Control.Lens.Lens

A Lens s t a b is a purely functional reference.

While a Traversal could be used for Getting like a valid Fold, it wasn't a valid Getter as Applicative wasn't a superclass of Gettable.

Functor, however is the superclass of both.

 type Lens s t a b = forall f. Functor f => (a -> f b) -> s -> f t

Every Lens is a valid Setter.

Every Lens can be used for Getting like a Fold that doesn't use the Applicative or Gettable.

Every Lens is a valid Traversal that only uses the Functor part of the Applicative it is supplied.

Every Lens can be used for Getting like a valid Getter, since Functor is a superclass of Gettable.

Since every Lens can be used for Getting like a valid Getter it follows that it must view exactly one element in the structure.

The Lens laws follow from this property and the desire for it to act like a Traversable when used as a Traversal.

In the examples below, getter and setter are supplied as example getters and setters, and are not actual functions supplied by this package.

Lenses

type Lens s t a b = forall f. Functor f => (a -> f b) -> s -> f t

A Lens is actually a lens family as described in http://comonad.com/reader/2012/mirrored-lenses/.

With great power comes great responsibility and a Lens is subject to the three common sense Lens laws:

1) You get back what you put in:

 view l (set l b a)  ≡ b

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

 set l (view l a) a  ≡ a

3) Setting twice is the same as setting once:

 set l c (set l b a) ≡ set l c a

These laws are strong enough that the 4 type parameters of a Lens cannot vary fully independently. For more on how they interact, read the "Why is it a Lens Family?" section of http://comonad.com/reader/2012/mirrored-lenses/.

Every Lens can be used directly as a Setter or Traversal.

You can also use a Lens for Getting as if it were a Fold or Getter.

Since every Lens is a valid Traversal, the Traversal laws are required of any Lens you create:

 l pure ≡ pure
 fmap (l f) . l g ≡ getCompose . l (Compose . fmap f . g)

 type Lens s t a b = forall f. Functor f => LensLike f s t a b

type Lens' s a = Lens s s a a

 type Lens' = Simple Lens

type IndexedLens i s t a b = forall f p. (Indexable i p, Functor f) => p a (f b) -> s -> f t

Every IndexedLens is a valid Lens and a valid IndexedTraversal.

type IndexedLens' i s a = IndexedLens i s s a a

 type IndexedLens' i = Simple (IndexedLens i)

Concrete Lenses

type ALens s t a b = LensLike (Pretext (->) a b) s t a b

When you see this as an argument to a function, it expects a Lens.

This type can also be used when you need to store a Lens in a container, since it is rank-1. You can turn them back into a Lens with cloneLens, or use it directly with combinators like storing and (^#).

type ALens' s a = ALens s s a a

 type ALens' = Simple ALens

type AnIndexedLens i s t a b = Overloading (Indexed i) (->) (Pretext (Indexed i) a b) s t a b

When you see this as an argument to a function, it expects an IndexedLens

type AnIndexedLens' i s a = AnIndexedLens i s s a a

 type AnIndexedLens' = Simple (AnIndexedLens i)

Combinators

lens :: (s -> a) -> (s -> b -> t) -> Lens s t a b

Build a Lens from a getter and a setter.

 lens :: Functor f => (s -> a) -> (s -> b -> t) -> (a -> f b) -> s -> f t

>>> s ^. lens getter setter
getter s

>>> s & lens getter setter .~ b
setter s b

>>> s & lens getter setter %~ f
setter s (f (getter s))

ilens :: (s -> (i, a)) -> (s -> b -> t) -> IndexedLens i s t a b

Build an IndexedLens from a Getter and a Setter.

iplens :: (Conjoined p, Functor f) => (s -> a) -> (s -> b -> t) -> Overloaded p f s t a b

Build an index-preserving Lens from a Getter and a Setter.

 lens :: (s -> a) -> (s -> b -> t) -> Lens s t a b
 lens sa sbt afb s = sbt s <$> afb (sa s)

(%%~) :: Overloading p q f s t a b -> p a (f b) -> q s (f t)

(%%~) can be used in one of two scenarios:

When applied to a Lens, it can edit the target of the Lens in a structure, extracting a functorial result.

When applied to a Traversal, it can edit the targets of the traversals, extracting an applicative summary of its actions.

