Re: [Haskell-beginners] Using Traversal as a kind of pointer

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

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