lens-3.0.6: Lenses, Folds and Traversals

Portability Rank2Types provisional Edward Kmett Safe-Inferred

Control.Lens.Setter

Description

A `Setter a b c d` is a generalization of `fmap` 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 :: Functor f => (c -> d) -> f c -> f d` we monomorphize the type to obtain `(c -> d) -> a -> b` and then decorate it with `Identity` to obtain

`type `Setter` 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 (`<\$`).

Synopsis

# 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`:

``` `over` l `id` ≡ `id`
`over` l f `.` `over` l g ≡ `over` l (f `.` g)
```

These an be stated more directly:

``` l `pure` ≡ `pure`
l f . `untainted` . l g ≡ l (f . `untainted` . g)
```

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.

# Building Setters

sets :: ((c -> d) -> a -> b) -> Setter a b c dSource

Build a Setter from a map-like function.

Your supplied function `f` is required to satisfy:

``` f `id` ≡ `id`
f g `.` f h ≡ f (g `.` h)
```

Equational reasoning:

``` `sets` `.` `over` ≡ `id`
`over` `.` `sets` ≡ `id`
```

Another way to view `sets` is that it takes a "semantic editor combinator" and transforms it into a `Setter`.

# Common Setters

mapped :: Functor f => Setter (f a) (f b) a bSource

This setter can be used to map over all of the values in a `Functor`.

``` `fmap` ≡ `over` `mapped`
`fmapDefault` ≡ `over` `traverse`
(`<\$`) ≡ `set` `mapped`
```
````>>> ````over mapped (+1) [1,2,3]
```[2,3,4]
```
````>>> ````set mapped () [1,2,3]
```[(),(),()]
```
````>>> ````mapped.mapped %~ (+1) \$ [[1,2],[3]]
```[[2,3],[4]]
```
````>>> ````over (mapped._2) length [("hello","world"),("leaders","!!!")]
```

# Functional Combinators

over :: 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` ≡ `over` `mapped`
`fmapDefault` ≡ `over` `traverse`
`sets` `.` `over` ≡ `id`
`over` `.` `sets` ≡ `id`
```
````>>> ````over mapped (*10) [1,2,3]
```[10,20,30]
```
````>>> ````over _1 show (10,20)
```("10",20)
```

Another way to view `over` is to say that it transformers a `Setter` into a "semantic editor combinator".

``over` :: `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 `over` that is provided for consistency.

``` `mapOf` ≡ `over`
`fmap` ≡ `mapOf` `mapped`
`fmapDefault` ≡ `mapOf` `traverse`
`sets` `.` `mapOf` ≡ `id`
`mapOf` `.` `sets` ≡ `id`
```
````>>> ````mapOf mapped (+1) [1,2,3,4]
```[2,3,4,5]
```
````>>> ````mapOf _1 (+1) (1,2)
```(2,2)
```
````>>> ````mapOf both (+1) (1,2)
```(2,3)
```
``` `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
```

set :: 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.

`(`<\$`) ≡ `set` `mapped``
````>>> ````set _2 "hello" (1,())
```(1,"hello")
```
````>>> ````set mapped () [1,2,3,4]
```[(),(),(),()]
```

Note: Attempting to `set` a `Fold` or `Getter` will fail at compile time with an relatively nice error message.

``` `set` :: `Setter` a b c d    -> d -> a -> b
`set` :: `Iso` a b c d       -> d -> a -> b
`set` :: `Lens` a b c d      -> d -> a -> b
`set` :: `Traversal` a b c d -> 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`
````>>> ````_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 `over`

``` `fmap` f ≡ `mapped` `%~` f
`fmapDefault` f ≡ `traverse` `%~` f
```
````>>> ````_2 %~ length \$ (1,"hello")
```(1,5)
```
````>>> ````traverse %~ (+1) \$ [1,2,3]
```[2,3,4]
```
````>>> ````_2 %~ (+1) \$ (3,4)
```(3,5)
```
````>>> ````traverse.traverse %~ length \$ [["hello","world"],["!!!"]]
```[[5,5],[3]]
```
``` (`%~`) :: `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
```

(+~) :: Num c => Setting a b c c -> c -> a -> bSource

Increment the target(s) of a numerically valued `Lens`, `Setter` or `Traversal`

