Portability  Rank2Types 

Stability  provisional 
Maintainer  Edward Kmett <ekmett@gmail.com> 
Safe Haskell  SafeInfered 
A
is a generalization of Setter
a b c dfmap
from Functor
. It allows you to map into a
structure and change out the contents, but it isn't strong enough to allow you to
enumerate those contents. Starting with fmap ::
we monomorphize the type to obtain Functor
f => (c > d) > f c > f d(c > d) > a > b
and then decorate it with Identity
to obtain
typeSetter
a b c d = (c >Identity
d) > a >Identity
b
Every Traversal
is a valid Setter
, since Identity
is Applicative
.
Everything you can do with a Functor
, you can do with a Setter
. There
are combinators that generalize fmap
and (<$
).
 type Setter a b c d = forall f. Settable f => (c > f d) > a > f b
 class Applicative f => Settable f where
 run :: f a > a
 type Setting a b c d = (c > Mutator d) > a > Mutator b
 newtype Mutator a = Mutator {
 runMutator :: a
 sets :: ((c > d) > a > b) > Setter a b c d
 mapped :: Functor f => Setter (f a) (f b) a b
 adjust :: Setting a b c d > (c > d) > a > b
 mapOf :: Setting a b c d > (c > d) > a > b
 set :: Setting a b c d > d > a > b
 (.~) :: Setting a b c d > d > a > b
 (%~) :: Setting a b c d > (c > d) > a > b
 (+~) :: Num c => Setting a b c c > c > a > b
 (~) :: Num c => Setting a b c c > c > a > b
 (*~) :: Num c => Setting a b c c > c > a > b
 (//~) :: Fractional c => Setting a b c c > c > a > b
 (^~) :: (Num c, Integral e) => Setting a b c c > e > a > b
 (^^~) :: (Fractional c, Integral e) => Setting a b c c > e > a > b
 (**~) :: Floating c => Setting a b c c > c > a > b
 (~) :: Setting a b Bool Bool > Bool > a > b
 (&&~) :: Setting a b Bool Bool > Bool > a > b
 (<>~) :: Monoid c => Setting a b c c > c > a > b
 (<.~) :: Setting a b c d > d > a > (d, b)
 (.=) :: MonadState a m => Setting a a c d > d > m ()
 (%=) :: MonadState a m => Setting a a c d > (c > d) > m ()
 (+=) :: (MonadState a m, Num b) => SimpleSetting a b > b > m ()
 (=) :: (MonadState a m, Num b) => SimpleSetting a b > b > m ()
 (*=) :: (MonadState a m, Num b) => SimpleSetting a b > b > m ()
 (//=) :: (MonadState a m, Fractional b) => SimpleSetting a b > b > m ()
 (^=) :: (MonadState a m, Fractional b, Integral c) => SimpleSetting a b > c > m ()
 (^^=) :: (MonadState a m, Fractional b, Integral c) => SimpleSetting a b > c > m ()
 (**=) :: (MonadState a m, Floating b) => SimpleSetting a b > b > m ()
 (=) :: MonadState a m => SimpleSetting a Bool > Bool > m ()
 (&&=) :: MonadState a m => SimpleSetting a Bool > Bool > m ()
 (<>=) :: (MonadState a m, Monoid b) => SimpleSetting a b > b > m ()
 (<.=) :: MonadState a m => Setting a a c d > d > m d
 (<~) :: MonadState a m => Setting a a c d > m d > m ()
 whisper :: (MonadWriter b m, Monoid a) => Setting a b c d > d > m ()
 type SimpleSetter a b = Setter a a b b
 type SimpleSetting a b = Setting a a b b
Setters
type Setter a b c d = forall f. Settable f => (c > f d) > a > f bSource
The only Lens
like law that can apply to a Setter
l
is that
set l c (set l b a) = set l c a
You can't view
a Setter
in general, so the other two laws are irrelevant.
However, two functor laws apply to a Setter
:
These an be stated more directly:
You can compose a Setter
with a Lens
or a Traversal
using (.
) from the Prelude
and the result is always only a Setter
and nothing more.
class Applicative f => Settable f whereSource
Anything Settable must be isomorphic to the Identity Functor.
Consuming Setters
type Setting a b c d = (c > Mutator d) > a > Mutator bSource
Running a Setter instantiates it to a concrete type.
When consuming a setter, use this type.
Mutator
is just a renamed Identity
functor to give better error
messages when someone attempts to use a getter as a setter.
Mutator  

Building Setters
Common Setters
Functional Combinators
adjust :: Setting a b c d > (c > d) > a > bSource
Modify the target of a Lens
or all the targets of a Setter
or Traversal
with a function.
fmap
=adjust
mapped
fmapDefault
=adjust
traverse
Free Theorems:
Another way to view adjust
is to say that it transformers a Setter
into a
"semantic editor combinator".
adjust
::Setter
a b c d > (c > d) > a > b
mapOf :: Setting a b c d > (c > d) > a > bSource
Modify the target of a Lens
or all the targets of a Setter
or Traversal
with a function. This is an alias for adjust that is provided for consistency.
mapOf
=adjust
fmap
=mapOf
mapped
fmapDefault
=mapOf
traverse
Free Theorems:
mapOf :: Setter a b c d > (c > d) > a > b mapOf :: Iso a b c d > (c > d) > a > b mapOf :: Lens a b c d > (c > d) > a > b mapOf :: Traversal a b c d > (c > d) > a > b
(.~) :: Setting a b c d > d > a > bSource
Replace the target of a Lens
or all of the targets of a Setter
or Traversal
with a constant value.
