Optics traversals

OpticsTraverse.hs

You can use *most* optics as though they're a custom `traverse`

>>> (each . _Right) print (Left 1, Right 2, Left 3)
2
-- We can ignore the new structure it returns
(Left 1,Right (),Left 3)

It works on other types of effects too!
Here we use lists as a non-determinism effect to get all possible combos!

>>> each (\x -> [x, x+5]) (0, 10)
[(0,10),(0,15),(5,10),(5,15)]

-- Helper
safeHead :: [a] -> Maybe a
safeHead (x:_) = Just x
safeHead _ = Nothing

If using them directly feels weird, we can use traverseOf to make it explicit:

>>> traverseOf (traversed . _2) safeHead [(1, "one"), (2, "two")]
Just [(1,'o'),(2,'t')]
--
>>> traverseOf (traversed . _2) safeHead [(1, ""), (2, "two")]
Nothing

`%%~` is the infix version of `traverseOf`:

>>> [(1, "one"), (2, "two")] & traversed . _2 %%~ safeHead
Just [(1,'o'),(2,'t')]

>>> [(1, ""), (2, "two")] & traversed . _2 %%~ safeHead
Nothing