Control.Lens.Internal.Getter

Internal Classes

Getters

class Functor f => Gettable f where

Generalizing Const so we can apply simple Applicative transformations to it and so we can get nicer error messages.

A Gettable Functor ignores its argument, which it carries solely as a phantom type parameter.

To ensure this, an instance of Gettable is required to satisfy:

id = fmap f = coerce

Which is equivalent to making a Gettable f an "anyvariant" Functor.

Methods

coerce :: f a -> f b

Replace the phantom type argument.

Instances

Functor (Const r) => Gettable (Const r)

(Functor (Backwards f), Gettable f) => Gettable (Backwards f)

Functor (Accessor r) => Gettable (Accessor r)

(Functor (Indexing64 f), Gettable f) => Gettable (Indexing64 f)

(Functor (Indexing f), Gettable f) => Gettable (Indexing f)

(Functor (Compose f g), Functor f, Gettable g) => Gettable (Compose f g)

Functor (Effect m r) => Gettable (Effect m r)

(Functor (PretextT p g a b), Profunctor p, Gettable g) => Gettable (PretextT p g a b)

(Functor (BazaarT p g a b), Profunctor p, Gettable g) => Gettable (BazaarT p g a b)

(Functor (TakingWhile p f a b), Gettable f) => Gettable (TakingWhile p f a b)

Functor (EffectRWS w st m s) => Gettable (EffectRWS w st m s)

noEffect :: (Applicative f, Gettable f) => f a

The mempty equivalent for a Gettable Applicative Functor.

newtype Accessor r a

Used instead of Const to report

No instance for (Settable Accessor)

when the user attempts to misuse a Setter as a Getter, rather than a monolithic unification error.

Constructors

Accessor

Fields

runAccessor :: r

Instances

(Monad Identity, Gettable (Accessor r)) => Effective Identity r (Accessor r)

Functor (Accessor r)

(Functor (Accessor r), Monoid r) => Applicative (Accessor r)

(Functor (Accessor r), Semigroup r) => Apply (Accessor r)

Functor (Accessor r) => Gettable (Accessor r)