````>>> ````_1 +~ 1 \$ (1,2)
```(2,2)
```
````>>> ````both +~ 2 \$ (5,6)
```(7,8)
```
``` (`+~`) :: Num b => `Simple` `Setter` a b -> b -> a -> a
(`+~`) :: Num b => `Simple` `Iso` a b -> b -> a -> a
(`+~`) :: Num b => `Simple` `Lens` a b -> b -> a -> a
(`+~`) :: Num b => `Simple` `Traversal` a b -> b -> a -> a
```

(-~) :: Num c => Setting a b c c -> c -> a -> bSource

Decrement the target(s) of a numerically valued `Lens`, `Iso`, `Setter` or `Traversal`

````>>> ````_1 -~ 2 \$ (1,2)
```(-1,2)
```
````>>> ````mapped.mapped -~ 1 \$ [[4,5],[6,7]]
```[[3,4],[5,6]]
```
``` (`-~`) :: `Num` b => `Simple` `Setter` a b -> b -> a -> a
(`-~`) :: `Num` b => `Simple` `Iso` a b -> b -> a -> a
(`-~`) :: `Num` b => `Simple` `Lens` a b -> b -> a -> a
(`-~`) :: `Num` b => `Simple` `Traversal` a b -> b -> a -> a
```

(*~) :: Num c => Setting a b c c -> c -> a -> bSource

Multiply the target(s) of a numerically valued `Lens`, `Iso`, `Setter` or `Traversal`

````>>> ````_2 *~ 4 \$ (1,2)
```(1,8)
```
````>>> ````mapped *~ 2 \$ Just 24
```Just 48
```
``` (`*~`) :: `Num` b => `Simple` `Setter` a b -> b -> a -> a
(`*~`) :: `Num` b => `Simple` `Iso` a b -> b -> a -> a
(`*~`) :: `Num` b => `Simple` `Lens` a b -> b -> a -> a
(`*~`) :: `Num` b => `Simple` `Traversal` a b -> b -> a -> a
```

(//~) :: Fractional c => Setting a b c c -> c -> a -> bSource

Divide the target(s) of a numerically valued `Lens`, `Iso`, `Setter` or `Traversal`

````>>> ````_2 //~ 2 \$ ("Hawaii",10)
```("Hawaii",5.0)
```
``` (`//~`) :: `Fractional` b => `Simple` `Setter` a b -> b -> a -> a
(`//~`) :: `Fractional` b => `Simple` `Iso` a b -> b -> a -> a
(`//~`) :: `Fractional` b => `Simple` `Lens` a b -> b -> a -> a
(`//~`) :: `Fractional` b => `Simple` `Traversal` a b -> b -> a -> a
```

(^~) :: (Num c, Integral e) => Setting a b c c -> e -> a -> bSource

Raise the target(s) of a numerically valued `Lens`, `Setter` or `Traversal` to a non-negative integral power

````>>> ````_2 ^~ 2 \$ (1,3)
```(1,9)
```
``` (`^~`) :: (`Num` b, `Integral` c) => `Simple` `Setter` a b -> c -> a -> a
(`^~`) :: (`Num` b, `Integral` c) => `Simple` `Iso` a b -> c -> a -> a
(`^~`) :: (`Num` b, `Integral` c) => `Simple` `Lens` a b -> c -> a -> a
(`^~`) :: (`Num` b, `Integral` c) => `Simple` `Traversal` a b -> c -> a -> a
```

(^^~) :: (Fractional c, Integral e) => Setting a b c c -> e -> a -> bSource

Raise the target(s) of a fractionally valued `Lens`, `Setter` or `Traversal` to an integral power

````>>> ````_2 ^^~ (-1) \$ (1,2)
```(1,0.5)
```
``` (`^^~`) :: (`Fractional` b, `Integral` c) => `Simple` `Setter` a b -> c -> a -> a
(`^^~`) :: (`Fractional` b, `Integral` c) => `Simple` `Iso` a b -> c -> a -> a
(`^^~`) :: (`Fractional` b, `Integral` c) => `Simple` `Lens` a b -> c -> a -> a
(`^^~`) :: (`Fractional` b, `Integral` c) => `Simple` `Traversal` a b -> c -> a -> a
```

