[Haskell-cafe] Using State monad with lens

[Henning Thielemann]

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.