[SOLVED] How to use IORef with lenses?

How to use IORef with lenses?

Wondering how best to combine the Control.Lens package with IORefs. Specifically I'd like to be able to use atomicModifyIORef with lenses so that i can supply a function of type a -> (a, b) and return a value from the operation. Code snippet:

let inc x = (x+1, x)
ior <- newIORef ((1, 1) :: (Int, Int))
thisShouldBe1 <- ior & atomicModifyIORef ?? _1 inc -- this is the bit I'm stuck on

Solution

In principle, the lens operator needed is actually %%~, which is just a convenience synonym for id. However, due to an annoying incompatibility in the tuple orderings used in atomicModifyIORef and the (,) a Functor, it needs some swapping around to work. I don't think the resulting operator is predefined, but I've given it the preliminary name swappedId below.

Note that the Lens type is defined as

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

It turns out that if you let f be the (,) a Functor, this almost perfectly fits the type you want to use to transform your inc, except that you'd really have wanted the a to be the last element of the tuple instead of the first. After fixing this up, here is what I ended up with:

import Data.IORef
import Control.Lens

l `swappedId` f = f & mapping swapped %~ l

main = do
    let inc x = (x+1, x)
    ior <- newIORef ((1, 1) :: (Int, Int))
    thisShouldBe1 <- atomicModifyIORef ior $ _1 `swappedId` inc
    print thisShouldBe1
    print =<< readIORef ior