For all that the definition of this combinator is just:

 (%%~) ≡ id

It may be beneficial to think about it as if it had these even more restricted types, however:

 (%%~) :: Functor f =>     Iso s t a b       -> (a -> f b) -> s -> f t
 (%%~) :: Functor f =>     Lens s t a b      -> (a -> f b) -> s -> f t
 (%%~) :: Applicative f => Traversal s t a b -> (a -> f b) -> s -> f t

When applied to a Traversal, it can edit the targets of the traversals, extracting a supplemental monoidal summary of its actions, by choosing f = ((,) m)

 (%%~) ::             Iso s t a b       -> (a -> (r, b)) -> s -> (r, t)
 (%%~) ::             Lens s t a b      -> (a -> (r, b)) -> s -> (r, t)
 (%%~) :: Monoid m => Traversal s t a b -> (a -> (m, b)) -> s -> (m, t)

(%%=) :: MonadState s m => Overloading p (->) ((,) r) s s a b -> p a (r, b) -> m r

Modify the target of a Lens in the current state returning some extra information of type r or modify all targets of a Traversal in the current state, extracting extra information of type r and return a monoidal summary of the changes.

>>> runState (_1 %%= \x -> (f x, g x)) (a,b)
(f a,(g a,b))

 (%%=) ≡ (state .)

It may be useful to think of (%%=), instead, as having either of the following more restricted type signatures:

 (%%=) :: MonadState s m             => Iso s s a b       -> (a -> (r, b)) -> m r
 (%%=) :: MonadState s m             => Lens s s a b      -> (a -> (r, b)) -> m r
 (%%=) :: (MonadState s m, Monoid r) => Traversal s s a b -> (a -> (r, b)) -> m r

(%%@~) :: IndexedLensLike i f s t a b -> (i -> a -> f b) -> s -> f t

Adjust the target of an IndexedLens returning a supplementary result, or adjust all of the targets of an IndexedTraversal and return a monoidal summary of the supplementary results and the answer.

(%%@~) ≡ withIndex

 (%%@~) :: Functor f => IndexedLens i s t a b      -> (i -> a -> f b) -> s -> f t
 (%%@~) :: Functor f => IndexedTraversal i s t a b -> (i -> a -> f b) -> s -> f t

In particular, it is often useful to think of this function as having one of these even more restricted type signatures:

 (%%@~) ::             IndexedLens i s t a b      -> (i -> a -> (r, b)) -> s -> (r, t)
 (%%@~) :: Monoid r => IndexedTraversal i s t a b -> (i -> a -> (r, b)) -> s -> (r, t)

(%%@=) :: MonadState s m => IndexedLensLike i ((,) r) s s a b -> (i -> a -> (r, b)) -> m r

Adjust the target of an IndexedLens returning a supplementary result, or adjust all of the targets of an IndexedTraversal within the current state, and return a monoidal summary of the supplementary results.

l %%@= f ≡ state (l %%@~ f)

 (%%@=) :: MonadState s m                IndexedLens i s s a b      -> (i -> a -> (r, b)) -> s -> m r
 (%%@=) :: (MonadState s m, Monoid r) => IndexedTraversal i s s a b -> (i -> a -> (r, b)) -> s -> m r

(<%@~) :: Overloading (Indexed i) q ((,) b) s t a b -> (i -> a -> b) -> q s (b, t)

Adjust the target of an IndexedLens returning the intermediate result, or adjust all of the targets of an IndexedTraversal and return a monoidal summary along with the answer.

l <%~ f ≡ l <%@~ const f

When you do not need access to the index then (<%~) is more liberal in what it can accept.

If you do not need the intermediate result, you can use (%@~) or even (%~).

 (<%@~) ::             IndexedLens i s t a b      -> (i -> a -> b) -> s -> (b, t)
 (<%@~) :: Monoid b => IndexedTraversal i s t a b -> (i -> a -> b) -> s -> (b, t)

(<%@=) :: MonadState s m => IndexedLensLike i ((,) b) s s a b -> (i -> a -> b) -> m b

Adjust the target of an IndexedLens returning the intermediate result, or adjust all of the targets of an IndexedTraversal within the current state, and return a monoidal summary of the intermediate results.

