And just remember filtered is only a valid traversal if you don't change the number of targets with the modification function! On 18 August 2014 14:17, Elise Huard wrote:

Hi Daniel,

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: *Main Graphics.Rendering.OpenGL Control.Lens> :t (liveforms . filtered (colliding p))

<interactive>:1:14: Couldn't match type _Lifeform_ with _[Lifeform]_ Expected type: (Lifeform -> f Lifeform) -> [Lifeform] -> f [Lifeform] Actual type: Optic' (->) f Lifeform Lifeform In the second argument of _(.)_, namely _filtered ((flip colliding) p)_ In the expression: (liveforms . filtered ((flip colliding) p)) *Main Graphics.Rendering.OpenGL Control.Lens>

Also, may I ask what the '&' is in your proposed solution? Thanks,

Elise

On 18 August 2014 13:30, Daniel Trstenjak wrote:

...

Hi Elise,

I'm not quite sure if I completely understood your use case, but if you only want to modify the lifeforms that are colliding with the player, then you could have something like:

world & liveforms . filtered (collidingWith player) %~ \liveform ->

...

...

with

liveforms :: Lens' World [Lifeform]

collidingWith :: Player -> Lifeform -> Bool

I would consider this as a quite nice solution.

Greetings, Daniel _______________________________________________ Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners

_______________________________________________ Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners

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 wrote:

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