Copyright  (C) 201216 Edward Kmett 

License  BSDstyle (see the file LICENSE) 
Maintainer  Edward Kmett <ekmett@gmail.com> 
Stability  provisional 
Portability  Rank2Types 
Safe Haskell  Trustworthy 
Language  Haskell98 
A
is a generalization of Traversal
s t a btraverse
from
Traversable
. It allows you to traverse
over a structure and change out
its contents with monadic or Applicative
sideeffects. Starting from
traverse
:: (Traversable
t,Applicative
f) => (a > f b) > t a > f (t b)
we monomorphize the contents and result to obtain
typeTraversal
s t a b = forall f.Applicative
f => (a > f b) > s > f t
A Traversal
can be used as a Fold
.
Any Traversal
can be used for Getting
like a Fold
,
because given a Monoid
m
, we have an Applicative
for
(
. Everything you know how to do with a Const
m)Traversable
container,
you can with a Traversal
, and here we provide combinators that generalize
the usual Traversable
operations.
 type Traversal s t a b = forall f. Applicative f => (a > f b) > s > f t
 type Traversal' s a = Traversal s s a a
 type Traversal1 s t a b = forall f. Apply f => (a > f b) > s > f t
 type Traversal1' s a = Traversal1 s s a a
 type IndexedTraversal i s t a b = forall p f. (Indexable i p, Applicative f) => p a (f b) > s > f t
 type IndexedTraversal' i s a = IndexedTraversal i s s a a
 type IndexedTraversal1 i s t a b = forall p f. (Indexable i p, Apply f) => p a (f b) > s > f t
 type IndexedTraversal1' i s a = IndexedTraversal1 i s s a a
 type ATraversal s t a b = LensLike (Bazaar (>) a b) s t a b
 type ATraversal' s a = ATraversal s s a a
 type ATraversal1 s t a b = LensLike (Bazaar1 (>) a b) s t a b
 type ATraversal1' s a = ATraversal1 s s a a
 type AnIndexedTraversal i s t a b = Over (Indexed i) (Bazaar (Indexed i) a b) s t a b
 type AnIndexedTraversal' i s a = AnIndexedTraversal i s s a a
 type AnIndexedTraversal1 i s t a b = Over (Indexed i) (Bazaar1 (Indexed i) a b) s t a b
 type AnIndexedTraversal1' i s a = AnIndexedTraversal1 i s s a a
 type Traversing p f s t a b = Over p (BazaarT p f a b) s t a b
 type Traversing' p f s a = Traversing p f s s a a
 type Traversing1 p f s t a b = Over p (BazaarT1 p f a b) s t a b
 type Traversing1' p f s a = Traversing1 p f s s a a
 traverseOf :: LensLike f s t a b > (a > f b) > s > f t
 forOf :: LensLike f s t a b > s > (a > f b) > f t
 sequenceAOf :: LensLike f s t (f b) b > s > f t
 mapMOf :: LensLike (WrappedMonad m) s t a b > (a > m b) > s > m t
 forMOf :: LensLike (WrappedMonad m) s t a b > s > (a > m b) > m t
 sequenceOf :: LensLike (WrappedMonad m) s t (m b) b > s > m t
 transposeOf :: LensLike ZipList s t [a] a > s > [t]
 mapAccumLOf :: LensLike (State acc) s t a b > (acc > a > (acc, b)) > acc > s > (acc, t)
 mapAccumROf :: LensLike (Backwards (State acc)) s t a b > (acc > a > (acc, b)) > acc > s > (acc, t)
 scanr1Of :: LensLike (Backwards (State (Maybe a))) s t a a > (a > a > a) > s > t
 scanl1Of :: LensLike (State (Maybe a)) s t a a > (a > a > a) > s > t
 failover :: Alternative m => LensLike ((,) Any) s t a b > (a > b) > s > m t
 ifailover :: Alternative m => Over (Indexed i) ((,) Any) s t a b > (i > a > b) > s > m t
 cloneTraversal :: ATraversal s t a b > Traversal s t a b
 cloneIndexPreservingTraversal :: ATraversal s t a b > IndexPreservingTraversal s t a b
 cloneIndexedTraversal :: AnIndexedTraversal i s t a b > IndexedTraversal i s t a b
 cloneTraversal1 :: ATraversal1 s t a b > Traversal1 s t a b
 cloneIndexPreservingTraversal1 :: ATraversal1 s t a b > IndexPreservingTraversal1 s t a b
 cloneIndexedTraversal1 :: AnIndexedTraversal1 i s t a b > IndexedTraversal1 i s t a b
 partsOf :: Functor f => Traversing (>) f s t a a > LensLike f s t [a] [a]
 partsOf' :: ATraversal s t a a > Lens s t [a] [a]
 unsafePartsOf :: Functor f => Traversing (>) f s t a b > LensLike f s t [a] [b]
 unsafePartsOf' :: ATraversal s t a b > Lens s t [a] [b]
 holesOf :: forall p s t a. Conjoined p => Over p (Bazaar p a a) s t a a > s > [Pretext p a a t]
 singular :: (Conjoined p, Functor f) => Traversing p f s t a a > Over p f s t a a
 unsafeSingular :: (Conjoined p, Functor f) => Traversing p f s t a b > Over p f s t a b
 class (Functor t, Foldable t) => Traversable t where
 traverse :: Applicative f => (a > f b) > t a > f (t b)
 class (Foldable1 t, Traversable t) => Traversable1 t where
 both :: Bitraversable r => Traversal (r a a) (r b b) a b
 beside :: (Representable q, Applicative (Rep q), Applicative f, Bitraversable r) => Optical p q f s t a b > Optical p q f s' t' a b > Optical p q f (r s s') (r t t') a b
 taking :: (Conjoined p, Applicative f) => Int > Traversing p f s t a a > Over p f s t a a
 dropping :: (Conjoined p, Applicative f) => Int > Over p (Indexing f) s t a a > Over p f s t a a
 failing :: (Conjoined p, Applicative f) => Traversing p f s t a b > Over p f s t a b > Over p f s t a b
 deepOf :: (Conjoined p, Applicative f) => LensLike f s t s t > Traversing p f s t a b > Over p f s t a b
 ignored :: Applicative f => pafb > s > f s
 class Ord k => TraverseMin k m  m > k where
 traverseMin :: IndexedTraversal' k (m v) v
 class Ord k => TraverseMax k m  m > k where
 traverseMax :: IndexedTraversal' k (m v) v
 traversed :: Traversable f => IndexedTraversal Int (f a) (f b) a b
 traversed1 :: Traversable1 f => IndexedTraversal1 Int (f a) (f b) a b
