Safe Haskell | Safe-Inferred |
---|---|

Language | Haskell98 |

Basic lens type and functions.

The lens package should be a drop-in replacement for this.

# Lens type

type Lens s t a b = forall f. Functor f => (a -> f b) -> s -> f t Source

**Purpose**

A value of type `Lens s t a b`

provides the following:

- A reference into the structure
`s`

to read and update the value`a`

inside it. - The possibility to change the type of
`s`

to`t`

and the type of`a`

to`b`

.

**The Functor constraint**

Operations may do something more interesting inside the `f`

functor. For the purpose of this module and package, all the
functions below (`view`

, `over`

, `set`

) use a no-op functor
and therefore the above type is equivalent to:

type Lens s t a b = (a -> b) -> (s -> t)

But it is left generic for forward compatibilty with the lens package.

**Example**

λ> data Person = Person Char Int deriving Show λ> let _age f (Person x a) = fmap (\b -> Person x b) (f a) λ> view _age (Person`a`

10) 10 λ> over _age (+1) (Person`a`

10) Person`a`

11 λ> set _age 100 (Person`a`

10) Person`a`

100 λ>

**Laws**

1) *Get-Put*: You get back what you put in.

view l (set l v s) ≡ v

2) *Put-Get*: Putting back what you got doesn't change anything.

set l (view l s) s ≡ s

3) *Put-Put*: Setting is idempotent.

set l v (set l v s) ≡ set l v s