Control.Eff.Reader.Strict

Strict read-only state

Documentation

data Reader e v where

The Reader monad

The request for a value of type e from the current environment This can be expressed as a GADT because the type of values returned in response to a (Reader e a) request is not any a; we expect in reply the value of type e, the value from the environment. So, the return type is restricted: 'a ~ e'

One can also define this as

data Reader e v = (e ~ v) => Reader

^ without GADTs, using explicit coercion as is done here.

newtype Reader e v = Reader (e->v)

^ In the latter case, when we make the request, we make it as Reader id. So, strictly speaking, GADTs are not really necessary.

Constructors

Ask :: Reader e e

Instances

(MonadBase m m, SetMember (* -> *) (Lift *) (Lift * m) s, MonadBaseControl m (Eff s)) => MonadBaseControl m (Eff ((:) (* -> *) (Reader * e) s))

Associated Types

type StM (Eff (((* -> *) ': Reader * e) s) :: * -> *) a :: *

Methods

liftBaseWith :: (RunInBase (Eff (((* -> *) ': Reader * e) s)) m -> m a) -> Eff (((* -> *) ': Reader * e) s) a

restoreM :: StM (Eff (((* -> *) ': Reader * e) s)) a -> Eff (((* -> *) ': Reader * e) s) a

type StM (Eff ((:) (* -> *) (Reader * e) s)) a

type StM (Eff ((:) (* -> *) (Reader * e) s)) a = StM (Eff s) a

ask :: Member (Reader e) r => Eff r e

Get the current value from a Reader. The signature is inferred (when using NoMonomorphismRestriction).

local :: forall e a r. Member (Reader e) r => (e -> e) -> Eff r a -> Eff r a

Locally rebind the value in the dynamic environment This function is like a relay; it is both an admin for Reader requests, and a requestor of them

reader :: Member (Reader e) r => (e -> a) -> Eff r a

Request the environment value using a transformation function.

runReader :: e -> Eff (Reader e ': r) w -> Eff r w

The handler of Reader requests. The return type shows that all Reader requests are fully handled.