8.15.0.2
2.2 Lenses
| (require ocular-patdown/optics/lens) | |
| package: ocular-patdown | |
This module’s bindings are also provided by ocular-patdown/optics.
A lens is a type of optic that has a single focus. Lenses can be used to focus on a field of a struct, the car of a pair, etc.
Examples:
> (define pair (cons 1 2)) > (lens-get car-lens pair) 1
> (lens-set car-lens pair #t) '(#t . 2)
> (struct posn [x y] #:transparent) > (define posn1 (posn 1 2)) > (define posn-x-lens (struct-lens posn x)) > (lens-get posn-x-lens posn1) 1
> (lens-set posn-x-lens posn1 3) (posn 3 2)
A predicate which recognizes lenses. To be more precise, it recognizes implementers of gen:lens.
Constructor for lenses. getter should extract the focus from the target.
setter should take in the target and the new value for the focus and return a copy of the target with the updated focus.
Examples:
There are a few laws lenses should obey:
- Getting the focus after setting the focus returns the new focus.
- Setting the focus using the current focus leaves the target unchanged.
- Setting the focus twice is the same as setting it once with the second value.
These laws should be obeyed for some reasonable definition of equality. If these laws are not obeyed, you may experience unexpected behavior.
Lenses should also be pure. In other words, lens operations should not mutate the target or the focus, or have any other side effects. For setters, it is recommended to create an updated copy of the original target rather than mutating it.
2.2.1 Lens Operations
Gets the focus from target under lens.
Returns an updated target with focus as the new focus under lens.
Examples:
Returns an updated target with proc applied to the focus under lens.
Examples:
> (lens-modify car-lens (cons 1 2) sub1) '(0 . 2)
> (lens-modify posn-x-lens (posn 3 4) -) (posn -3 4)
procedure
(lens-compose lens ...) → lens?
lens : lens?
Compose lenses like optic-compose.
Examples:
> (struct tree [val children] #:transparent) > (define tree-first-child-lens (lens-compose (struct-lens tree children) car-lens))
> (lens-set tree-first-child-lens (tree 1 (list (tree 2 '()) (tree 3 '()))) (tree #t '())) (tree 1 (list (tree #t '()) (tree 3 '())))
> (define second-lens (lens-compose cdr-lens car-lens)) > (lens-set second-lens (list 1 2) #t) '(1 #t)
> (define first-of-second-lens (lens-compose second-lens car-lens)) > (lens-set first-of-second-lens (list 1 (list 2 3)) #t) '(1 (#t 3))
2.2.2 Library Lenses
value
A lens that focuses on the entire target. The identity of lens-compose.
Examples:
> (lens-get identity-lens 1) 1
> (lens-set identity-lens 1 2) 2
> (lens-modify identity-lens 1 add1) 2
A lens that focuses on the car of a pair.
A lens that focuses on the cdr of a pair.
A lens that focuses on the caar of a pair.
syntax
(struct-lens struct-id field-id maybe-parent)
maybe-parent =
| #:parent parent-struct-id
A lens that focuses on a field of a struct. Supply #:parent if the field is a field of a supertype.
Examples:
> (struct posn [x y] #:transparent) > (define posn-x-lens (struct-lens posn x)) > (lens-get posn-x-lens (posn 3 4)) 3
> (lens-set posn-x-lens (posn 3 4) 9) (posn 9 4)
> (struct posn3 posn [z] #:transparent) > (define posn3-z-lens (struct-lens posn3 z)) > (define posn3-x-lens (struct-lens posn3 x #:parent posn)) > (lens-set posn3-z-lens (posn3 1 2 3) 9) (posn3 1 2 9)
> (lens-set posn3-x-lens (posn3 1 2 3) 9) (posn3 9 2 3)
> (lens-set posn-x-lens (posn3 1 2 3) 9) (posn 9 2)
If the supertype is not supplied for fields of supertypes, an update will yield an instance of the supertype.
2.2.3 Generic Lens Interface
syntax
A generic interface for lenses.