Control.Reference.Representation

This module declares the representation and basic classes of references.

This module should not be imported directly.

Documentation

data Reference w r s t a b

A reference is an accessor to a part or different view of some data. The referenc has a separate getter, setter and updater. In some cases, the semantics are a bit different

Reference laws

As the references are generalizations of lenses, they should conform to the lens laws:

1) You get back what you put in:

refSet l a s >>= refGet l return ≡ a

2) Putting back what you got doesn't change anything:

refGet l return a >>= \b -> refSet l b s ≡ s

3) Setting twice is the same as setting once:

refSet l a s >>= refSet l b ≡ refSet l b s

But because update, set and get are different operations, .

4) Updating something is the same as getting and then setting (if the reader and writer monads are the same, or one can be converted into the other):

refGet l a >>= f >>= \b -> refSet l b s ≡ refUpdate l f s

This has some consequences. For example lensUpdate l id = return.

Type arguments of Reference

w

Writer monad, controls how the value can be reassembled when the part is changed. See differences between Lens, IOLens and StateLens

r

Reader monad. Controls how part of the value can be asked. See differences between Lens, Partial and Traversal

s

The type of the original context.

t

The after replacing the accessed part to something of type b the type of the context changes to t.

a

The type of the accessed part.

b

The accessed part can be changed to something of this type.

Usually s and b determines t, t and a determines s.

The reader monad usually have more information (MMorph w r).

Constructors

Reference

Fields

refGet :: forall x. (a -> r x) -> s -> r x

Getter for the lens. Takes a monadic function and runs it on the accessed value. This is necessary to run actions after a read.

refSet :: b -> s -> w t

Setter for the lens

refUpdate :: (a -> w b) -> s -> w t

Updater for the lens. Handles monadic update functions.

reference

Arguments

:: (Functor w, Applicative w, Monad w, Functor r, Applicative r, Monad r)

=> (s -> r a)

Getter

-> (b -> s -> w t)

Setter

-> ((a -> w b) -> s -> w t)

Updater

-> Reference w r s t a b

Creates a reference.

referenceWithClose

Arguments

:: (Functor w, Applicative w, Monad w, Functor r, Applicative r, Monad r)

=> (s -> r a)

Getter

-> (s -> r ())

Close after getting

-> (b -> s -> w t)

Setter

-> (s -> w ())

Close after setting

-> ((a -> w b) -> s -> w t)

Updater

-> (s -> w ())

Close after updating

-> Reference w r s t a b

Creates a reference with explicit close operations that are executed after the data is accessed.

class (Functor w, Applicative w, Monad w, Functor r, Applicative r, Monad r) => RefMonads w r

A simple class to enforce that both reader and writer semantics of the reference are Monads (as well as Applicatives and Functors)

Instances

(Functor w, Applicative w, Monad w, Functor r, Applicative r, Monad r) => RefMonads w r

type Simple t s a = t s s a a

A monomorph Lens, Traversal, Partial, etc... Setting or updating does not change the type of the base.

Pure references

type Lens s t a b = forall w r. RefMonads w r => Reference w r s t a b

A Reference that can access a part of data that exists in the context. Every well-formed Reference is a Lens.

type Lens' = Reference Identity Identity

Strict lens. A Reference that must access a part of data that surely exists in the context.

type RefPlus s t a b = forall w r. (RefMonads w r, MonadPlus r) => Reference w r s t a b

A reference that may not have the accessed element, and that can look for the accessed element in multiple locations.

type Partial s t a b = forall w r. (Functor w, Applicative w, Monad w, Functor r, Applicative r, MonadPlus r, MMorph Maybe r) => Reference w r s t a b

Partial lens. A Reference that can access data that may not exist in the context. Every lens is a partial lens.

Any reference that is a partial lens should only perform the action given to its updateRef function if it can get a value (the value returned by getRef is not the lifted form of Nothing).

type Partial' = Reference Identity Maybe

Strict partial lens. A Reference that must access data that may not exist in the context.

type Traversal s t a b = forall w r. (RefMonads w r, MonadPlus r, MMorph [] r) => Reference w r s t a b

A reference that can access data that is available in a number of instances inside the contexts.

Any reference that is a Traversal should perform the action given to its updater in the exactly the same number of times that is the number of the values returned by it's getRef function.

type Traversal' = Reference Identity []

Strict traversal. A reference that must access data that is available in a number of instances inside the context.

