are passing as first argument to case', not a tuple of functions, but a tuple of objects of type (Either i j). This is what the interpreter is telling you:

--ERROR sec2-5.hs:8 - Type error in application --*** Expression : case' (Left' f,Right' g) --*** Term : (Left' f,Right' g) --*** Type : (Either' (d -> e) f,Either' g (h -> i)) ^^^^^^^ ^^^^^^^ --*** Does not match : (a -> b,c -> b) ^^^^^^^^^^^^^

Look at the types of plus' and case', and at case' 's definition to figure out the solution. It's not difficult. Sengan