LambdaCase is useful for things like this. On Sat, Mar 21, 2020 at 10:50 PM Henning Thielemann < lemming@henning-thielemann.de> wrote:

On Sat, 21 Mar 2020, Michael Hull wrote:

...

Hi,I use the State monad in a lot of places, but when I use it with the lens library, I find myself writing code like:

myFunc :: StateT MyState ([ResultType]) myFunc = do st <- get case st ^. some_property of True -> do my_value .= "Hello" True -> do my_value .= "Goodbye"

I want to do something based on a 'state-variable', but I have to write 'get' -- is there a pattern that allows me to avoid the intermediate 'st' variable?

gets (^. some_property)

But you still need a local variable for the result of 'gets'.

...

I want to write something like:

myFunc = do case (get ^. some_property) of True -> do my_value .= "Hello" True -> do my_value .= "Goodbye"

but that won't compile.

There cannot be an according accessor, because getting the state needs access to the state in the monad. One could at least think of using LambdaCase._______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.

Which as others have mentioned, works well with lambda case, here's an example from a game I am writing: ``` runPhase :: (MonadIO m, MonadState GameState m) => Card -> m () runPhase c@Card {..} = use phase >>= \case HeadlinePhase -> liftIO . putStrLn $ "Drew: " ++ show c EventResolutionPhase -> liftIO . putStrLn $ "Event" ArmyMovementPhase -> traverse_ moveArmy armyMovements ActionPhase -> liftIO . putStrLn $ "Show me your moves!" KaiserschlachtPhase -> pure () NationalWillPhase -> do nw <- nationalWill if nw <= -5 then gameOver else pure () GameOver -> liftIO . putStrLn $ "Game over" ``` On Sat, Mar 21, 2020 at 11:34 AM Emily Pillmore wrote:

Normally, you'd write `gets` to access entries in your state environment according to some function (e.g. record accessors), but since `some_property` is itself (presumably) a `Lens` or `Getter`, you should be able to use the lensy-State accessor `use`( https://github.com/ekmett/lens/blob/e06c171840bbe86c789b8dff8c8211b88ac9211e...). Then, your script becomes:

``` myFunc :: StateT MyState [ResultType] myFunc = do p ← use some_property case p of True → do my_value .= "Hello" False → do my_value .= "Goodbye" ```

`use` works in the same sense that `view` does for `MonadReader r m ⇒ m`. It even has an analog to `preview` called `preuse`, which you can use for `Prism`s in your state environment.

Cheers, hope that helps, Emily

On Sat, Mar 21, 2020 at 10:42 AM, Michael Hull wrote:

...

Hi, I use the State monad in a lot of places, but when I use it with the lens library, I find myself writing code like:

myFunc :: StateT MyState ([ResultType]) myFunc = do st <- get case st ^. some_property of True -> do my_value .= "Hello" True -> do my_value .= "Goodbye"

I want to do something based on a 'state-variable', but I have to write 'get' -- is there a pattern that allows me to avoid the intermediate 'st' variable? I want to write something like:

myFunc = do case (get ^. some_property) of True -> do my_value .= "Hello" True -> do my_value .= "Goodbye"

but that won't compile.

Best wishes,

Mike

_______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.

_______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.

Thankyou everybody - this is really helpful, especially the \case technique. @Juan I am not sure my Haskell is strong enough to use zoom yet, but I will look into it! Thanks again. Mike On Sat, 21 Mar 2020 at 21:12, Juan Casanova wrote:

I'm surprised noone has mentioned 'zoom' yet. It does not make your original example work directly, but almost. It may be very useful to keep in mind as a way to do whatever you want.

http://hackage.haskell.org/package/lens-4.19.1/docs/Control-Lens-Zoom.html#v...

The idea of zoom is that you use it like so:

zoom my_lens my_monadic_action

where my_monadic_action is a monadic action that works on the sub-element (call it 's') of the state, and the entire thing 'zoom my_lens my_monadic_action' is then a monadic action that works on the whole element (call it 't'). my_lens is a lens that focuses on 's' within 't'.

So instead of using the lens all the time in the monadic action, you're basically saying: "Put this everywhere that you need to", and then you can do things with the sub-element 's'.

Juan.

PS: I hope I did not severely misunderstand what you wanted, but it's still a possibility.

-- The University of Edinburgh is a charitable body, registered in Scotland, with registration number SC005336.

_______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.