 (<%@=) :: MonadState s m                IndexedLens i s s a b      -> (i -> a -> b) -> m b
 (<%@=) :: (MonadState s m, Monoid b) => IndexedTraversal i s s a b -> (i -> a -> b) -> m b

Lateral Composition

choosing :: Functor f => LensLike f s t a b -> LensLike f s' t' a b -> LensLike f (Either s s') (Either t t') a b

Merge two lenses, getters, setters, folds or traversals.

 chosen ≡ choosing id id

 choosing :: Getter s a     -> Getter s' a     -> Getter (Either s s') a
 choosing :: Fold s a       -> Fold s' a       -> Fold (Either s s') a
 choosing :: Lens' s a      -> Lens' s' a      -> Lens' (Either s s') a
 choosing :: Traversal' s a -> Traversal' s' a -> Traversal' (Either s s') a
 choosing :: Setter' s a    -> Setter' s' a    -> Setter' (Either s s') a

chosen :: IndexPreservingLens (Either a a) (Either b b) a b

This is a Lens that updates either side of an Either, where both sides have the same type.

 chosen ≡ choosing id id

>>> Left a^.chosen
a

>>> Right a^.chosen
a

>>> Right "hello"^.chosen
"hello"

>>> Right a & chosen *~ b
Right (a * b)

 chosen :: Lens (Either a a) (Either b b) a b
 chosen f (Left a)  = Left <$> f a
 chosen f (Right a) = Right <$> f a

alongside :: ALens s t a b -> ALens s' t' a' b' -> Lens (s, s') (t, t') (a, a') (b, b')

alongside makes a Lens from two other lenses.

>>> (Left a, Right b)^.alongside chosen chosen
(a,b)

>>> (Left a, Right b) & alongside chosen chosen .~ (c,d)
(Left c,Right d)

 alongside :: Lens s t a b -> Lens s' t' a' b' -> Lens (s,s') (t,t') (a,a') (b,b')

inside :: ALens s t a b -> Lens (e -> s) (e -> t) (e -> a) (e -> b)

Lift a Lens so it can run under a function.

Setting Functionally with Passthrough

(<%~) :: Profunctor p => Overloading p q ((,) b) s t a b -> p a b -> q s (b, t)

Modify the target of a Lens and return the result.

When you do not need the result of the addition, (%~) is more flexible.

 (<%~) ::             Lens s t a b      -> (a -> b) -> s -> (b, t)
 (<%~) ::             Iso s t a b       -> (a -> b) -> s -> (b, t)
 (<%~) :: Monoid b => Traversal s t a b -> (a -> b) -> s -> (b, t)

(<+~) :: Num a => Overloading (->) q ((,) a) s t a a -> a -> q s (a, t)

Increment the target of a numerically valued Lens and return the result.

When you do not need the result of the addition, (+~) is more flexible.

 (<+~) :: Num a => Lens' s a -> a -> s -> (a, s)
 (<+~) :: Num a => Iso' s a  -> a -> s -> (a, s)

(<-~) :: Num a => Overloading (->) q ((,) a) s t a a -> a -> q s (a, t)

Decrement the target of a numerically valued Lens and return the result.

When you do not need the result of the subtraction, (-~) is more flexible.

 (<-~) :: Num a => Lens' s a -> a -> s -> (a, s)
 (<-~) :: Num a => Iso' s a  -> a -> s -> (a, s)

(<*~) :: Num a => Overloading (->) q ((,) a) s t a a -> a -> q s (a, t)

Multiply the target of a numerically valued Lens and return the result.

When you do not need the result of the multiplication, (*~) is more flexible.

 (<*~) :: Num b => Lens' s a -> a -> a -> (s, a)
 (<*~) :: Num b => Iso'  s a -> a -> a -> (s, a)

(<//~) :: Fractional a => Overloading (->) q ((,) a) s t a a -> a -> q s (a, t)

Divide the target of a fractionally valued Lens and return the result.

When you do not need the result of the division, (//~) is more flexible.

 (<//~) :: Fractional b => Lens' s a -> a -> a -> (s, a)
 (<//~) :: Fractional b => Iso'  s a -> a -> a -> (s, a)

(<^~) :: (Num a, Integral e) => Overloading (->) q ((,) a) s t a a -> e -> q s (a, t)

Raise the target of a numerically valued Lens to a non-negative Integral power and return the result.

When you do not need the result of the division, (^~) is more flexible.

 (<^~) :: (Num b, Integral e) => Lens' s a -> e -> a -> (a, s)
 (<^~) :: (Num b, Integral e) => Iso' s a -> e -> a -> (a, s)

(<^^~) :: (Fractional a, Integral e) => Overloading (->) q ((,) a) s t a a -> e -> q s (a, t)

Raise the target of a fractionally valued Lens to an Integral power and return the result.

When you do not need the result of the division, (^^~) is more flexible.