(**~) :: Floating c => Setting a b c c -> c -> a -> bSource

Raise the target(s) of a floating-point valued `Lens`, `Setter` or `Traversal` to an arbitrary power.

````>>> ````_2 **~ pi \$ (1,3)
```(1,31.54428070019754)
```
``` (`**~`) :: `Floating` b => `Simple` `Setter` a b -> b -> a -> a
(`**~`) :: `Floating` b => `Simple` `Iso` a b -> b -> a -> a
(`**~`) :: `Floating` b => `Simple` `Lens` a b -> b -> a -> a
(`**~`) :: `Floating` b => `Simple` `Traversal` a b -> b -> a -> a
```

(||~) :: Setting a b Bool Bool -> Bool -> a -> bSource

Logically `||` the target(s) of a `Bool`-valued `Lens` or `Setter`

````>>> ````both ||~ True \$ (False,True)
```(True,True)
```
````>>> ````both ||~ False \$ (False,True)
```(False,True)
```
``` (`||~`) :: `Simple` `Setter` a `Bool` -> `Bool` -> a -> a
(`||~`) :: `Simple` `Iso` a `Bool` -> `Bool` -> a -> a
(`||~`) :: `Simple` `Lens` a `Bool` -> `Bool` -> a -> a
(`||~`) :: `Simple` `Traversal` a `Bool` -> `Bool` -> a -> a
```

(&&~) :: Setting a b Bool Bool -> Bool -> a -> bSource

Logically `&&` the target(s) of a `Bool`-valued `Lens` or `Setter`

````>>> ````both &&~ True \$ (False, True)
```(False,True)
```
````>>> ````both &&~ False \$ (False, True)
```(False,False)
```
``` (`&&~`) :: `Simple` `Setter` a `Bool` -> `Bool` -> a -> a
(`&&~`) :: `Simple` `Iso` a `Bool` -> `Bool` -> a -> a
(`&&~`) :: `Simple` `Lens` a `Bool` -> `Bool` -> a -> a
(`&&~`) :: `Simple` `Traversal` a `Bool` -> `Bool` -> a -> a
```

(<.~) :: Setting a b c d -> d -> a -> (d, b)Source

Set with pass-through

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.

````>>> ````_3 <.~ "world" \$ ("good","morning","vietnam")
```("world",("good","morning","world"))
```
````>>> ````import Data.Map as Map
````>>> ````_2.at "hello" <.~ Just "world" \$ (42,Map.fromList [("goodnight","gracie")])
```(Just "world",(42,fromList [("goodnight","gracie"),("hello","world")]))
```
``` (`<.~`) :: `Setter` a b c d    -> d -> a -> (d, b)
(`<.~`) :: `Iso` a b c d       -> d -> a -> (d, b)
(`<.~`) :: `Lens` a b c d      -> d -> a -> (d, b)
(`<.~`) :: `Traversal` a b c d -> d -> a -> (d, b)
```

(?~) :: Setting a b c (Maybe d) -> d -> a -> bSource

Set the target of a `Lens`, `Traversal` or `Setter` to `Just` a value.

`l `?~` d ≡ `set` l (`Just` d)`
``` (`?~`) :: `Setter` a b c (`Maybe` d)    -> d -> a -> b
(`?~`) :: `Iso` a b c (`Maybe` d)       -> d -> a -> b
(`?~`) :: `Lens` a b c (`Maybe` d)      -> d -> a -> b
(`?~`) :: `Traversal` a b c (`Maybe` d) -> d -> a -> b
```

(<?~) :: Setting a b c (Maybe d) -> d -> a -> (d, b)Source

Set to `Just` a value with pass-through

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.

````>>> ````import Data.Map as Map
````>>> ````_2.at "hello" <?~ "world" \$ (42,Map.fromList [("goodnight","gracie")])
```("world",(42,fromList [("goodnight","gracie"),("hello","world")]))
```
``` (`<?~`) :: `Setter` a b c d    -> (`Maybe` d) -> a -> (d, b)
(`<?~`) :: `Iso` a b c (`Maybe` d)       -> d -> a -> (d, b)
(`<?~`) :: `Lens` a b c (`Maybe` d)      -> d -> a -> (d, b)
(`<?~`) :: `Traversal` a b c (`Maybe` d) -> d -> a -> (d, b)
```

# State Combinators

assign :: 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.

