Portability | Rank2Types |
---|---|
Stability | provisional |
Maintainer | Edward Kmett <ekmett@gmail.com> |
Safe Haskell | Safe-Infered |
A
is just any function Getter
a c(a -> c)
, which we've flipped into continuation
passing style, (c -> r) -> a -> r
and decorated with Accessor
to obtain
typeGetting
r a c = (c ->Accessor
r c) -> a ->Accessor
r a
If we restrict access to knowledge about the type r
and can work for any d and b, we could get:
typeGetter
a c = forall r.Getting
r a c
But we actually hide the use of Accessor
behind a class Gettable
to error messages from
type class resolution rather than at unification time, where they are much uglier.
typeGetter
a c = forall f.Gettable
f => (c -> f c) -> a -> f a
Everything you can do with a function, you can do with a Getter
, but note that because of the
continuation passing style (.
) composes them in the opposite order.
Since it is only a function, every Getter
obviously only retrieves a single value for a given
input.
- type Getter a c = forall f. Gettable f => (c -> f c) -> a -> f a
- type Getting r a c = (c -> Accessor r c) -> a -> Accessor r a
- class Functor f => Gettable f where
- coerce :: f a -> f b
- newtype Accessor r a = Accessor {
- runAccessor :: r
- to :: (a -> c) -> Getter a c
- (^.) :: a -> Getting c a c -> c
- (^$) :: Getting c a c -> a -> c
- view :: Getting c a c -> a -> c
- views :: Getting m a c -> (c -> m) -> a -> m
- use :: MonadState a m => Getting c a c -> m c
- uses :: MonadState a m => Getting e a c -> (c -> e) -> m e
- query :: MonadReader a m => Getting c a c -> m c
- queries :: MonadReader a m => Getting e a c -> (c -> e) -> m e
Getters
type Getter a c = forall f. Gettable f => (c -> f c) -> a -> f aSource
A Getter
describes how to retrieve a single value in a way that can be composed with
other lens-like constructions.
Unlike a Lens
a Getter
is read-only. Since a Getter
cannot be used to write back
there are no lens laws that can be applied to it. In fact, it is isomorphic to an arbitrary function from (a -> c)
.
Moreover, a Getter
can be used directly as a Fold
, since it just ignores the Applicative
.
type Getting r a c = (c -> Accessor r c) -> a -> Accessor r aSource
Most Getter
combinators are able to be used with both a Getter
or a Fold
in
limited situations, to do so, they need to be monomorphic in what we are going to
extract with Const
. To be compatible with Lens
, Traversal
and Iso
we also
restricted choices of the irrelevant b and d parameters.
If a function accepts a
, then when Getting
r a cr
is a Monoid
, then you can pass a Fold
(or Traversal
), otherwise you can only pass this a
Getter
or Lens
.
Used instead of Const
to report
No instance of (Settable
Accessor
)
when the user attempts to misuse a Setter
as a Getter
,
rather than a monolithic unification error.
Accessor | |
|
Building Getters
Combinators for Getters and Folds
(^.) :: a -> Getting c a c -> cSource
View the value pointed to by a Getter
or Lens
or the result of folding over
all the results of a Fold
or Traversal
that points at a monoidal values.
This is the same operation as view
with the arguments flipped.
The fixity and semantics are such that subsequent field accesses can be
performed with (.
)
>>>
:m + Data.Complex Control.Lens
>>>
((0, 1 :+ 2), 3)^._1._2.to magnitude
2.23606797749979
(^.) :: a ->Getter
a c -> c (^.) ::Monoid
m => a ->Fold
a m -> m (^.) :: a ->Simple
Iso
a c -> c (^.) :: a ->Simple
Lens
a c -> c (^.) ::Monoid
m => a ->Simple
Traversal
a m -> m
(^$) :: Getting c a c -> a -> cSource
View the value pointed to by a Getter
, Iso
or Lens
or the result of folding over
all the results of a Fold
or Traversal
that points at a monoidal values.
This is the same operation as view
, only infix.