 (<^^~) :: (Fractional b, Integral e) => Lens' s a -> e -> a -> (a, s)
 (<^^~) :: (Fractional b, Integral e) => Iso' s a -> e -> a -> (a, s)

(<**~) :: Floating a => Overloading (->) q ((,) a) s t a a -> a -> q s (a, t)

Raise the target of a floating-point valued Lens to an arbitrary power and return the result.

When you do not need the result of the division, (**~) is more flexible.

 (<**~) :: Floating a => Lens' s a -> a -> s -> (a, s)
 (<**~) :: Floating a => Iso' s a  -> a -> s -> (a, s)

(<||~) :: Overloading (->) q ((,) Bool) s t Bool Bool -> Bool -> q s (Bool, t)

Logically || a Boolean valued Lens and return the result.

When you do not need the result of the operation, (||~) is more flexible.

 (<||~) :: Lens' s Bool -> Bool -> s -> (Bool, s)
 (<||~) :: Iso' s Bool  -> Bool -> s -> (Bool, s)

(<&&~) :: Overloading (->) q ((,) Bool) s t Bool Bool -> Bool -> q s (Bool, t)

Logically && a Boolean valued Lens and return the result.

When you do not need the result of the operation, (&&~) is more flexible.

 (<&&~) :: Lens' s Bool -> Bool -> s -> (Bool, s)
 (<&&~) :: Iso' s Bool  -> Bool -> s -> (Bool, s)

(<<>~) :: Monoid m => Overloading (->) q ((,) m) s t m m -> m -> q s (m, t)

mappend a monoidal value onto the end of the target of a Lens and return the result.

When you do not need the result of the operation, (<>~) is more flexible.

(<<%~) :: Strong p => Overloading p q ((,) a) s t a b -> p a b -> q s (a, t)

Modify the target of a Lens, but return the old value.

When you do not need the result of the addition, (%~) is more flexible.

 (<<%~) ::             Lens s t a b      -> (a -> b) -> s -> (b, t)
 (<<%~) ::             Iso s t a b       -> (a -> b) -> s -> (b, t)
 (<<%~) :: Monoid b => Traversal s t a b -> (a -> b) -> s -> (b, t)

(<<.~) :: Overloading (->) q ((,) a) s t a b -> b -> q s (a, t)

Modify the target of a Lens, but return the old value.

When you do not need the old value, (%~) is more flexible.

 (<<%~) ::             Lens s t a b      -> b -> s -> (a, t)
 (<<%~) ::             Iso s t a b       -> b -> s -> (a, t)
 (<<%~) :: Monoid b => Traversal s t a b -> b -> s -> (a, t)

Setting State with Passthrough

(<%=) :: (Profunctor p, MonadState s m) => Overloading p (->) ((,) b) s s a b -> p a b -> m b

Modify the target of a Lens into your 'Monad'\'s state by a user supplied function and return the result.

When applied to a Traversal, it this will return a monoidal summary of all of the intermediate results.

When you do not need the result of the operation, (%=) is more flexible.

 (<%=) :: MonadState s m             => Lens' s a      -> (a -> a) -> m a
 (<%=) :: MonadState s m             => Iso' s a       -> (a -> a) -> m a
 (<%=) :: (MonadState s m, Monoid a) => Traversal' s a -> (a -> a) -> m a

(<+=) :: (MonadState s m, Num a) => LensLike' ((,) a) s a -> a -> m a

Add to the target of a numerically valued Lens into your 'Monad'\'s state and return the result.

When you do not need the result of the addition, (+=) is more flexible.

 (<+=) :: (MonadState s m, Num a) => Lens' s a -> a -> m a
 (<+=) :: (MonadState s m, Num a) => Iso' s a -> a -> m a

(<-=) :: (MonadState s m, Num a) => LensLike' ((,) a) s a -> a -> m a

Subtract from the target of a numerically valued Lens into your 'Monad'\'s state and return the result.

When you do not need the result of the subtraction, (-=) is more flexible.

 (<-=) :: (MonadState s m, Num a) => Lens' s a -> a -> m a
 (<-=) :: (MonadState s m, Num a) => Iso' s a -> a -> m a

(<*=) :: (MonadState s m, Num a) => LensLike' ((,) a) s a -> a -> m a

Multiply the target of a numerically valued Lens into your 'Monad'\'s state and return the result.

When you do not need the result of the multiplication, (*=) is more flexible.