References for IO

class (MMorph IO w, MMorph IO r, MonadBaseControl IO w, MonadBaseControl IO r) => IOMonads w r

Instances

(MMorph IO w, MMorph IO r, MonadBaseControl IO w, MonadBaseControl IO r) => IOMonads w r

type IOLens s t a b = forall w r. (RefMonads w r, IOMonads w r) => Reference w r s t a b

A reference that can access mutable data.

type IOLens' = Reference IO IO

A reference that must access mutable data that is available in the context.

type IOPartial s t a b = forall w r. (RefMonads w r, IOMonads w r, MonadPlus r, MMorph Maybe r) => Reference w r s t a b

A reference that can access mutable data that may not exist in the context.

type IOPartial' = Reference IO (MaybeT IO)

A reference that must access mutable data that may not exist in the context.

type IOTraversal s t a b = forall w r. (RefMonads w r, IOMonads w r, MonadPlus r, MMorph [] r) => Reference w r s t a b

type IOTraversal' = Reference IO (ListT IO)

A reference that can access mutable data that is available in a number of instances inside the contexts.

References for StateT

type StateLens st m s t a b = forall w r. (RefMonads w r, MMorph (StateT st m) w, MMorph (StateT st m) r) => Reference w r s t a b

A reference that can access a value inside a StateT transformed monad.

type StateLens' s m = Reference (StateT s m) (StateT s m)

A reference that must access a value inside a StateT transformed monad.

type StatePartial st m s t a b = forall w r. (RefMonads w r, MMorph (StateT st m) w, MonadPlus r, MMorph Maybe r, MMorph (StateT st m) r) => Reference w r s t a b

A reference that can access a value inside a StateT transformed monad that may not exist.

type StatePartial' s m = Reference (StateT s m) (MaybeT (StateT s m))

A reference that must access a value inside a StateT transformed monad that may not exist.

type StateTraversal st m s t a b = forall w r. (RefMonads w r, MMorph (StateT st m) w, MonadPlus r, MMorph [] r, MMorph (StateT st m) r) => Reference w r s t a b

A reference that can access a value inside a StateT transformed monad that may exist in multiple instances.

type StateTraversal' s m = Reference (StateT s m) (ListT (StateT s m))

A reference that must access a value inside a StateT transformed monad that may exist in multiple instances.

References for WriterT

type WriterLens st m s t a b = forall w r. (RefMonads w r, MMorph (WriterT st m) w, MMorph (WriterT st m) r) => Reference w r s t a b

A reference that can access a value inside a WriterT transformed monad.

type WriterLens' s m = Reference (WriterT s m) (WriterT s m)

A reference that must access a value inside a WriterT transformed monad.

type WriterPartial st m s t a b = forall w r. (RefMonads w r, MMorph (WriterT st m) w, MonadPlus r, MMorph Maybe r, MMorph (WriterT st m) r) => Reference w r s t a b

A reference that can access a value inside a WriterT transformed monad that may not exist.

type WriterPartial' s m = Reference (WriterT s m) (MaybeT (WriterT s m))

A reference that must access a value inside a WriteT transformed monad that may not exist.

type WriterTraversal st m s t a b = forall w r. (RefMonads w r, MMorph (WriterT st m) w, MonadPlus r, MMorph [] r, MMorph (WriterT st m) r) => Reference w r s t a b

A reference that can access a value inside a WriteT transformed monad that may exist in multiple instances.

type WriterTraversal' s m = Reference (WriterT s m) (ListT (WriterT s m))

A reference that must access a value inside a WriteT transformed monad that may exist in multiple instances.

class MMorph m1 m2 where

States that m1 can be represented with m2. That is because m2 contains more infromation than m1.

The MMorph relation defines a natural transformation from m1 to m2 that keeps the following laws:

morph (return x)  =  return x

morph (m >>= f)   =  morph m >>= morph . f

It is a reflexive and transitive relation.

Methods

morph :: m1 a -> m2 a

Lifts the first monad into the second.

Instances

MMorph [] []

MMorph IO IO

MMorph Maybe []

MMorph Maybe Maybe

MMorph Identity []

MMorph Identity IO

MMorph Identity Maybe

MMorph Identity Identity

MMorph [] (ListT IO)

MMorph [] (ListT (StateT s Q))

MMorph IO (ListT IO)

MMorph IO (MaybeT IO)

MMorph Maybe (ListT IO)

MMorph Maybe (MaybeT IO)

MMorph Identity (ListT IO)

MMorph Identity (MaybeT IO)

MMorph (ListT IO) (ListT IO)

MMorph (MaybeT IO) (ListT IO)

MMorph (MaybeT IO) (MaybeT IO)

Monad m => MMorph (StateT s m) (ListT (StateT s m))