| Safe Haskell | None |
|---|---|
| Language | Haskell2010 |
Optics.AffineTraversal
Description
An
AffineTraversal
is a
Traversal
that
applies to at most one element.
These arise most frequently as the composition of a
Lens
with a
Prism
.
Synopsis
- type AffineTraversal s t a b = Optic An_AffineTraversal NoIx s t a b
- type AffineTraversal' s a = Optic' An_AffineTraversal NoIx s a
- atraversal :: (s -> Either t a) -> (s -> b -> t) -> AffineTraversal s t a b
- matching :: Is k An_AffineTraversal => Optic k is s t a b -> s -> Either t a
- unsafeFiltered :: (a -> Bool ) -> AffineTraversal' a a
- withAffineTraversal :: Is k An_AffineTraversal => Optic k is s t a b -> ((s -> Either t a) -> (s -> b -> t) -> r) -> r
- data An_AffineTraversal :: OpticKind
- type AffineTraversalVL s t a b = forall f. Functor f => ( forall r. r -> f r) -> (a -> f b) -> s -> f t
- type AffineTraversalVL' s a = AffineTraversalVL s s a a
- atraversalVL :: AffineTraversalVL s t a b -> AffineTraversal s t a b
- atraverseOf :: ( Is k An_AffineTraversal , Functor f) => Optic k is s t a b -> ( forall r. r -> f r) -> (a -> f b) -> s -> f t
Formation
type AffineTraversal s t a b = Optic An_AffineTraversal NoIx s t a b Source #
Type synonym for a type-modifying affine traversal.
type AffineTraversal' s a = Optic' An_AffineTraversal NoIx s a Source #
Type synonym for a type-preserving affine traversal.
Introduction
atraversal :: (s -> Either t a) -> (s -> b -> t) -> AffineTraversal s t a b Source #
Build an affine traversal from a matcher and an updater.
If you want to build an
AffineTraversal
from the van Laarhoven
representation, use
atraversalVL
.
Elimination
An
AffineTraversal
is in particular an
AffineFold
and a
Setter
, therefore you can specialise types to obtain:
preview::AffineTraversals t a b -> s -> Maybe a
over::AffineTraversals t a b -> (a -> b) -> s -> tset::AffineTraversals t a b -> b -> s -> t
Computation
matching(atraversalf g) ≡ fisRight(f s) =>set(atraversalf g) b s ≡ g s b
Additional introduction forms
See
_head
,
_tail
,
_init
and
_last
for
AffineTraversal
s for container types.
unsafeFiltered :: (a -> Bool ) -> AffineTraversal' a a Source #
Filter result(s) of a traversal that don't satisfy a predicate.
Note:
This is
not
a legal
Traversal
, unless you are
very careful not to invalidate the predicate on the target.
As a counter example, consider that given
evens =
the second
unsafeFiltered
even
Traversal
law is violated:
overevenssucc.overevenssucc/=overevens (succ.succ)
So, in order for this to qualify as a legal
Traversal
you
can only use it for actions that preserve the result of the predicate!
For a safe variant see
indices
(or
filtered
for read-only optics).
Additional elimination forms
withAffineTraversal :: Is k An_AffineTraversal => Optic k is s t a b -> ((s -> Either t a) -> (s -> b -> t) -> r) -> r Source #
Work with an affine traversal as a matcher and an updater.
Subtyping
data An_AffineTraversal :: OpticKind Source #
Tag for an affine traversal.
Instances
van Laarhoven encoding
type AffineTraversalVL s t a b = forall f. Functor f => ( forall r. r -> f r) -> (a -> f b) -> s -> f t Source #
Type synonym for a type-modifying van Laarhoven affine traversal.
Note: this isn't exactly van Laarhoven representation as there is
no
Pointed
class (which would be a superclass of
Applicative
that contains
pure
but not
<*>
). You can interpret the first
argument as a dictionary of
Pointed
that supplies the
point
function (i.e. the implementation of
pure
).
A
TraversalVL
has
Applicative
available and
hence can combine the effects arising from multiple elements using
<*>
. In contrast, an
AffineTraversalVL
has no way to combine
effects from multiple elements, so it must act on at most one
element. (It can act on none at all thanks to the availability of
point
.)
type AffineTraversalVL' s a = AffineTraversalVL s s a a Source #
Type synonym for a type-preserving van Laarhoven affine traversal.
atraversalVL :: AffineTraversalVL s t a b -> AffineTraversal s t a b Source #
Build an affine traversal from the van Laarhoven representation.
Example:
>>>:{azSnd = atraversalVL $ \point f ab@(a, b) -> if a >= 'a' && a <= 'z' then (a, ) <$> f b else point ab :}
>>>preview azSnd ('a', "Hi")Just "Hi"
>>>preview azSnd ('@', "Hi")Nothing
>>>over azSnd (++ "!!!") ('f', "Hi")('f',"Hi!!!")
>>>set azSnd "Bye" ('Y', "Hi")('Y',"Hi")
atraverseOf :: ( Is k An_AffineTraversal , Functor f) => Optic k is s t a b -> ( forall r. r -> f r) -> (a -> f b) -> s -> f t Source #
Traverse over the target of an
AffineTraversal
and compute a
Functor
-based answer.
Since: 0.3