There's a possible programming idiom that I repeatedly find myself thinking about as I write my code. It feels like a kind of dual of fold, except that it's not necessarily confined to lists. I'm wondering if anything like this is discussed in the literature, or implemented in the libraries... Scene-setting: broadly speaking, fold takes a function and applies it to several values, and combines the results in some way. What I'm thinking of is a combinator or suchlike that takes two or more functions, applies them (separately) to a single value, then combines the results of those combinations. One function that I have defined and use in my code is called 'flist': flist :: [a->b] -> a -> [b] flist fs a = map ($ a) fs (which is similar to Monad.ap, except that the 'a' parameter is not a list/monad). I could imagine combining this with a fold in some cases. A more generalized form of this that works with arbitrary monads was suggested by Derek Elkin: fmonad :: Monad m => m (a->b) -> a -> m b fmonad fm a = do { f <- fm ; return $ f a } But not all cases I encounter involve lists or monads. A different case might look like this:
eval :: (b->c->d) -> (a->b) -> (a->c) -> (a->d) eval f g1 g2 a = f (g1 a) (g2 a)
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
That's about as far as I've taken this, but it feels as if it might be part of a more general pattern. Hence my question: is there anything like this in the libraries? #g ------------ Graham Klyne For email: http://www.ninebynine.org/#Contact