 traversed64 :: Traversable f => IndexedTraversal Int64 (f a) (f b) a b
 elementOf :: Applicative f => LensLike (Indexing f) s t a a > Int > IndexedLensLike Int f s t a a
 element :: Traversable t => Int > IndexedTraversal' Int (t a) a
 elementsOf :: Applicative f => LensLike (Indexing f) s t a a > (Int > Bool) > IndexedLensLike Int f s t a a
 elements :: Traversable t => (Int > Bool) > IndexedTraversal' Int (t a) a
 ipartsOf :: forall i p f s t a. (Indexable [i] p, Functor f) => Traversing (Indexed i) f s t a a > Over p f s t [a] [a]
 ipartsOf' :: forall i p f s t a. (Indexable [i] p, Functor f) => Over (Indexed i) (Bazaar' (Indexed i) a) s t a a > Over p f s t [a] [a]
 iunsafePartsOf :: forall i p f s t a b. (Indexable [i] p, Functor f) => Traversing (Indexed i) f s t a b > Over p f s t [a] [b]
 iunsafePartsOf' :: forall i s t a b. Over (Indexed i) (Bazaar (Indexed i) a b) s t a b > IndexedLens [i] s t [a] [b]
 itraverseOf :: (Indexed i a (f b) > s > f t) > (i > a > f b) > s > f t
 iforOf :: (Indexed i a (f b) > s > f t) > s > (i > a > f b) > f t
 imapMOf :: Over (Indexed i) (WrappedMonad m) s t a b > (i > a > m b) > s > m t
 iforMOf :: (Indexed i a (WrappedMonad m b) > s > WrappedMonad m t) > s > (i > a > m b) > m t
 imapAccumROf :: Over (Indexed i) (Backwards (State acc)) s t a b > (i > acc > a > (acc, b)) > acc > s > (acc, t)
 imapAccumLOf :: Over (Indexed i) (State acc) s t a b > (i > acc > a > (acc, b)) > acc > s > (acc, t)
 traverseBy :: Traversable t => (forall x. x > f x) > (forall x y. f (x > y) > f x > f y) > (a > f b) > t a > f (t b)
 traverseByOf :: Traversal s t a b > (forall x. x > f x) > (forall x y. f (x > y) > f x > f y) > (a > f b) > s > f t
 sequenceBy :: Traversable t => (forall x. x > f x) > (forall x y. f (x > y) > f x > f y) > t (f a) > f (t a)
 sequenceByOf :: Traversal s t (f b) b > (forall x. x > f x) > (forall x y. f (x > y) > f x > f y) > s > f t
 newtype Bazaar p a b t = Bazaar {
 runBazaar :: forall f. Applicative f => p a (f b) > f t
 type Bazaar' p a = Bazaar p a a
 newtype Bazaar1 p a b t = Bazaar1 {
 runBazaar1 :: forall f. Apply f => p a (f b) > f t
 type Bazaar1' p a = Bazaar1 p a a
 loci :: Traversal (Bazaar (>) a c s) (Bazaar (>) b c s) a b
 iloci :: IndexedTraversal i (Bazaar (Indexed i) a c s) (Bazaar (Indexed i) b c s) a b
 confusing :: Applicative f => LensLike (Curried (Yoneda f) (Yoneda f)) s t a b > LensLike f s t a b
Traversals
type Traversal s t a b = forall f. Applicative f => (a > f b) > s > f t Source
A Traversal
can be used directly as a Setter
or a Fold
(but not as a Lens
) and provides
the ability to both read and update multiple fields, subject to some relatively weak Traversal
laws.
These have also been known as multilenses, but they have the signature and spirit of
traverse
::Traversable
f =>Traversal
(f a) (f b) a b
and the more evocative name suggests their application.
Most of the time the Traversal
you will want to use is just traverse
, but you can also pass any
Lens
or Iso
as a Traversal
, and composition of a Traversal
(or Lens
or Iso
) with a Traversal
(or Lens
or Iso
)
using (.
) forms a valid Traversal
.
The laws for a Traversal
t
follow from the laws for Traversable
as stated in "The Essence of the Iterator Pattern".
tpure
≡pure
fmap
(t f).
t g ≡getCompose
.
t (Compose
.
fmap
f.
g)
One consequence of this requirement is that a Traversal
needs to leave the same number of elements as a
candidate for subsequent Traversal
that it started with. Another testament to the strength of these laws
is that the caveat expressed in section 5.5 of the "Essence of the Iterator Pattern" about exotic
Traversable
instances that traverse
the same entry multiple times was actually already ruled out by the
second law in that same paper!
type Traversal' s a = Traversal s s a a Source
typeTraversal'
=Simple
Traversal
type Traversal1 s t a b = forall f. Apply f => (a > f b) > s > f t Source
type Traversal1' s a = Traversal1 s s a a Source
type IndexedTraversal i s t a b = forall p f. (Indexable i p, Applicative f) => p a (f b) > s > f t Source
Every IndexedTraversal
is a valid Traversal
or
IndexedFold
.
The Indexed
constraint is used to allow an IndexedTraversal
to be used
directly as a Traversal
.
The Traversal
laws are still required to hold.
In addition, the index i
should satisfy the requirement that it stays
unchanged even when modifying the value a
, otherwise traversals like
indices
break the Traversal
laws.
type IndexedTraversal' i s a = IndexedTraversal i s s a a Source
typeIndexedTraversal'
i =Simple
(IndexedTraversal
i)
type IndexedTraversal1 i s t a b = forall p f. (Indexable i p, Apply f) => p a (f b) > s > f t Source
type IndexedTraversal1' i s a = IndexedTraversal1 i s s a a Source
type ATraversal s t a b = LensLike (Bazaar (>) a b) s t a b Source
When you see this as an argument to a function, it expects a Traversal
.
type ATraversal' s a = ATraversal s s a a Source
typeATraversal'
=Simple
ATraversal
type ATraversal1 s t a b = LensLike (Bazaar1 (>) a b) s t a b Source
When you see this as an argument to a function, it expects a Traversal1
.
type ATraversal1' s a = ATraversal1 s s a a Source
typeATraversal1'
=Simple
ATraversal1
type AnIndexedTraversal i s t a b = Over (Indexed i) (Bazaar (Indexed i) a b) s t a b Source
When you see this as an argument to a function, it expects an IndexedTraversal
.
