Add `evalState` (to `Control.Functor.Linear`)

[andreasabel]

evalState is a common use-case of the state monad.

import Control.Functor.Linear
import Data.Tuple.Linear

evalState :: Consumable s => State s a %1 -> s %1 -> a
evalState m s = fst (runState m s)

Probably there should also be evalStateT.

[aspiwack]

I'm not sure that evalState is quite as useful as in the case of the unrestricted-state monad. That being said, I'll welcome a PR, but see my thoughts in #405 (comment).

[andreasabel]

Maybe I can do without it, but I didn't see how because of the Ur in the type of HashMap.empty. I'd be happy for advice, see my code at: https://gist.github.com/andreasabel/f5c877491d32b653efba44cb283ba490#file-interpretermain-hs

[andreasabel]

I just tried to generalize the response type of HashMap.empty from Ur b to b and the type checker allowed it:

• More polymorphic type for Array.alloc and HashMap.empty #410

So if this is an admissible generalization, the need for evalState would be reduced considerably, and one could decide of the fate of the final state later. As it stands (without #410), one seems to have to consume the final state in the continuation passed to HashMap.empty already.

[andreasabel]

So if this is an admissible generalization, ...

I ain't...

[andreasabel]

@aspiwack : I am back to evalState, see

• Add evalState(T) #411

If you have other ideas how to write https://gist.github.com/andreasabel/f5c877491d32b653efba44cb283ba490#file-interpretermain-hs , please let me know.