>>>
import Control.Lens
>>>
_2 ^$ (1, "hello")
"hello"
(^$) ::Getter
a c -> a -> c (^$) ::Monoid
m =>Fold
a m -> a -> m (^$) ::Simple
Iso
a c -> a -> c (^$) ::Simple
Lens
a c -> a -> c (^$) ::Monoid
m =>Simple
Traversal
a m -> a -> m
view :: Getting c a c -> a -> cSource
View the value pointed to by a Getter
, Iso
or Lens
or the result of folding over
all the results of a Fold
or Traversal
that points at a monoidal values.
It may be useful to think of view
as having these more restrictive signatures:
view
.to
=id
>>>
import Control.Lens
>>>
view _2 (1,"hello")
"hello"
view ::Getter
a c -> a -> c view ::Monoid
m =>Fold
a m -> a -> m view ::Simple
Iso
a c -> a -> c view ::Simple
Lens
a c -> a -> c view ::Monoid
m =>Simple
Traversal
a m -> a -> m
views :: Getting m a c -> (c -> m) -> a -> mSource
View the value of a Getter
, Iso
, Lens
or the result of folding over the
result of mapping the targets of a Fold
or Traversal
.
It may be useful to think of views
as having these more restrictive signatures:
>>>
import Control.Lens
>>>
views _2 length (1,"hello")
5
views ::Getter
a c -> (c -> d) -> a -> d views ::Monoid
m =>Fold
a c -> (c -> m) -> a -> m views ::Simple
Iso
a c -> (c -> d) -> a -> d views ::Simple
Lens
a c -> (c -> d) -> a -> d views ::Monoid
m =>Simple
Traversal
a c -> (c -> m) -> a -> m
use :: MonadState a m => Getting c a c -> m cSource
Use the target of a Lens
, Iso
, or Getter
in the current state, or use a
summary of a Fold
or Traversal
that points to a monoidal value.
use ::MonadState
a m =>Getter
a c -> m c use :: (MonadState
a m,Monoid
r) =>Fold
a r -> m r use ::MonadState
a m =>Simple
Iso
a c -> m c use ::MonadState
a m =>Simple
Lens
a c -> m c use :: (MonadState
a m,Monoid
r) =>Simple
Traversal
a r -> m r
uses :: MonadState a m => Getting e a c -> (c -> e) -> m eSource
Use the target of a Lens
, Iso
or Getter
in the current state, or use a
summary of a Fold
or Traversal
that points to a monoidal value.
uses ::MonadState
a m =>Getter
a c -> (c -> e) -> m e uses :: (MonadState
a m,Monoid
r) =>Fold
a c -> (c -> r) -> m r uses ::MonadState
a m =>Simple
Lens
a c -> (c -> e) -> m e uses ::MonadState
a m =>Simple
Iso
a c -> (c -> e) -> m e uses :: (MonadState
a m,Monoid
r) =>Simple
Traversal
a c -> (c -> r) -> m r
query :: MonadReader a m => Getting c a c -> m cSource
Query the target of a Lens
, Iso
or Getter
in the current state, or use a
summary of a Fold
or Traversal
that points to a monoidal value.
query ::MonadReader
a m =>Getter
a c -> m c query :: (MonadReader
a m,Monoid
c) =>Fold
a c -> m c query ::MonadReader
a m =>Simple
Iso
a c -> m c query ::MonadReader
a m =>Simple
Lens
a c -> m c query :: (MonadReader
a m,Monoid
c) =>Simple
Traversal
a c -> m c
queries :: MonadReader a m => Getting e a c -> (c -> e) -> m eSource
Use the target of a Lens
, Iso
or Getter
in the current state, or use a
summary of a Fold
or Traversal
that points to a monoidal value.
queries ::MonadReader
a m =>Getter
a c -> (c -> e) -> m e queries :: (MonadReader
a m,Monoid
c) =>Fold
a c -> (c -> e) -> m e queries ::MonadReader
a m =>Simple
Iso
a c -> (c -> e) -> m e queries ::MonadReader
a m =>Simple
Lens
a c -> (c -> e) -> m e queries :: (MonadReader
a m,Monoid
c) =>Simple
Traversal
a c -> (c -> e) -> m e