type AnIndexedTraversal' i s a = AnIndexedTraversal i s s a a Source
typeAnIndexedTraversal'
=Simple
(AnIndexedTraversal
i)
type AnIndexedTraversal1 i s t a b = Over (Indexed i) (Bazaar1 (Indexed i) a b) s t a b Source
When you see this as an argument to a function, it expects an IndexedTraversal1
.
type AnIndexedTraversal1' i s a = AnIndexedTraversal1 i s s a a Source
typeAnIndexedTraversal1'
=Simple
(AnIndexedTraversal1
i)
type Traversing p f s t a b = Over p (BazaarT p f a b) s t a b Source
When you see this as an argument to a function, it expects
 to be indexed if
p
is an instance ofIndexed
i,  to be unindexed if
p
is(>)
,  a
Traversal
iff
isApplicative
,  a
Getter
iff
is only aFunctor
andContravariant
,  a
Lens
ifp
is only aFunctor
,  a
Fold
iff
isFunctor
,Contravariant
andApplicative
.
type Traversing' p f s a = Traversing p f s s a a Source
typeTraversing'
f =Simple
(Traversing
f)
type Traversing1 p f s t a b = Over p (BazaarT1 p f a b) s t a b Source
type Traversing1' p f s a = Traversing1 p f s s a a Source
Traversing and Lensing
traverseOf :: LensLike f s t a b > (a > f b) > s > f t Source
Map each element of a structure targeted by a Lens
or Traversal
,
evaluate these actions from left to right, and collect the results.
This function is only provided for consistency, id
is strictly more general.
>>>
traverseOf each print (1,2,3)
1 2 3 ((),(),())
traverseOf
≡id
itraverseOf
l ≡traverseOf
l.
Indexed
itraverseOf
itraversed
≡itraverse
This yields the obvious law:
traverse
≡traverseOf
traverse
traverseOf
::Functor
f =>Iso
s t a b > (a > f b) > s > f ttraverseOf
::Functor
f =>Lens
s t a b > (a > f b) > s > f ttraverseOf
::Applicative
f =>Traversal
s t a b > (a > f b) > s > f t
forOf :: LensLike f s t a b > s > (a > f b) > f t Source
A version of traverseOf
with the arguments flipped, such that:
>>>
forOf each (1,2,3) print
1 2 3 ((),(),())
This function is only provided for consistency, flip
is strictly more general.
forOf
≡flip
forOf
≡flip
.traverseOf
for
≡forOf
traverse
ifor
l s ≡for
l s.
Indexed
forOf
::Functor
f =>Iso
s t a b > s > (a > f b) > f tforOf
::Functor
f =>Lens
s t a b > s > (a > f b) > f tforOf
::Applicative
f =>Traversal
s t a b > s > (a > f b) > f t
sequenceAOf :: LensLike f s t (f b) b > s > f t Source
Evaluate each action in the structure from left to right, and collect the results.
>>>
sequenceAOf both ([1,2],[3,4])
[(1,3),(1,4),(2,3),(2,4)]
sequenceA
≡sequenceAOf
traverse
≡traverse
id
sequenceAOf
l ≡traverseOf
lid
≡ lid
sequenceAOf
::Functor
f =>Iso
s t (f b) b > s > f tsequenceAOf
::Functor
f =>Lens
s t (f b) b > s > f tsequenceAOf
::Applicative
f =>Traversal
s t (f b) b > s > f t
mapMOf :: LensLike (WrappedMonad m) s t a b > (a > m b) > s > m t Source
Map each element of a structure targeted by a Lens
to a monadic action,
evaluate these actions from left to right, and collect the results.
>>>
mapMOf both (\x > [x, x + 1]) (1,3)
[(1,3),(1,4),(2,3),(2,4)]
mapM
≡mapMOf
traverse
imapMOf
l ≡forM
l.
Indexed
mapMOf
::Monad
m =>Iso
s t a b > (a > m b) > s > m tmapMOf
::Monad
m =>Lens
s t a b > (a > m b) > s > m tmapMOf
::Monad
m =>Traversal
s t a b > (a > m b) > s > m t
forMOf :: LensLike (WrappedMonad m) s t a b > s > (a > m b) > m t Source
forMOf
is a flipped version of mapMOf
, consistent with the definition of forM
.
>>>
forMOf both (1,3) $ \x > [x, x + 1]
[(1,3),(1,4),(2,3),(2,4)]
forM
≡forMOf
traverse
forMOf
l ≡flip
(mapMOf
l)iforMOf
l s ≡forM
l s.
Indexed
forMOf
::Monad
m =>Iso
s t a b > s > (a > m b) > m tforMOf
::Monad
m =>Lens
s t a b > s > (a > m b) > m tforMOf
::Monad
m =>Traversal
s t a b > s > (a > m b) > m t
sequenceOf :: LensLike (WrappedMonad m) s t (m b) b > s > m t Source
Sequence the (monadic) effects targeted by a Lens
in a container from left to right.
>>>
sequenceOf each ([1,2],[3,4],[5,6])
[(1,3,5),(1,3,6),(1,4,5),(1,4,6),(2,3,5),(2,3,6),(2,4,5),(2,4,6)]
sequence
≡sequenceOf
traverse
sequenceOf
l ≡mapMOf
lid
sequenceOf
l ≡unwrapMonad
.
lWrapMonad
sequenceOf
::Monad
m =>Iso
s t (m b) b > s > m tsequenceOf
::Monad
m =>Lens
s t (m b) b > s > m tsequenceOf
::Monad
m =>Traversal
s t (m b) b > s > m t
transposeOf :: LensLike ZipList s t [a] a > s > [t] Source
This generalizes transpose
to an arbitrary Traversal
.
