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 <emilypi@cohomolo.gy> 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/src/Control/Lens/Getter.hs#L347). 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 <mikehulluk@gmail.com> 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

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