On Thu, 27 Nov 2003 14:56:03 +0000
Graham Klyne
But not all cases I encounter involve lists or monads. A different case might look like this:
Are you sure this doesn't involve monads?
eval :: (b->c->d) -> (a->b) -> (a->c) -> (a->d) eval f g1 g2 a = f (g1 a) (g2 a)
eval :: Monad m => (b -> c -> d) -> m b -> m c -> m d eval = liftM2
So, for example, a function to test of the two elements of a pair are the same might be:
pairSame = eval (==) fst snd
giving:
pairSame (1,2) -- false pairSame (3,3) -- true
Or a function to subtract the second and subsequent elements of a list from the first:
firstDiffRest = eval (-) head (sum . tail)
firstDiffRest [10,4,3,2,1] -- 0
(these work fine with a Monad instance ((->) r) which is a Reader monad)