The lens operator that does this is %%~, also called `traverseOf`:

λ: ("Hello", "World") & _1 %%~ (\a -> putStrLn a >> return a)

Hello

("Hello","World")

Also see the documentation: http://hackage.haskell.org/package/lens-4.4.0.1/docs/Control-Lens-Traversal.html#v:traverseOf

--

Benno

2014-08-30 9:44 GMT+02:00 Nikolay Amiantov <nikoamia@gmail.com>:

Hello Cafe,

I'm trying to wrap my head around 'lens' library. My current exercise is to modify something using Lens in monad. Say,

("Hello", "World") & _1 `myOp` (\a -> putStrLn a >> return a)

in IO, where myOp would be of type:

myOp :: Monad m => Lens s t a b -> (a -> m b) -> s -> m t

Of course I can write it myself, using combination of "view" and "set":

myOp lens f v = f (view lens v) >>= flip (set lens) v

(have not checked this, but something like that should do), but is there a more elegant way?

Nikolay.

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