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

[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