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

[Elise Huard]

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 <daniel.trstenjak at gmail.com> 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
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