Effect for a set of stateful values indexed by a type of keys.

The KeyedState k effect provides access to a set of stateful values indexed by some key type k :: Type -> Type, where a key of type k a can be used to access a stateful value of type a.

polysemy-keyed-state

This package provides the KeyedState effect for use with polysemy. For any key type k :: Type -> Type, the KeyedState k effect provides access to a set of stateful values indexed by k, where a key of type k a can be used to access a stateful value of type a.

In the most direct use case, the KeyedState effect can be used as an interface to low-level reference types like IORef and STRef: for example, getAt can be used with the type Member (KeyedState IORef) r => IORef a -> Sem r a.

At a higher level, key types defined as GADTs can be used with KeyedState to represent sets of stateful variables in a single effect. For example, with the GADT definition of K below, the effect KeyedState K provides access to an Int value with key X and a Bool value with key Y.

data K a where
  X :: K Int
  Y :: K Bool

By mapping high-level keys to low-level references, the KeyedState effect can be used to implement a high-level interface to a set of low-level variables.

Some of the interpreters for KeyedState require instances of Has for certain typeclasses, which can be generated for most GADT key types with deriveArgDict from the constraints-extras package.