 (<*=) :: (MonadState s m, Num a) => Lens' s a -> a -> m a
 (<*=) :: (MonadState s m, Num a) => Iso' s a -> a -> m a

(<//=) :: (MonadState s m, Fractional a) => LensLike' ((,) a) s a -> a -> m a

Divide the target of a fractionally valued Lens into your 'Monad'\'s state and return the result.

When you do not need the result of the division, (//=) is more flexible.

 (<//=) :: (MonadState s m, Fractional a) => Lens' s a -> a -> m a
 (<//=) :: (MonadState s m, Fractional a) => Iso' s a -> a -> m a

(<^=) :: (MonadState s m, Num a, Integral e) => LensLike' ((,) a) s a -> e -> m a

Raise the target of a numerically valued Lens into your 'Monad'\'s state to a non-negative Integral power and return the result.

When you do not need the result of the operation, (**=) is more flexible.

 (<^=) :: (MonadState s m, Num a, Integral e) => Lens' s a -> e -> m a
 (<^=) :: (MonadState s m, Num a, Integral e) => Iso' s a -> e -> m a

(<^^=) :: (MonadState s m, Fractional a, Integral e) => LensLike' ((,) a) s a -> e -> m a

Raise the target of a fractionally valued Lens into your 'Monad'\'s state to an Integral power and return the result.

When you do not need the result of the operation, (^^=) is more flexible.

 (<^^=) :: (MonadState s m, Fractional b, Integral e) => Lens' s a -> e -> m a
 (<^^=) :: (MonadState s m, Fractional b, Integral e) => Iso' s a  -> e -> m a

(<**=) :: (MonadState s m, Floating a) => LensLike' ((,) a) s a -> a -> m a

Raise the target of a floating-point valued Lens into your 'Monad'\'s state to an arbitrary power and return the result.

When you do not need the result of the operation, (**=) is more flexible.

 (<**=) :: (MonadState s m, Floating a) => Lens' s a -> a -> m a
 (<**=) :: (MonadState s m, Floating a) => Iso' s a -> a -> m a

(<||=) :: MonadState s m => LensLike' ((,) Bool) s Bool -> Bool -> m Bool

Logically || a Boolean valued Lens into your 'Monad'\'s state and return the result.

When you do not need the result of the operation, (||=) is more flexible.

 (<||=) :: MonadState s m => Lens' s Bool -> Bool -> m Bool
 (<||=) :: MonadState s m => Iso' s Bool  -> Bool -> m Bool

(<&&=) :: MonadState s m => LensLike' ((,) Bool) s Bool -> Bool -> m Bool

Logically && a Boolean valued Lens into your 'Monad'\'s state and return the result.

When you do not need the result of the operation, (&&=) is more flexible.

 (<&&=) :: MonadState s m => Lens' s Bool -> Bool -> m Bool
 (<&&=) :: MonadState s m => Iso' s Bool  -> Bool -> m Bool

(<<>=) :: (MonadState s m, Monoid r) => LensLike' ((,) r) s r -> r -> m r

mappend a monoidal value onto the end of the target of a Lens into your 'Monad'\'s state and return the result.

When you do not need the result of the operation, (<>=) is more flexible.

(<<%=) :: (Strong p, MonadState s m) => Overloading p (->) ((,) a) s s a b -> p a b -> m a

Modify the target of a Lens into your 'Monad'\'s state by a user supplied function and return the old value that was replaced.

When applied to a Traversal, it this will return a monoidal summary of all of the old values present.

When you do not need the result of the operation, (%=) is more flexible.

 (<<%=) :: MonadState s m             => Lens' s a      -> (a -> a) -> m a
 (<<%=) :: MonadState s m             => Iso' s a       -> (a -> a) -> m a
 (<<%=) :: (MonadState s m, Monoid b) => Traversal' s a -> (a -> a) -> m a

(<<%=) :: MonadState s m => LensLike ((,)a) s s a b -> (a -> b) -> m a

(<<.=) :: MonadState s m => LensLike ((,) a) s s a b -> b -> m a

Modify the target of a Lens into your 'Monad'\'s state by a user supplied function and return the old value that was replaced.

When applied to a Traversal, it this will return a monoidal summary of all of the old values present.

When you do not need the result of the operation, (%=) is more flexible.