This is an infix version of set
, provided for consistency with (.=
)
f<$
a =mapped
.~
f$
a
>>>
import Control.Lens
>>>
_1 .~ "hello" $ (42,"world")
("hello","world")
(.~) :: Setter a b c d > d > a > b (.~) :: Iso a b c d > d > a > b (.~) :: Lens a b c d > d > a > b (.~) :: Traversal a b c d > d > a > b
(%~) :: Setting a b c d > (c > d) > a > bSource
Modifies the target of a Lens
or all of the targets of a Setter
or
Traversal
with a user supplied function.
This is an infix version of adjust
fmap
f =mapped
%~
f
fmapDefault
f =traverse
%~
f
>>>
import Control.Lens
>>>
_2 %~ length $ (1,"hello")
(1,5)
(%~) :: Setter a b c d > (c > d) > a > b (%~) :: Iso a b c d > (c > d) > a > b (%~) :: Lens a b c d > (c > d) > a > b (%~) :: Traversal a b c d > (c > d) > a > b
(//~) :: Fractional c => Setting a b c c > c > a > bSource
(^^~) :: (Fractional c, Integral e) => Setting a b c c > e > a > bSource
(<>~) :: Monoid c => Setting a b c c > c > a > bSource
Modify the target of a monoidally valued by mappend
ing another value.
(<.~) :: Setting a b c d > d > a > (d, b)Source
Set with passthrough
This is mostly present for consistency, but may be useful for for chaining assignments
If you do not need a copy of the intermediate result, then using l .~ d
directly is a good idea.
State Combinators
(.=) :: MonadState a m => Setting a a c d > d > m ()Source
Replace the target of a Lens
or all of the targets of a Setter
or Traversal
in our monadic
state with a new value, irrespective of the old.
(.=) :: MonadState a m => Iso a a c d > d > m () (.=) :: MonadState a m => Lens a a c d > d > m () (.=) :: MonadState a m => Traversal a a c d > d > m () (.=) :: MonadState a m => Setter a a c d > d > m ()
(%=) :: MonadState a m => Setting a a c d > (c > d) > m ()Source
Map over the target of a Lens
or all of the targets of a Setter
or Traversal
in our monadic state.
(%=) :: MonadState a m => Iso a a c d > (c > d) > m () (%=) :: MonadState a m => Lens a a c d > (c > d) > m () (%=) :: MonadState a m => Traversal a a c d > (c > d) > m () (%=) :: MonadState a m => Setter a a c d > (c > d) > m ()
(+=) :: (MonadState a m, Num b) => SimpleSetting a b > b > m ()Source
(=) :: (MonadState a m, Num b) => SimpleSetting a b > b > m ()Source
(*=) :: (MonadState a m, Num b) => SimpleSetting a b > b > m ()Source
(//=) :: (MonadState a m, Fractional b) => SimpleSetting a b > b > m ()Source
(^=) :: (MonadState a m, Fractional b, Integral c) => SimpleSetting a b > c > m ()Source
(^^=) :: (MonadState a m, Fractional b, Integral c) => SimpleSetting a b > c > m ()Source
(**=) :: (MonadState a m, Floating b) => SimpleSetting a b > b > m ()Source
(=) :: MonadState a m => SimpleSetting a Bool > Bool > m ()Source
(&&=) :: MonadState a m => SimpleSetting a Bool > Bool > m ()Source
(<>=) :: (MonadState a m, Monoid b) => SimpleSetting a b > b > m ()Source
(<.=) :: MonadState a m => Setting a a c d > d > m dSource
Set with passthrough
This is useful for chaining assignment
do x < _2 <.= (an expensive expression)
If you do not need a copy of the intermediate result, then using l .= d
will avoid unused binding warnings
(<~) :: MonadState a m => Setting a a c d > m d > m ()Source
Run a monadic action, and set all of the targets of a Lens
, Setter
or Traversal
to its result.
(<~) :: MonadState a m => Iso a a c d > m d > m () (<~) :: MonadState a m => Lens a a c d > m d > m () (<~) :: MonadState a m => Traversal a a c d > m d > m () (<~) :: MonadState a m => Setter a a c d > m d > m ()
As a reasonable mnemonic, this lets you store the result of a monadic action in a lens rather than in a local variable.
do foo < bar ...
will store the result in a variable, while
do foo <~ bar ...
MonadWriter
whisper :: (MonadWriter b m, Monoid a) => Setting a b c d > d > m ()Source
Tell a part of a value to a MonadWriter
, filling in the rest from mempty
whisper l d = tell (set l d mempty)
Simplicity
type SimpleSetter a b = Setter a a b bSource
'SimpleSetter' = 'Simple' 'Setter'
type SimpleSetting a b = Setting a a b bSource
'SimpleSetting' m = 'Simple' 'Setting'