Re: [Haskell-cafe] Using 'lens' for "over"-like computation in Monad

Thank you! I'm still lost in the forest of all lens functions and generality, and haven't expected this signature. Can someone recommend good reading on all the lens-related types and classes, what they represent and how can they be used? (interested not only in "how", but also "why") 30 авг. 2014 г. 11:20 пользователь "Benno Fünfstück" < benno.fuenfstueck@gmail.com> написал:

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....

-- Benno

2014-08-30 9:44 GMT+02:00 Nikolay Amiantov :

...

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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