Thanks for the example! I found another example of filtered here
http://www.haskellforall.com/2013/05/program-imperatively-using-haskell.html
I'm aware that filtered traversals should only ever be used with
functions that will not change the number of elements filtered.
Thank you!
Elise
On 18 August 2014 16:03, Daniel Trstenjak
Hi Elise,
thanks for your answer. That looks like something that might work - however, having never worked with Optics, I'm not entirely sure whether I'm doing it right, getting an error:
oh sorry, there has to be a 'traversed' before the 'filtered'.
Also, may I ask what the '&' is in your proposed solution?
The '&' is just applying a lens to a variable.
So here's a working example:
{-# LANGUAGE Rank2Types #-}
import Control.Lens
type Background = Int type Lifeform = Int type Player = Int
data World = World Background [Lifeform] deriving (Show)
lifeforms :: Lens' World [Lifeform] lifeforms = lens getLifeforms setLifeforms where getLifeforms (World _ lifeforms) = lifeforms setLifeforms (World bg _) lifeforms = World bg lifeforms
colliding :: Player -> Traversal' [Lifeform] Lifeform colliding player = traversed . filtered (== player)
If you load this into ghci und can write:
World 1 [1, 2] & lifeforms . colliding 1 %~ (+ 10) World 1 [11,2]
Greetings, Daniel _______________________________________________ Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners