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 < 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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