Copyright  (C) 201215 Edward Kmett 

License  BSDstyle (see the file LICENSE) 
Maintainer  Edward Kmett <ekmett@gmail.com> 
Stability  provisional 
Portability  nonportable 
Safe Haskell  Trustworthy 
Language  Haskell98 
 type Prism s t a b = forall p f. (Choice p, Applicative f) => p a (f b) > p s (f t)
 type Prism' s a = Prism s s a a
 type APrism s t a b = Market a b a (Identity b) > Market a b s (Identity t)
 type APrism' s a = APrism s s a a
 prism :: (b > t) > (s > Either t a) > Prism s t a b
 prism' :: (b > s) > (s > Maybe a) > Prism s s a b
 withPrism :: APrism s t a b > ((b > t) > (s > Either t a) > r) > r
 clonePrism :: APrism s t a b > Prism s t a b
 outside :: Representable p => APrism s t a b > Lens (p t r) (p s r) (p b r) (p a r)
 aside :: APrism s t a b > Prism (e, s) (e, t) (e, a) (e, b)
 without :: APrism s t a b > APrism u v c d > Prism (Either s u) (Either t v) (Either a c) (Either b d)
 below :: Traversable f => APrism' s a > Prism' (f s) (f a)
 isn't :: APrism s t a b > s > Bool
 matching :: APrism s t a b > s > Either t a
 _Left :: Prism (Either a c) (Either b c) a b
 _Right :: Prism (Either c a) (Either c b) a b
 _Just :: Prism (Maybe a) (Maybe b) a b
 _Nothing :: Prism' (Maybe a) ()
 _Void :: Prism s s a Void
 _Show :: (Read a, Show a) => Prism' String a
 only :: Eq a => a > Prism' a ()
 nearly :: a > (a > Bool) > Prism' a ()
 class Profunctor p => Choice p where
Prisms
type Prism s t a b = forall p f. (Choice p, Applicative f) => p a (f b) > p s (f t) Source
A Prism
l
is a Traversal
that can also be turned
around with re
to obtain a Getter
in the
opposite direction.
There are two laws that a Prism
should satisfy:
First, if I re
or review
a value with a Prism
and then preview
or use (^?
), I will get it back:
preview
l (review
l b) ≡Just
b
Second, if you can extract a value a
using a Prism
l
from a value s
, then the value s
is completely described by l
and a
:
If
then preview
l s ≡ Just
areview
l a ≡ s
These two laws imply that the Traversal
laws hold for every Prism
and that we traverse
at most 1 element:
lengthOf
l x<=
1
It may help to think of this as a Iso
that can be partial in one direction.
Every Prism
is a valid Traversal
.
For example, you might have a
allows you to always
go from a Prism'
Integer
Natural
Natural
to an Integer
, and provide you with tools to check if an Integer
is
a Natural
and/or to edit one if it is.
nat
::Prism'
Integer
Natural
nat
=prism
toInteger
$
\ i > if i<
0 thenLeft
i elseRight
(fromInteger
i)
Now we can ask if an Integer
is a Natural
.
>>>
5^?nat
Just 5
>>>
(5)^?nat
Nothing
We can update the ones that are:
>>>
(3,4) & both.nat *~ 2
(3,8)
And we can then convert from a Natural
to an Integer
.
>>>
5 ^. re nat  :: Natural
5
Similarly we can use a Prism
to traverse
the Left
half of an Either
:
>>>
Left "hello" & _Left %~ length
Left 5
or to construct an Either
:
>>>
5^.re _Left
Left 5
such that if you query it with the Prism
, you will get your original input back.
>>>
5^.re _Left ^? _Left
Just 5
Another interesting way to think of a Prism
is as the categorical dual of a Lens
 a coLens
, so to speak. This is what permits the construction of outside
.
Note: Composition with a Prism
is indexpreserving.
type APrism s t a b = Market a b a (Identity b) > Market a b s (Identity t) Source
If you see this in a signature for a function, the function is expecting a Prism
.
Constructing Prisms
Consuming Prisms
withPrism :: APrism s t a b > ((b > t) > (s > Either t a) > r) > r Source
Convert APrism
to the pair of functions that characterize it.
clonePrism :: APrism s t a b > Prism s t a b Source
Clone a Prism
so that you can reuse the same monomorphically typed Prism
for different purposes.