This is an alias for (`.=`).

``` `assign` :: `MonadState` a m => `Simple` `Iso` a b       -> b -> m ()
`assign` :: `MonadState` a m => `Simple` `Lens` a b      -> b -> m ()
`assign` :: `MonadState` a m => `Simple` `Traversal` a b -> b -> m ()
`assign` :: `MonadState` a m => `Simple` `Setter` a b    -> b -> m ()
```

(.=) :: 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.

This is an infix version of `assign`.

``` (`.=`) :: `MonadState` a m => `Simple` `Iso` a b       -> b -> m ()
(`.=`) :: `MonadState` a m => `Simple` `Lens` a b      -> b -> m ()
(`.=`) :: `MonadState` a m => `Simple` `Traversal` a b -> b -> m ()
(`.=`) :: `MonadState` a m => `Simple` `Setter` a b    -> b -> 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 => `Simple` `Iso` a b       -> (b -> b) -> m ()
(`%=`) :: `MonadState` a m => `Simple` `Lens` a b      -> (b -> b) -> m ()
(`%=`) :: `MonadState` a m => `Simple` `Traversal` a b -> (b -> b) -> m ()
(`%=`) :: `MonadState` a m => `Simple` `Setter` a b    -> (b -> b) -> m ()
```

(+=) :: (MonadState a m, Num b) => SimpleSetting a b -> b -> m ()Source

Modify the target(s) of a `Simple` `Lens`, `Iso`, `Setter` or `Traversal` by adding a value

Example:

``` fresh :: MonadState Int m => m Int
fresh = do
`id` `+=` 1
`use` `id`
```
``` (`+=`) :: (`MonadState` a m, `Num` b) => `Simple` `Setter` a b -> b -> m ()
(`+=`) :: (`MonadState` a m, `Num` b) => `Simple` `Iso` a b -> b -> m ()
(`+=`) :: (`MonadState` a m, `Num` b) => `Simple` `Lens` a b -> b -> m ()
(`+=`) :: (`MonadState` a m, `Num` b) => `Simple` `Traversal` a b -> b -> m ()
```

(-=) :: (MonadState a m, Num b) => SimpleSetting a b -> b -> m ()Source

Modify the target(s) of a `Simple` `Lens`, `Iso`, `Setter` or `Traversal` by subtracting a value

``` (`-=`) :: (`MonadState` a m, `Num` b) => `Simple` `Setter` a b -> b -> m ()
(`-=`) :: (`MonadState` a m, `Num` b) => `Simple` `Iso` a b -> b -> m ()
(`-=`) :: (`MonadState` a m, `Num` b) => `Simple` `Lens` a b -> b -> m ()
(`-=`) :: (`MonadState` a m, `Num` b) => `Simple` `Traversal` a b -> b -> m ()
```

(*=) :: (MonadState a m, Num b) => SimpleSetting a b -> b -> m ()Source

Modify the target(s) of a `Simple` `Lens`, `Iso`, `Setter` or `Traversal` by multiplying by value.

``` (`*=`) :: (`MonadState` a m, `Num` b) => `Simple` `Setter` a b -> b -> m ()
(`*=`) :: (`MonadState` a m, `Num` b) => `Simple` `Iso` a b -> b -> m ()
(`*=`) :: (`MonadState` a m, `Num` b) => `Simple` `Lens` a b -> b -> m ()
(`*=`) :: (`MonadState` a m, `Num` b) => `Simple` `Traversal` a b -> b -> m ()
```