Note: transpose
handles ragged inputs more intelligently, but for nonragged inputs:
>>>
transposeOf traverse [[1,2,3],[4,5,6]]
[[1,4],[2,5],[3,6]]
transpose
≡transposeOf
traverse
Since every Lens
is a Traversal
, we can use this as a form of
monadic strength as well:
transposeOf
_2
:: (b, [a]) > [(b, a)]
mapAccumLOf :: LensLike (State acc) s t a b > (acc > a > (acc, b)) > acc > s > (acc, t) Source
This generalizes mapAccumL
to an arbitrary Traversal
.
mapAccumL
≡mapAccumLOf
traverse
mapAccumLOf
accumulates State
from left to right.
mapAccumLOf
::Iso
s t a b > (acc > a > (acc, b)) > acc > s > (acc, t)mapAccumLOf
::Lens
s t a b > (acc > a > (acc, b)) > acc > s > (acc, t)mapAccumLOf
::Traversal
s t a b > (acc > a > (acc, b)) > acc > s > (acc, t)
mapAccumLOf
::LensLike
(State
acc) s t a b > (acc > a > (acc, b)) > acc > s > (acc, t)mapAccumLOf
l f acc0 s =swap
(runState
(l (a >state
(acc >swap
(f acc a))) s) acc0)
mapAccumROf :: LensLike (Backwards (State acc)) s t a b > (acc > a > (acc, b)) > acc > s > (acc, t) Source
This generalizes mapAccumR
to an arbitrary Traversal
.
mapAccumR
≡mapAccumROf
traverse
mapAccumROf
accumulates State
from right to left.
mapAccumROf
::Iso
s t a b > (acc > a > (acc, b)) > acc > s > (acc, t)mapAccumROf
::Lens
s t a b > (acc > a > (acc, b)) > acc > s > (acc, t)mapAccumROf
::Traversal
s t a b > (acc > a > (acc, b)) > acc > s > (acc, t)
mapAccumROf
::LensLike
(Backwards
(State
acc)) s t a b > (acc > a > (acc, b)) > acc > s > (acc, t)
failover :: Alternative m => LensLike ((,) Any) s t a b > (a > b) > s > m t Source
Try to map a function over this Traversal
, failing if the Traversal
has no targets.
>>>
failover (element 3) (*2) [1,2] :: Maybe [Int]
Nothing
>>>
failover _Left (*2) (Right 4) :: Maybe (Either Int Int)
Nothing
>>>
failover _Right (*2) (Right 4) :: Maybe (Either Int Int)
Just (Right 8)
failover
:: Alternative m => Traversal s t a b > (a > b) > s > m t
ifailover :: Alternative m => Over (Indexed i) ((,) Any) s t a b > (i > a > b) > s > m t Source
Try to map a function which uses the index over this IndexedTraversal
, failing if the IndexedTraversal
has no targets.
ifailover
:: Alternative m => IndexedTraversal i s t a b > (i > a > b) > s > m t
Monomorphic Traversals
cloneTraversal :: ATraversal s t a b > Traversal s t a b Source
A Traversal
is completely characterized by its behavior on a Bazaar
.
Cloning a Traversal
is one way to make sure you aren't given
something weaker, such as a Fold
and can be
used as a way to pass around traversals that have to be monomorphic in f
.
Note: This only accepts a proper Traversal
(or Lens
). To clone a Lens
as such, use cloneLens
.
Note: It is usually better to use ReifiedTraversal
and
runTraversal
than to cloneTraversal
. The
former can execute at full speed, while the latter needs to round trip through
the Bazaar
.
>>>
let foo l a = (view (getting (cloneTraversal l)) a, set (cloneTraversal l) 10 a)
>>>
foo both ("hello","world")
("helloworld",(10,10))
cloneTraversal
::LensLike
(Bazaar
(>) a b) s t a b >Traversal
s t a b
cloneIndexPreservingTraversal :: ATraversal s t a b > IndexPreservingTraversal s t a b Source
Clone a Traversal
yielding an IndexPreservingTraversal
that passes through
whatever index it is composed with.
cloneIndexedTraversal :: AnIndexedTraversal i s t a b > IndexedTraversal i s t a b Source
Clone an IndexedTraversal
yielding an IndexedTraversal
with the same index.
cloneTraversal1 :: ATraversal1 s t a b > Traversal1 s t a b Source
A Traversal1
is completely characterized by its behavior on a Bazaar1
.
cloneIndexPreservingTraversal1 :: ATraversal1 s t a b > IndexPreservingTraversal1 s t a b Source
Clone a Traversal1
yielding an IndexPreservingTraversal1
that passes through
whatever index it is composed with.
cloneIndexedTraversal1 :: AnIndexedTraversal1 i s t a b > IndexedTraversal1 i s t a b Source
Clone an IndexedTraversal1
yielding an IndexedTraversal1
with the same index.
Parts and Holes
partsOf :: Functor f => Traversing (>) f s t a a > LensLike f s t [a] [a] Source
partsOf
turns a Traversal
into a Lens
that resembles an early version of the uniplate
(or biplate
) type.
Note: You should really try to maintain the invariant of the number of children in the list.
>>>
(a,b,c) & partsOf each .~ [x,y,z]
(x,y,z)
Any extras will be lost. If you do not supply enough, then the remainder will come from the original structure.
>>>
(a,b,c) & partsOf each .~ [w,x,y,z]
(w,x,y)
>>>
(a,b,c) & partsOf each .~ [x,y]
(x,y,c)
>>>
('b', 'a', 'd', 'c') & partsOf each %~ sort
('a','b','c','d')
So technically, this is only a Lens
if you do not change the number of results it returns.
When applied to a Fold
the result is merely a Getter
.
partsOf
::Iso'
s a >Lens'
s [a]partsOf
::Lens'
s a >Lens'
s [a]partsOf
::Traversal'
s a >Lens'
s [a]partsOf
::Fold
s a >Getter
s [a]partsOf
::Getter
s a >Getter
s [a]
partsOf' :: ATraversal s t a a > Lens s t [a] [a] Source
unsafePartsOf :: Functor f => Traversing (>) f s t a b > LensLike f s t [a] [b] Source
unsafePartsOf
turns a Traversal
into a uniplate
(or biplate
) family.
If you do not need the types of s
and t
to be different, it is recommended that
you use partsOf
.
It is generally safer to traverse with the Bazaar
rather than use this
combinator. However, it is sometimes convenient.
This is unsafe because if you don't supply at least as many b
's as you were
given a
's, then the reconstruction of t
will result in an error!
When applied to a Fold
the result is merely a Getter
(and becomes safe).
unsafePartsOf
::Iso
s t a b >Lens
s t [a] [b]unsafePartsOf
::Lens
s t a b >Lens
s t [a] [b]unsafePartsOf
::Traversal
s t a b >Lens
s t [a] [b]unsafePartsOf
::Fold
s a >Getter
s [a]unsafePartsOf
::Getter
s a >Getter
s [a]
unsafePartsOf' :: ATraversal s t a b > Lens s t [a] [b] Source
holesOf :: forall p s t a. Conjoined p => Over p (Bazaar p a a) s t a a > s > [Pretext p a a t] Source
The onelevel version of contextsOf
. This extracts a list of the immediate children according to a given Traversal
as editable contexts.
