How to "change" single values in large objects in elegant way?

Question

For example, i have

data ShipDesign = ShipDesign {
      offense :: Offense
    , defense :: Defense
    , maxHealth :: Integer
    , repairRate :: Integer
    , stealth :: Integer
    , radar :: Integer
    , speed :: Integer
    , shipType :: String
    ...
    }

Now i want to change the defense. The known way to do this is:

changeDefense :: (Defense -> Defense) -> ShipDesign -> ShipDesign
changeDefense fDef sd@(ShipDesign o d m rr s r sp st ...) = ShipDesign o (fDef d) m rr s r sp st ...

which isn't elegant. Especially in games its common to change just a few values per step.

My Question is: is there a library, design pattern or other way to change a single value in a more elegant manner?

Answer 1

Yes, you can use record update notation:

 changeDefense :: (Defense -> Defense) -> ShipDesign -> ShipDesign
 changeDefense fDef sd = sd { defense = fDef (defense sd) }

As you work with it, though, the limitations of record update notation will show themselves, and you will want something more powerful. At that point, you should start to learn about lenses.

Answer 2

I wonder why nobody proposed lens(es)?

I recommend this short introduction by Gabriel Gonzalez: http://www.haskellforall.com/2013/05/program-imperatively-using-haskell.html?m=1

Edit: oh ... I over read the last sentence. But the suggestion still stands.