(//=) :: (MonadState a m, Fractional b) => SimpleSetting a b -> b -> m ()Source

Modify the target(s) of a `Simple` `Lens`, `Iso`, `Setter` or `Traversal` by dividing by a value.

``` (`//=`) :: (`MonadState` a m, `Fractional` b) => `Simple` `Setter` a b -> b -> m ()
(`//=`) :: (`MonadState` a m, `Fractional` b) => `Simple` `Iso` a b -> b -> m ()
(`//=`) :: (`MonadState` a m, `Fractional` b) => `Simple` `Lens` a b -> b -> m ()
(`//=`) :: (`MonadState` a m, `Fractional` b) => `Simple` `Traversal` a b -> b -> m ()
```

(^=) :: (MonadState a m, Num b, Integral c) => SimpleSetting a b -> c -> m ()Source

Raise the target(s) of a numerically valued `Lens`, `Setter` or `Traversal` to a non-negative integral power.

``` (`^=`) ::  (`MonadState` a m, `Num` b, `Integral` c) => `Simple` `Setter` a b -> c -> m ()
(`^=`) ::  (`MonadState` a m, `Num` b, `Integral` c) => `Simple` `Iso` a b -> c -> m ()
(`^=`) ::  (`MonadState` a m, `Num` b, `Integral` c) => `Simple` `Lens` a b -> c -> m ()
(`^=`) ::  (`MonadState` a m, `Num` b, `Integral` c) => `Simple` `Traversal` a b -> c -> m ()
```

(^^=) :: (MonadState a m, Fractional b, Integral c) => SimpleSetting a b -> c -> m ()Source

Raise the target(s) of a numerically valued `Lens`, `Setter` or `Traversal` to an integral power.

``` (`^^=`) ::  (`MonadState` a m, `Fractional` b, `Integral` c) => `Simple` `Setter` a b -> c -> m ()
(`^^=`) ::  (`MonadState` a m, `Fractional` b, `Integral` c) => `Simple` `Iso` a b -> c -> m ()
(`^^=`) ::  (`MonadState` a m, `Fractional` b, `Integral` c) => `Simple` `Lens` a b -> c -> m ()
(`^^=`) ::  (`MonadState` a m, `Fractional` b, `Integral` c) => `Simple` `Traversal` a b -> c -> m ()
```

(**=) :: (MonadState a m, Floating b) => SimpleSetting a b -> b -> m ()Source

Raise the target(s) of a numerically valued `Lens`, `Setter` or `Traversal` to an arbitrary power

``` (`**=`) ::  (`MonadState` a m, `Floating` b) => `Simple` `Setter` a b -> b -> m ()
(`**=`) ::  (`MonadState` a m, `Floating` b) => `Simple` `Iso` a b -> b -> m ()
(`**=`) ::  (`MonadState` a m, `Floating` b) => `Simple` `Lens` a b -> b -> m ()
(`**=`) ::  (`MonadState` a m, `Floating` b) => `Simple` `Traversal` a b -> b -> m ()
```

(||=) :: MonadState a m => SimpleSetting a Bool -> Bool -> m ()Source

Modify the target(s) of a `Simple` `Lens`, 'Iso, `Setter` or `Traversal` by taking their logical `||` with a value

``` (`||=`) :: `MonadState` a m => `Simple` `Setter` a `Bool` -> `Bool` -> m ()
(`||=`) :: `MonadState` a m => `Simple` `Iso` a `Bool` -> `Bool` -> m ()
(`||=`) :: `MonadState` a m => `Simple` `Lens` a `Bool` -> `Bool` -> m ()
(`||=`) :: `MonadState` a m => `Simple` `Traversal` a `Bool` -> `Bool` -> m ()
```

(&&=) :: MonadState a m => SimpleSetting a Bool -> Bool -> m ()Source

Modify the target(s) of a `Simple` `Lens`, `Iso`, `Setter` or `Traversal` by taking their logical `&&` with a value

``` (`&&=`) :: `MonadState` a m => `Simple` `Setter` a `Bool` -> `Bool` -> m ()
(`&&=`) :: `MonadState` a m => `Simple` `Iso` a `Bool` -> `Bool` -> m ()
(`&&=`) :: `MonadState` a m => `Simple` `Lens` a `Bool` -> `Bool` -> m ()
(`&&=`) :: `MonadState` a m => `Simple` `Traversal` a `Bool` -> `Bool` -> m ()
```

(<.=) :: MonadState a m => Setting a a c d -> d -> m dSource

Set with pass-through

This is useful for chaining assignment without round-tripping through your monad stack.