Given a context you can use pos
to see the values, peek
at what the structure would be like with an edited result, or simply extract
the original structure.
propChildren l x = childrenOf l x==
map
pos
(holesOf
l x) propId l x =all
(==
x) [extract
w  w <holesOf
l x]
holesOf
::Iso'
s a > s > [Pretext'
(>) a s]holesOf
::Lens'
s a > s > [Pretext'
(>) a s]holesOf
::Traversal'
s a > s > [Pretext'
(>) a s]holesOf
::IndexedLens'
i s a > s > [Pretext'
(Indexed
i) a s]holesOf
::IndexedTraversal'
i s a > s > [Pretext'
(Indexed
i) a s]
singular :: (Conjoined p, Functor f) => Traversing p f s t a a > Over p f s t a a Source
This converts a Traversal
that you "know" will target one or more elements to a Lens
. It can
also be used to transform a nonempty Fold
into a Getter
.
The resulting Lens
or Getter
will be partial if the supplied Traversal
returns
no results.
>>>
[1,2,3] ^. singular _head
1
>>>
[] ^. singular _head
*** Exception: singular: empty traversal
>>>
Left 4 ^. singular _Left
4
>>>
[1..10] ^. singular (ix 7)
8
>>>
[] & singular traverse .~ 0
[]
singular
::Traversal
s t a a >Lens
s t a asingular
::Fold
s a >Getter
s asingular
::IndexedTraversal
i s t a a >IndexedLens
i s t a asingular
::IndexedFold
i s a >IndexedGetter
i s a
unsafeSingular :: (Conjoined p, Functor f) => Traversing p f s t a b > Over p f s t a b Source
This converts a Traversal
that you "know" will target only one element to a Lens
. It can also be
used to transform a Fold
into a Getter
.
The resulting Lens
or Getter
will be partial if the Traversal
targets nothing
or more than one element.
>>>
[] & unsafeSingular traverse .~ 0
*** Exception: unsafeSingular: empty traversal
unsafeSingular
::Traversal
s t a b >Lens
s t a bunsafeSingular
::Fold
s a >Getter
s aunsafeSingular
::IndexedTraversal
i s t a b >IndexedLens
i s t a bunsafeSingular
::IndexedFold
i s a >IndexedGetter
i s a
Common Traversals
class (Functor t, Foldable t) => Traversable t where
Functors representing data structures that can be traversed from left to right.
A definition of traverse
must satisfy the following laws:
 naturality
t .
for every applicative transformationtraverse
f =traverse
(t . f)t
 identity
traverse
Identity = Identity composition
traverse
(Compose .fmap
g . f) = Compose .fmap
(traverse
g) .traverse
f
A definition of sequenceA
must satisfy the following laws:
 naturality
t .
for every applicative transformationsequenceA
=sequenceA
.fmap
tt
 identity
sequenceA
.fmap
Identity = Identity composition
sequenceA
.fmap
Compose = Compose .fmap
sequenceA
.sequenceA
where an applicative transformation is a function
t :: (Applicative f, Applicative g) => f a > g a
preserving the Applicative
operations, i.e.
and the identity functor Identity
and composition of functors Compose
are defined as
newtype Identity a = Identity a instance Functor Identity where fmap f (Identity x) = Identity (f x) instance Applicative Indentity where pure x = Identity x Identity f <*> Identity x = Identity (f x) newtype Compose f g a = Compose (f (g a)) instance (Functor f, Functor g) => Functor (Compose f g) where fmap f (Compose x) = Compose (fmap (fmap f) x) instance (Applicative f, Applicative g) => Applicative (Compose f g) where pure x = Compose (pure (pure x)) Compose f <*> Compose x = Compose ((<*>) <$> f <*> x)
(The naturality law is implied by parametricity.)
Instances are similar to Functor
, e.g. given a data type
data Tree a = Empty  Leaf a  Node (Tree a) a (Tree a)
a suitable instance would be
instance Traversable Tree where traverse f Empty = pure Empty traverse f (Leaf x) = Leaf <$> f x traverse f (Node l k r) = Node <$> traverse f l <*> f k <*> traverse f r
This is suitable even for abstract types, as the laws for <*>
imply a form of associativity.
The superclass instances should satisfy the following:
 In the
Functor
instance,fmap
should be equivalent to traversal with the identity applicative functor (fmapDefault
).  In the
Foldable
instance,foldMap
should be equivalent to traversal with a constant applicative functor (foldMapDefault
).
traverse :: Applicative f => (a > f b) > t a > f (t b)
Map each element of a structure to an action, evaluate these actions
from left to right, and collect the results. For a version that ignores
the results see traverse_
.
class (Foldable1 t, Traversable t) => Traversable1 t where
Traversable1 Identity  
Traversable1 Tree  
Traversable1 NonEmpty  
Traversable1 ((,) a)  
Traversable1 f => Traversable1 (IdentityT f)  
Traversable1 f => Traversable1 (Cofree f)  
Traversable1 f => Traversable1 (Free f)  
Traversable1 f => Traversable1 (Yoneda f)  
Traversable1 f => Traversable1 (Reverse f)  
Traversable1 f => Traversable1 (Backwards f)  
Traversable1 f => Traversable1 (Lift f)  
(Traversable1 f, Traversable1 g) => Traversable1 (Compose f g)  
(Traversable1 f, Traversable1 g) => Traversable1 (Sum f g)  
(Traversable1 f, Traversable1 g) => Traversable1 (Product f g)  
Bitraversable1 p => Traversable1 (Join * p)  
Traversable1 f => Traversable1 (AlongsideRight f a)  
Traversable1 f => Traversable1 (AlongsideLeft f b)  
Traversable1 g => Traversable1 (Joker * * g a)  
both :: Bitraversable r => Traversal (r a a) (r b b) a b Source
Traverse both parts of a Bitraversable
container with matching types.
Usually that type will be a pair.
