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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