[Haskell-cafe] Is this haskelly enough?
rendel at rbg.informatik.tu-darmstadt.de
Wed Jul 18 10:43:00 EDT 2007
Johan Tibell wrote:
> I found myself wanting a map that looks at neighboring elements. This is
> where I used explicit recursion the most. Something like this:
> f  = 
> f ((Foo a) : (Bar b) : xs)
> | fooBar a b = Foo a : f xs
> | otherwise = Bar b : f xs
> This is almost a map. A variation is when filtering and you want some
> look-ahead to make the filtering decision. There's probably a good way
> to do this I'm not aware of.
If you want to map over all elements, but need to look ahead in the
mapped function, you can map over the tails:
map' :: ([a] -> b) -> [a] -> b
map' f = map f . tails
f should be something like
f (a:b:c:_) = ...
If you want to handle groups of n elements together, producing only one
element per group, you can use unfoldr with splitAt:
map'' :: Int -> ([a] -> b) -> [a] -> [b]
map'' n f =
map f . unfoldr (((not . null . fst) `guarding`) . splitAt n)
guarding p x = guard (p x) >> return x
If you want to decide in the mapped function how many elements to
consume, you can use unfoldr directly.
More information about the Haskell-Cafe