>>>
(1,2) & both *~ 10
(10,20)
>>>
over both length ("hello","world")
(5,5)
>>>
("hello","world")^.both
"helloworld"
both
::Traversal
(a, a) (b, b) a bboth
::Traversal
(Either
a a) (Either
b b) a b
beside :: (Representable q, Applicative (Rep q), Applicative f, Bitraversable r) => Optical p q f s t a b > Optical p q f s' t' a b > Optical p q f (r s s') (r t t') a b Source
Apply a different Traversal
or Fold
to each side of a Bitraversable
container.
beside
::Traversal
s t a b >Traversal
s' t' a b >Traversal
(r s s') (r t t') a bbeside
::IndexedTraversal
i s t a b >IndexedTraversal
i s' t' a b >IndexedTraversal
i (r s s') (r t t') a bbeside
::IndexPreservingTraversal
s t a b >IndexPreservingTraversal
s' t' a b >IndexPreservingTraversal
(r s s') (r t t') a b
beside
::Traversal
s t a b >Traversal
s' t' a b >Traversal
(s,s') (t,t') a bbeside
::Lens
s t a b >Lens
s' t' a b >Traversal
(s,s') (t,t') a bbeside
::Fold
s a >Fold
s' a >Fold
(s,s') abeside
::Getter
s a >Getter
s' a >Fold
(s,s') a
beside
::IndexedTraversal
i s t a b >IndexedTraversal
i s' t' a b >IndexedTraversal
i (s,s') (t,t') a bbeside
::IndexedLens
i s t a b >IndexedLens
i s' t' a b >IndexedTraversal
i (s,s') (t,t') a bbeside
::IndexedFold
i s a >IndexedFold
i s' a >IndexedFold
i (s,s') abeside
::IndexedGetter
i s a >IndexedGetter
i s' a >IndexedFold
i (s,s') a
beside
::IndexPreservingTraversal
s t a b >IndexPreservingTraversal
s' t' a b >IndexPreservingTraversal
(s,s') (t,t') a bbeside
::IndexPreservingLens
s t a b >IndexPreservingLens
s' t' a b >IndexPreservingTraversal
(s,s') (t,t') a bbeside
::IndexPreservingFold
s a >IndexPreservingFold
s' a >IndexPreservingFold
(s,s') abeside
::IndexPreservingGetter
s a >IndexPreservingGetter
s' a >IndexPreservingFold
(s,s') a
>>>
("hello",["world","!!!"])^..beside id traverse
["hello","world","!!!"]
taking :: (Conjoined p, Applicative f) => Int > Traversing p f s t a a > Over p f s t a a Source
Visit the first n targets of a Traversal
, Fold
, Getter
or Lens
.
>>>
[("hello","world"),("!!!","!!!")]^.. taking 2 (traverse.both)
["hello","world"]
>>>
timingOut $ [1..] ^.. taking 3 traverse
[1,2,3]
>>>
over (taking 5 traverse) succ "hello world"
"ifmmp world"
taking
::Int
>Traversal'
s a >Traversal'
s ataking
::Int
>Lens'
s a >Traversal'
s ataking
::Int
>Iso'
s a >Traversal'
s ataking
::Int
>Prism'
s a >Traversal'
s ataking
::Int
>Getter
s a >Fold
s ataking
::Int
>Fold
s a >Fold
s ataking
::Int
>IndexedTraversal'
i s a >IndexedTraversal'
i s ataking
::Int
>IndexedLens'
i s a >IndexedTraversal'
i s ataking
::Int
>IndexedGetter
i s a >IndexedFold
i s ataking
::Int
>IndexedFold
i s a >IndexedFold
i s a
dropping :: (Conjoined p, Applicative f) => Int > Over p (Indexing f) s t a a > Over p f s t a a Source
Visit all but the first n targets of a Traversal
, Fold
, Getter
or Lens
.
>>>
("hello","world") ^? dropping 1 both
Just "world"
Dropping works on infinite traversals as well:
>>>
[1..] ^? dropping 1 folded
Just 2
dropping
::Int
>Traversal'
s a >Traversal'
s adropping
::Int
>Lens'
s a >Traversal'
s adropping
::Int
>Iso'
s a >Traversal'
s adropping
::Int
>Prism'
s a >Traversal'
s adropping
::Int
>Getter
s a >Fold
s adropping
::Int
>Fold
s a >Fold
s adropping
::Int
>IndexedTraversal'
i s a >IndexedTraversal'
i s adropping
::Int
>IndexedLens'
i s a >IndexedTraversal'
i s adropping
::Int
>IndexedGetter
i s a >IndexedFold
i s adropping
::Int
>IndexedFold
i s a >IndexedFold
i s a
failing :: (Conjoined p, Applicative f) => Traversing p f s t a b > Over p f s t a b > Over p f s t a b infixl 5 Source
Try the first Traversal
(or Fold
), falling back on the second Traversal
(or Fold
) if it returns no entries.
This is only a valid Traversal
if the second Traversal
is disjoint from the result of the first or returns
exactly the same results. These conditions are trivially met when given a Lens
, Iso
, Getter
, Prism
or "affine" Traversal  one that
has 0 or 1 target.
Mutatis mutandis for Fold
.
>>>
[0,1,2,3] ^? failing (ix 1) (ix 2)
Just 1
>>>
[0,1,2,3] ^? failing (ix 42) (ix 2)
Just 2
failing
::Traversal
s t a b >Traversal
s t a b >Traversal
s t a bfailing
::Prism
s t a b >Prism
s t a b >Traversal
s t a bfailing
::Fold
s a >Fold
s a >Fold
s a
These cases are also supported, trivially, but are boring, because the left hand side always succeeds.
failing
::Lens
s t a b >Traversal
s t a b >Traversal
s t a bfailing
::Iso
s t a b >Traversal
s t a b >Traversal
s t a bfailing
::Equality
s t a b >Traversal
s t a b >Traversal
s t a bfailing
::Getter
s a >Fold
s a >Fold
s a
If both of the inputs are indexed, the result is also indexed, so you can apply this to a pair of indexed traversals or indexed folds, obtaining an indexed traversal or indexed fold.
failing
::IndexedTraversal
i s t a b >IndexedTraversal
i s t a b >IndexedTraversal
i s t a bfailing
::IndexedFold
i s a >IndexedFold
i s a >IndexedFold
i s a
These cases are also supported, trivially, but are boring, because the left hand side always succeeds.
failing
::IndexedLens
i s t a b >IndexedTraversal
i s t a b >IndexedTraversal
i s t a bfailing
::IndexedGetter
i s a >IndexedGetter
i s a >IndexedFold
i s a
deepOf :: (Conjoined p, Applicative f) => LensLike f s t s t > Traversing p f s t a b > Over p f s t a b Source
Try the second traversal. If it returns no entries, try again with all entries from the first traversal, recursively.
deepOf
::Fold
s s >Fold
s a >Fold
s adeepOf
::Traversal'
s s >Traversal'
s a >Traversal'
s adeepOf
::Traversal
s t s t >Traversal
s t a b >Traversal
s t a bdeepOf
::Fold
s s >IndexedFold
i s a >IndexedFold
i s adeepOf
::Traversal
s t s t >IndexedTraversal
i s t a b >IndexedTraversal
i s t a b
Indexed Traversals
Common
ignored :: Applicative f => pafb > s > f s Source
class Ord k => TraverseMin k m  m > k where Source
Allows IndexedTraversal
the value at the smallest index.
traverseMin :: IndexedTraversal' k (m v) v Source
IndexedTraversal
of the element with the smallest index.