 (<<%=) :: MonadState s m             => Lens' s a      -> (a -> a) -> m a
 (<<%=) :: MonadState s m             => Iso' s a       -> (a -> a) -> m a
 (<<%=) :: (MonadState s m, Monoid t) => Traversal' s a -> (a -> a) -> m a

(<<~) :: MonadState s m => ALens s s a b -> m b -> m b

Run a monadic action, and set the target of Lens to its result.

 (<<~) :: MonadState s m => Iso s s a b   -> m b -> m b
 (<<~) :: MonadState s m => Lens s s a b  -> m b -> m b

NB: This is limited to taking an actual Lens than admitting a Traversal because there are potential loss of state issues otherwise.

Cloning Lenses

cloneLens :: ALens s t a b -> Lens s t a b

Cloning a Lens is one way to make sure you aren't given something weaker, such as a Traversal and can be used as a way to pass around lenses that have to be monomorphic in f.

Note: This only accepts a proper Lens.

>>> let example l x = set (cloneLens l) (x^.cloneLens l + 1) x in example _2 ("hello",1,"you")
("hello",2,"you")

cloneIndexPreservingLens :: ALens s t a b -> IndexPreservingLens s t a b

Clone a Lens as an IndexedPreservingLens that just passes through whatever index is on any IndexedLens, IndexedFold, IndexedGetter or IndexedTraversal it is composed with.

cloneIndexedLens :: AnIndexedLens i s t a b -> IndexedLens i s t a b

Clone an IndexedLens as an IndexedLens with the same index.

ALens Combinators

storing :: ALens s t a b -> b -> s -> t

A version of set that works on ALens.

>>> storing _2 "world" ("hello","there")
("hello","world")

(^#) :: s -> ALens s t a b -> a

A version of (^.) that works on ALens.

>>> ("hello","world")^#_2
"world"

(#~) :: ALens s t a b -> b -> s -> t

A version of (.~) that works on ALens.

>>> ("hello","there") & _2 #~ "world"
("hello","world")

(#%~) :: ALens s t a b -> (a -> b) -> s -> t

A version of (%~) that works on ALens.

>>> ("hello","world") & _2 #%~ length
("hello",5)

(#%%~) :: Functor f => ALens s t a b -> (a -> f b) -> s -> f t

A version of (%%~) that works on ALens.

>>> ("hello","world") & _2 #%%~ \x -> (length x, x ++ "!")
(5,("hello","world!"))

(<#~) :: ALens s t a b -> b -> s -> (b, t)

A version of (<.~) that works on ALens.

>>> ("hello","there") & _2 <#~ "world"
("world",("hello","world"))

(<#%~) :: ALens s t a b -> (a -> b) -> s -> (b, t)

A version of (<%~) that works on ALens.

>>> ("hello","world") & _2 <#%~ length
(5,("hello",5))

(#=) :: MonadState s m => ALens s s a b -> b -> m ()

A version of (.=) that works on ALens.

(#%=) :: MonadState s m => ALens s s a b -> (a -> b) -> m ()

A version of (%=) that works on ALens.

(#%%=) :: MonadState s m => ALens s s a b -> (a -> (r, b)) -> m r

A version of (%%=) that works on ALens.

(<#=) :: MonadState s m => ALens s s a b -> b -> m b

A version of (<#=) that works on ALens.

(<#%=) :: MonadState s m => ALens s s a b -> (a -> b) -> m b

A version of (<%=) that works on ALens.

Context

data Context a b t

The indexed store can be used to characterize a Lens and is used by clone.

Context a b t is isomorphic to newtype Context a b t = Context { runContext :: forall f. Functor f => (a -> f b) -> f t }, and to exists s. (s, Lens s t a b).

A Context is like a Lens that has already been applied to a some structure.

Constructors

Context (b -> t) a

Instances

IndexedComonadStore Context

IndexedComonad Context

IndexedFunctor Context

Sellable (->) Context

(Comonad (Context a b), ~ * a b) => ComonadStore a (Context a b)

Functor (Context a b)

(Functor (Context a b), ~ * a b) => Comonad (Context a b)

type Context' a = Context a a

type Context' a s = Context a a s

locus :: IndexedComonadStore p => Lens (p a c s) (p b c s) a b

This Lens lets you view the current pos of any indexed store comonad and seek to a new position. This reduces the API for working these instances to a single Lens.

 ipos w ≡ w ^. locus
 iseek s w ≡ w & locus .~ s
 iseeks f w ≡ w & locus %~ f

 locus :: Lens' (Context' a s) a
 locus :: Lens' (Pretext' p a s) a
 locus :: Lens' (PretextT' p g a s) a