lens-4.12.3: Lenses, Folds and Traversals

Copyright(C) 2012-15 Edward Kmett
LicenseBSD-style (see the file LICENSE)
MaintainerEdward Kmett <ekmett@gmail.com>
Stabilityprovisional
Portabilitynon-portable
Safe HaskellSafe-Inferred
LanguageHaskell98

Control.Lens.Review

Contents

Description

A Review is a type-restricted form of a Prism that can only be used for writing back via re, review, reuse.

Synopsis

Reviewing

type Review t b = forall p f. (Choice p, Bifunctor p, Settable f) => Optic' p f t b Source

This is a limited form of a Prism that can only be used for re operations.

Like with a Getter, there are no laws to state for a Review.

You can generate a Review by using unto. You can also use any Prism or Iso directly as a Review.

type AReview t b = Optic' Tagged Identity t b Source

If you see this in a signature for a function, the function is expecting a Review (in practice, this usually means a Prism).

unto :: (Profunctor p, Bifunctor p, Functor f) => (b -> t) -> Optic p f s t a b Source

An analogue of to for review.

unto :: (b -> t) -> Review' t b
unto = un . to

un :: (Profunctor p, Bifunctor p, Functor f) => Getting a s a -> Optic' p f a s Source

Turn a Getter around to get a Review

un = unto . view
unto = un . to
>>> un (to length) # [1,2,3]
3

re :: Contravariant f => AReview t b -> LensLike' f b t Source

Turn a Prism or Iso around to build a Getter.

If you have an Iso, from is a more powerful version of this function that will return an Iso instead of a mere Getter.

>>> 5 ^.re _Left
Left 5
>>> 6 ^.re (_Left.unto succ)
Left 7
reviewview  . re
reviewsviews . re
reuseuse   . re
reusesuses  . re
re :: Prism s t a b -> Getter b t
re :: Iso s t a b   -> Getter b t

review :: MonadReader b m => AReview t b -> m t Source

This can be used to turn an Iso or Prism around and view a value (or the current environment) through it the other way.

reviewview . re
review . untoid
>>> review _Left "mustard"
Left "mustard"
>>> review (unto succ) 5
6

Usually review is used in the (->) Monad with a Prism or Iso, in which case it may be useful to think of it as having one of these more restricted type signatures:

review :: Iso' s a   -> a -> s
review :: Prism' s a -> a -> s

However, when working with a Monad transformer stack, it is sometimes useful to be able to review the current environment, in which case it may be beneficial to think of it as having one of these slightly more liberal type signatures:

review :: MonadReader a m => Iso' s a   -> m s
review :: MonadReader a m => Prism' s a -> m s

reviews :: MonadReader b m => AReview t b -> (t -> r) -> m r Source

This can be used to turn an Iso or Prism around and view a value (or the current environment) through it the other way, applying a function.

reviewsviews . re
reviews (unto f) g ≡ g . f
>>> reviews _Left isRight "mustard"
False
>>> reviews (unto succ) (*2) 3
8

Usually this function is used in the (->) Monad with a Prism or Iso, in which case it may be useful to think of it as having one of these more restricted type signatures:

reviews :: Iso' s a   -> (s -> r) -> a -> r
reviews :: Prism' s a -> (s -> r) -> a -> r

However, when working with a Monad transformer stack, it is sometimes useful to be able to review the current environment, in which case it may be beneficial to think of it as having one of these slightly more liberal type signatures:

reviews :: MonadReader a m => Iso' s a   -> (s -> r) -> m r
reviews :: MonadReader a m => Prism' s a -> (s -> r) -> m r

reuse :: MonadState b m => AReview t b -> m t Source

This can be used to turn an Iso or Prism around and use a value (or the current environment) through it the other way.

reuseuse . re
reuse . untogets
>>> evalState (reuse _Left) 5
Left 5
>>> evalState (reuse (unto succ)) 5
6
reuse :: MonadState a m => Prism' s a -> m s
reuse :: MonadState a m => Iso' s a   -> m s

reuses :: MonadState b m => AReview t b -> (t -> r) -> m r Source

This can be used to turn an Iso or Prism around and use the current state through it the other way, applying a function.

reusesuses . re
reuses (unto f) g ≡ gets (g . f)
>>> evalState (reuses _Left isLeft) (5 :: Int)
True
reuses :: MonadState a m => Prism' s a -> (s -> r) -> m r
reuses :: MonadState a m => Iso' s a   -> (s -> r) -> m r

(#) :: AReview t b -> b -> t infixr 8 Source

An infix alias for review.

unto f # x ≡ f x
l # x ≡ x ^. re l

This is commonly used when using a Prism as a smart constructor.

>>> _Left # 4
Left 4

But it can be used for any Prism

>>> base 16 # 123
"7b"
(#) :: Iso'      s a -> a -> s
(#) :: Prism'    s a -> a -> s
(#) :: Review    s a -> a -> s
(#) :: Equality' s a -> a -> s

class Bifunctor p where

Minimal definition either bimap or first and second

Formally, the class Bifunctor represents a bifunctor from Hask -> Hask.

Intuitively it is a bifunctor where both the first and second arguments are covariant.

You can define a Bifunctor by either defining bimap or by defining both first and second.

If you supply bimap, you should ensure that:

bimap id idid

If you supply first and second, ensure:

first idid
second idid

If you supply both, you should also ensure:

bimap f g ≡ first f . second g

These ensure by parametricity:

bimap  (f . g) (h . i) ≡ bimap f h . bimap g i
first  (f . g) ≡ first  f . first  g
second (f . g) ≡ second f . second g

Minimal complete definition

bimap | first, second

Methods

bimap :: (a -> b) -> (c -> d) -> p a c -> p b d

Map over both arguments at the same time.

bimap f g ≡ first f . second g

retagged :: (Profunctor p, Bifunctor p) => p a b -> p s b Source

This is a profunctor used internally to implement Review

It plays a role similar to that of Accessor or Const do for Control.Lens.Getter

class (Profunctor p, Bifunctor p) => Reviewable p Source

This class is provided mostly for backwards compatibility with lens 3.8, but it can also shorten type signatures.

Instances