[Haskell-cafe] Using State monad with lens

[Emily Pillmore]

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 at 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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