See cloneLens
and cloneTraversal
for examples of why you might want to do this.
outside :: Representable p => APrism s t a b > Lens (p t r) (p s r) (p b r) (p a r) Source
aside :: APrism s t a b > Prism (e, s) (e, t) (e, a) (e, b) Source
Use a Prism
to work over part of a structure.
without :: APrism s t a b > APrism u v c d > Prism (Either s u) (Either t v) (Either a c) (Either b d) Source
below :: Traversable f => APrism' s a > Prism' (f s) (f a) Source
lift
a Prism
through a Traversable
functor, giving a Prism that matches only if all the elements of the container match the Prism
.
>>>
[Left 1, Right "foo", Left 4, Right "woot"]^..below _Right
[]
>>>
[Right "hail hydra!", Right "foo", Right "blah", Right "woot"]^..below _Right
[["hail hydra!","foo","blah","woot"]]
isn't :: APrism s t a b > s > Bool Source
Check to see if this Prism
doesn't match.
>>>
isn't _Left (Right 12)
True
>>>
isn't _Left (Left 12)
False
>>>
isn't _Empty []
False
matching :: APrism s t a b > s > Either t a Source
Retrieve the value targeted by a Prism
or return the
original value while allowing the type to change if it does
not match.
>>>
matching _Just (Just 12)
Right 12
>>>
matching _Just (Nothing :: Maybe Int) :: Either (Maybe Bool) Int
Left Nothing
Common Prisms
_Left :: Prism (Either a c) (Either b c) a b Source
This Prism
provides a Traversal
for tweaking the Left
half of an Either
:
>>>
over _Left (+1) (Left 2)
Left 3
>>>
over _Left (+1) (Right 2)
Right 2
>>>
Right 42 ^._Left :: String
""
>>>
Left "hello" ^._Left
"hello"
It also can be turned around to obtain the embedding into the Left
half of an Either
:
>>>
_Left # 5
Left 5
>>>
5^.re _Left
Left 5
_Right :: Prism (Either c a) (Either c b) a b Source
This Prism
provides a Traversal
for tweaking the Right
half of an Either
:
>>>
over _Right (+1) (Left 2)
Left 2
>>>
over _Right (+1) (Right 2)
Right 3
>>>
Right "hello" ^._Right
"hello"
>>>
Left "hello" ^._Right :: [Double]
[]
It also can be turned around to obtain the embedding into the Right
half of an Either
:
>>>
_Right # 5
Right 5
>>>
5^.re _Right
Right 5
_Just :: Prism (Maybe a) (Maybe b) a b Source
This Prism
provides a Traversal
for tweaking the target of the value of Just
in a Maybe
.
>>>
over _Just (+1) (Just 2)
Just 3
Unlike traverse
this is a Prism
, and so you can use it to inject as well:
>>>
_Just # 5
Just 5
>>>
5^.re _Just
Just 5
Interestingly,
m^?
_Just
≡ m
>>>
Just x ^? _Just
Just x
>>>
Nothing ^? _Just
Nothing
_Show :: (Read a, Show a) => Prism' String a Source
This is an improper prism for text formatting based on Read
and Show
.
This Prism
is "improper" in the sense that it normalizes the text formatting, but round tripping
is idempotent given sane 'Read'/'Show' instances.
>>>
_Show # 2
"2"
>>>
"EQ" ^? _Show :: Maybe Ordering
Just EQ
_Show
≡prism'
show
readMaybe
nearly :: a > (a > Bool) > Prism' a () Source
This Prism
compares for approximate equality with a given value and a predicate for testing.
To comply with the Prism
laws the arguments you supply to nearly a p
are somewhat constrained.
We assume p x
holds iff x ≡ a
. Under that assumption then this is a valid Prism
.
This is useful when working with a type where you can test equality for only a subset of its values, and the prism selects such a value.
Prismatic profunctors
class Profunctor p => Choice p where
The generalization of Costar
of Functor
that is strong with respect
to Either
.
Note: This is also a notion of strength, except with regards to another monoidal structure that we can choose to equip Hask with: the cocartesian coproduct.
Choice (>)  
Choice ReifiedFold  
Choice ReifiedGetter  
Monad m => Choice (Kleisli m)  
Comonad w => Choice (Cokleisli w) 

Applicative f => Choice (Star f)  
Traversable w => Choice (Costar w)  
ArrowChoice p => Choice (WrappedArrow p)  
Monoid r => Choice (Forget r)  
Choice (Tagged *)  
Choice (Indexed i)  
Choice (Market a b) 