TraverseMin Int IntMap Source  
Ord k => TraverseMin k (Map k) Source  
class Ord k => TraverseMax k m  m > k where Source
Allows IndexedTraversal
of the value at the largest index.
traverseMax :: IndexedTraversal' k (m v) v Source
IndexedTraversal
of the element at the largest index.
TraverseMax Int IntMap Source  
Ord k => TraverseMax k (Map k) Source  
traversed :: Traversable f => IndexedTraversal Int (f a) (f b) a b Source
Traverse any Traversable
container. This is an IndexedTraversal
that is indexed by ordinal position.
traversed1 :: Traversable1 f => IndexedTraversal1 Int (f a) (f b) a b Source
Traverse any Traversable1
container. This is an IndexedTraversal1
that is indexed by ordinal position.
traversed64 :: Traversable f => IndexedTraversal Int64 (f a) (f b) a b Source
Traverse any Traversable
container. This is an IndexedTraversal
that is indexed by ordinal position.
elementOf :: Applicative f => LensLike (Indexing f) s t a a > Int > IndexedLensLike Int f s t a a Source
Traverse the nth elementOf
a Traversal
, Lens
or
Iso
if it exists.
>>>
[[1],[3,4]] & elementOf (traverse.traverse) 1 .~ 5
[[1],[5,4]]
>>>
[[1],[3,4]] ^? elementOf (folded.folded) 1
Just 3
>>>
timingOut $ ['a'..] ^?! elementOf folded 5
'f'
>>>
timingOut $ take 10 $ elementOf traverse 3 .~ 16 $ [0..]
[0,1,2,16,4,5,6,7,8,9]
elementOf
::Traversal'
s a >Int
>IndexedTraversal'
Int
s aelementOf
::Fold
s a >Int
>IndexedFold
Int
s a
element :: Traversable t => Int > IndexedTraversal' Int (t a) a Source
Traverse the nth element of a Traversable
container.
element
≡elementOf
traverse
elementsOf :: Applicative f => LensLike (Indexing f) s t a a > (Int > Bool) > IndexedLensLike Int f s t a a Source
Traverse (or fold) selected elements of a Traversal
(or Fold
) where their ordinal positions match a predicate.
elementsOf
::Traversal'
s a > (Int
>Bool
) >IndexedTraversal'
Int
s aelementsOf
::Fold
s a > (Int
>Bool
) >IndexedFold
Int
s a
elements :: Traversable t => (Int > Bool) > IndexedTraversal' Int (t a) a Source
Traverse elements of a Traversable
container where their ordinal positions match a predicate.
elements
≡elementsOf
traverse
Combinators
ipartsOf :: forall i p f s t a. (Indexable [i] p, Functor f) => Traversing (Indexed i) f s t a a > Over p f s t [a] [a] Source
An indexed version of partsOf
that receives the entire list of indices as its index.
ipartsOf' :: forall i p f s t a. (Indexable [i] p, Functor f) => Over (Indexed i) (Bazaar' (Indexed i) a) s t a a > Over p f s t [a] [a] Source
A typerestricted version of ipartsOf
that can only be used with an IndexedTraversal
.
iunsafePartsOf :: forall i p f s t a b. (Indexable [i] p, Functor f) => Traversing (Indexed i) f s t a b > Over p f s t [a] [b] Source
An indexed version of unsafePartsOf
that receives the entire list of indices as its index.
iunsafePartsOf' :: forall i s t a b. Over (Indexed i) (Bazaar (Indexed i) a b) s t a b > IndexedLens [i] s t [a] [b] Source
itraverseOf :: (Indexed i a (f b) > s > f t) > (i > a > f b) > s > f t Source
Traversal with an index.
NB: When you don't need access to the index then you can just apply your IndexedTraversal
directly as a function!
itraverseOf
≡withIndex
traverseOf
l =itraverseOf
l.
const
=id
itraverseOf
::Functor
f =>IndexedLens
i s t a b > (i > a > f b) > s > f titraverseOf
::Applicative
f =>IndexedTraversal
i s t a b > (i > a > f b) > s > f titraverseOf
::Apply
f =>IndexedTraversal1
i s t a b > (i > a > f b) > s > f t
iforOf :: (Indexed i a (f b) > s > f t) > s > (i > a > f b) > f t Source
Traverse with an index (and the arguments flipped).
forOf
l a ≡iforOf
l a.
const
iforOf
≡flip
.
itraverseOf
iforOf
::Functor
f =>IndexedLens
i s t a b > s > (i > a > f b) > f tiforOf
::Applicative
f =>IndexedTraversal
i s t a b > s > (i > a > f b) > f tiforOf
::Apply
f =>IndexedTraversal1
i s t a b > s > (i > a > f b) > f t
imapMOf :: Over (Indexed i) (WrappedMonad m) s t a b > (i > a > m b) > s > m t Source
Map each element of a structure targeted by a Lens
to a monadic action,
evaluate these actions from left to right, and collect the results, with access
its position.