`do x <- `_2` <.= ninety_nine_bottles_of_beer_on_the_wall`

If you do not need a copy of the intermediate result, then using `l .= d` will avoid unused binding warnings

``` (`<.=`) :: `MonadState` a m => `Setter` a a c d -> d -> m d
(`<.=`) :: `MonadState` a m => `Iso` a a c d -> d -> m d
(`<.=`) :: `MonadState` a m => `Lens` a a c d -> d -> m d
(`<.=`) :: `MonadState` a m => `Traversal` a a c d -> d -> m d
```

(?=) :: MonadState a m => Setting a a c (Maybe 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 `Just` a new value, irrespective of the old.

``` (`?=`) :: `MonadState` a m => `Simple` `Iso` a (`Maybe` b)       -> b -> m ()
(`?=`) :: `MonadState` a m => `Simple` `Lens` a (`Maybe` b)      -> b -> m ()
(`?=`) :: `MonadState` a m => `Simple` `Traversal` a (`Maybe` b) -> b -> m ()
(`?=`) :: `MonadState` a m => `Simple` `Setter` a (`Maybe` b)    -> b -> m ()
```

(<?=) :: MonadState a m => Setting a a c (Maybe d) -> d -> m dSource

Set `Just` a value with pass-through

This is useful for chaining assignment without round-tripping through your monad stack.

`do x <- at foo <?= ninety_nine_bottles_of_beer_on_the_wall`

If you do not need a copy of the intermediate result, then using `l ?= d` will avoid unused binding warnings

``` (`<?=`) :: `MonadState` a m => `Setter` a a c (`Maybe` d) -> d -> m d
(`<?=`) :: `MonadState` a m => `Iso` a a c (`Maybe` d) -> d -> m d
(`<?=`) :: `MonadState` a m => `Lens` a a c (`Maybe` d) -> d -> m d
(`<?=`) :: `MonadState` a m => `Traversal` a a c (`Maybe` d) -> d -> m d
```

(<~) :: 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
...
```

will store the result in a `Lens`, `Setter`, or `Traversal`.

# Storing Setters

newtype ReifiedSetter a b c d Source

Reify a setter so it can be stored safely in a container.

Constructors

 ReifySetter FieldsreflectSetter :: Setter a b c d

# Setter Internals

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 directly to perform a mapping, you can use this type, but most user code will not need to use this type.

By choosing `Mutator` rather than `Identity`, we get nicer error messages.

type SimpleSetting a b = Setting a a b bSource

This is a useful alias for use when consuming a `SimpleSetter`.

Most user code will never have to use this type.

`type `SimpleSetting` m = `Simple` `Setting``

# Simplicity

type SimpleSetter a b = Setter a a b bSource

A Simple Setter is just a `Setter` that doesn't change the types.

These are particularly common when talking about monomorphic containers. e.g.

``sets` Data.Text.map :: `SimpleSetter` `Text` `Char``
`type `SimpleSetter` = `Simple` `Setter``

type SimpleReifiedSetter a b = ReifiedSetter a a b bSource

`type `SimpleReifiedSetter` = `Simple` `ReifiedSetter``

# Exported for legible error messages

class Applicative f => Settable f Source

Anything `Settable` must be isomorphic to the `Identity` `Functor`.

Instances

 Settable Identity so you can pass our a `Setter` into combinators from other lens libraries Settable Mutator (Applicative (Backwards f), Settable f) => Settable (Backwards f) `backwards` (Applicative (Compose f g), Settable f, Settable g) => Settable (Compose f g)

data Mutator a Source

`Mutator` is just a renamed `Identity` functor to give better error messages when someone attempts to use a getter as a setter.

Most user code will never need to see this type.

Instances

 Functor Mutator Applicative Mutator Settable Mutator