Hello there,

I'm starting to use a bit more extensively the lens package. It is clearly very powerful, but it feels like a maze of related components (Setting/Setter/Lens'/LensLike/Lens/Traversal/Traversal') which can be swapped for one another except when they can't.

The first (concrete) problem I ran into is how to update the members of a set with the result of an IO action. I have managed to do this with a pure function (prefixName) but I'm not sure of how to do this with promptName.

Full program below:

{-# LANGUAGE TemplateHaskell #-}
import Control.Lens
import Data.Set
import Data.Set.Lens

data Dog = Dog { _name :: String, _legs :: Int }
         deriving (…)
makeLenses ''Dog

data Dogs = Dogs { _dogs :: Set Dog }
         deriving Show
makeLenses ''Dogs

fourLegs :: Traversal' Dog Dog
fourLegs = filtered (ëd -> d^.legs == 4)

promptName :: String -> IO String
promptName dogName = do
putStr $ "New name for " ++ dogName
      getLine

prefixName :: Dog -> Dog
prefixName dog = set name ("PREFIXED: " ++ dog^.name) dog

main :: IO ()
main = do
        let fido = Dog "fido" 4
        let milou = Dog "milou" 4
        let cripple = Dog "cripple" 3
        let doggies = Dogs $ fromList [fido, milou, cripple]

-- prefix dog names via a pure function

let doggies' = over (dogs.setmapped) prefixName doggies
print doggies'

-- change dog names by prompting the user ?

return ()

Help would be appreciated (in particular, 'cripple' would love to be renamed).

Since I was struggling with the library, I had the idea to look at the internals, but got stuck at the definition of Lens:

type Lens s t a b = forall f. Functor f => (a -> f b) -> s -> f t

a) I'm not sure why the explicit forall is needed here (isn't this equivalent to just Functor f => ...)?

b) My understanding is that a lens packs both getter and setters, but I don't know which is supposed to be which here...

c) Is there any kind of in-depth guide to Control.Lens somewhere? I have found some examples and tutorials but nothing that seemed to do more than scratch the surface.

Thanks,

Emm