When you don't need access to the index mapMOf
is more liberal in what it can accept.
mapMOf
l ≡imapMOf
l.
const
imapMOf
::Monad
m =>IndexedLens
i s t a b > (i > a > m b) > s > m timapMOf
::Monad
m =>IndexedTraversal
i s t a b > (i > a > m b) > s > m timapMOf
::Bind
m =>IndexedTraversal1
i s t a b > (i > a > m b) > s > m t
iforMOf :: (Indexed i a (WrappedMonad m b) > s > WrappedMonad m t) > s > (i > a > m b) > m t Source
Map each element of a structure targeted by a Lens
to a monadic action,
evaluate these actions from left to right, and collect the results, with access
its position (and the arguments flipped).
forMOf
l a ≡iforMOf
l a.
const
iforMOf
≡flip
.
imapMOf
iforMOf
::Monad
m =>IndexedLens
i s t a b > s > (i > a > m b) > m tiforMOf
::Monad
m =>IndexedTraversal
i s t a b > s > (i > a > m b) > m t
imapAccumROf :: Over (Indexed i) (Backwards (State acc)) s t a b > (i > acc > a > (acc, b)) > acc > s > (acc, t) Source
Generalizes mapAccumR
to an arbitrary IndexedTraversal
with access to the index.
imapAccumROf
accumulates state from right to left.
mapAccumROf
l ≡imapAccumROf
l.
const
imapAccumROf
::IndexedLens
i s t a b > (i > acc > a > (acc, b)) > acc > s > (acc, t)imapAccumROf
::IndexedTraversal
i s t a b > (i > acc > a > (acc, b)) > acc > s > (acc, t)
imapAccumLOf :: Over (Indexed i) (State acc) s t a b > (i > acc > a > (acc, b)) > acc > s > (acc, t) Source
Generalizes mapAccumL
to an arbitrary IndexedTraversal
with access to the index.
imapAccumLOf
accumulates state from left to right.
mapAccumLOf
l ≡imapAccumLOf
l.
const
imapAccumLOf
::IndexedLens
i s t a b > (i > acc > a > (acc, b)) > acc > s > (acc, t)imapAccumLOf
::IndexedTraversal
i s t a b > (i > acc > a > (acc, b)) > acc > s > (acc, t)
Reflection
traverseBy :: Traversable t => (forall x. x > f x) > (forall x y. f (x > y) > f x > f y) > (a > f b) > t a > f (t b)
Traverse a container using its Traversable
instance using
explicitly provided Applicative
operations. This is like traverse
where the Applicative
instance can be manually specified.
traverseByOf :: Traversal s t a b > (forall x. x > f x) > (forall x y. f (x > y) > f x > f y) > (a > f b) > s > f t Source
Traverse a container using a specified Applicative
.
This is like traverseBy
where the Traversable
instance can be specified by any Traversal
traverseByOf
traverse
≡traverseBy
sequenceBy :: Traversable t => (forall x. x > f x) > (forall x y. f (x > y) > f x > f y) > t (f a) > f (t a)
Sequence a container using its Traversable
instance using
explicitly provided Applicative
operations. This is like sequence
where the Applicative
instance can be manually specified.
sequenceByOf :: Traversal s t (f b) b > (forall x. x > f x) > (forall x y. f (x > y) > f x > f y) > s > f t Source
Sequence a container using a specified Applicative
.
This is like traverseBy
where the Traversable
instance can be specified by any Traversal
sequenceByOf
traverse
≡sequenceBy
Implementation Details
This is used to characterize a Traversal
.
a.k.a. indexed Cartesian store comonad, indexed Kleene store comonad, or an indexed FunList
.
http://twanvl.nl/blog/haskell/nonregular1
A Bazaar
is like a Traversal
that has already been applied to some structure.
Where a
holds an Context
a b ta
and a function from b
to
t
, a
holds Bazaar
a b tN
a
s and a function from N
b
s to t
, (where N
might be infinite).
Mnemonically, a Bazaar
holds many stores and you can easily add more.
This is a final encoding of Bazaar
.
Bazaar  

Corepresentable p => Sellable p (Bazaar p) Source  
Profunctor p => Bizarre p (Bazaar p) Source  
Conjoined p => IndexedComonad (Bazaar p) Source  
IndexedFunctor (Bazaar p) Source  
Functor (Bazaar p a b) Source  
Applicative (Bazaar p a b) Source  
((~) * a b, Conjoined p) => Comonad (Bazaar p a b) Source  
((~) * a b, Conjoined p) => ComonadApply (Bazaar p a b) Source  
Apply (Bazaar p a b) Source  
newtype Bazaar1 p a b t Source
This is used to characterize a Traversal
.
a.k.a. indexed Cartesian store comonad, indexed Kleene store comonad, or an indexed FunList
.
http://twanvl.nl/blog/haskell/nonregular1
A Bazaar1
is like a Traversal
that has already been applied to some structure.
Where a
holds an Context
a b ta
and a function from b
to
t
, a
holds Bazaar1
a b tN
a
s and a function from N
b
s to t
, (where N
might be infinite).
Mnemonically, a Bazaar1
holds many stores and you can easily add more.
This is a final encoding of Bazaar1
.
Bazaar1  

Corepresentable p => Sellable p (Bazaar1 p) Source  
Profunctor p => Bizarre1 p (Bazaar1 p) Source  
Conjoined p => IndexedComonad (Bazaar1 p) Source  
IndexedFunctor (Bazaar1 p) Source  
Functor (Bazaar1 p a b) Source  
((~) * a b, Conjoined p) => Comonad (Bazaar1 p a b) Source  
((~) * a b, Conjoined p) => ComonadApply (Bazaar1 p a b) Source  
Apply (Bazaar1 p a b) Source  
iloci :: IndexedTraversal i (Bazaar (Indexed i) a c s) (Bazaar (Indexed i) b c s) a b Source
This IndexedTraversal
allows you to traverse
the individual stores in
a Bazaar
with access to their indices.
Fusion
confusing :: Applicative f => LensLike (Curried (Yoneda f) (Yoneda f)) s t a b > LensLike f s t a b Source
Fuse a Traversal
by reassociating all of the \<*\>
operations to the
left and fusing all of the fmap
calls into one. This is particularly
useful when constructing a Traversal
using operations from GHC.Generics.
Given a pair of Traversal
s foo
and bar
,
confusing
(foo.bar) = foo.bar
However, foo
and bar
are each going to use the Applicative
they are given.
confusing
exploits the Yoneda
lemma to merge their separate uses of fmap
into a single fmap
.
and it further exploits an interesting property of the right Kan lift (or Curried
) to left associate
all of the uses of '(*)' to make it possible to fuse together more fmaps.
This is particularly effective when the choice of functor f
is unknown at compile
time or when the Traversal
foo.bar
in the above description is recursive or complex
enough to prevent inlining.
fusing
is a version of this combinator suitable for fusing lenses.
confusing
::Traversal
s t a b >Traversal
s t a b