Can I make a Lens with a Monad constraint?

Question

Context: This question is specifically in reference to Control.Lens (version 3.9.1 at the time of this writing)

I\'ve been using the le

Answer 1

I had the same problem. I tried the methods in Petr and Cirdec's answers but never got to the point I wanted to. Started working on the problem, and at the end, I published the references library on hackage with a generalization of lenses.

I followed the idea of the yall library to parameterize the references with monad types. As a result there is an mvar reference in Control.Reference.Predefined. It is an IO reference, so an access to the referenced value is done in an IO action.

There are also other applications of this library, it is not restricted to IO. An additional feature is to add references (so adding _1 and _2 tuple accessors will give a both traversal, that accesses both fields). It can also be used to release resources after accessing them, so it can be used to manipulate files safely.

The usage is like this:

test = 
  do result <- newEmptyMVar
     terminator <- newEmptyMVar
     forkIO $ (result ^? mvar) >>= print >> (mvar .= ()) terminator >> return ()
     hello <- newMVar (Just "World")
     forkIO $ ((mvar & just & _tail & _tail) %~= ('_':) $ hello) >> return ()
     forkIO $ ((mvar & just & element 1) .= 'u' $ hello) >> return ()
     forkIO $ ((mvar & just) %~= ("Hello" ++) $ hello) >> return ()

     x <- runMaybeT $ hello ^? (mvar & just) 
     mvar .= x $ result
     terminator ^? mvar

The operator & combines lenses, ^? is generalized to handle references of any monad, not just a referenced value that may not exist. The %~= operator is an update of a monadic reference with a pure function.