Lenses for Generic Collections

7.7

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Lenses for Generic Collections

(require alexis/collection/lens)
	package: alexis-collection-lens

This provides lenses that examine and modify generic collections.

procedure
(ref-lens key) → lens?
  key : any/c
(ref-lens key default) → lens?
  key : any/c
  default : any/c

Constructs a lens for a keyed value of an indexable? instance.

Examples:

> (lens-transform (ref-lens 'key)
                  (hash 'key 2)
                  (λ (x) (* 10 x)))
'#hash((key . 20))
> (lens-view (ref-lens 'key 'default)
             (hash))
'default

procedure
(nth-lens index) → lens?
index : exact-nonnegative-integer?

Constructs a lens for a single element of a sequence?.

Example:

> (lens-transform (nth-lens 1)
'(1 2 3)
(λ (x) (* 10 x)))
'(1 20 3)

procedure
(map-lens lens) → lens?
lens : lens?

Constructs a lens that maps over sequence? instances.

Examples:

> (lens-view (map-lens (ref-lens 'key))
             (list (hash 'key 1 'a 2) (hash 'key 2 'b 3)))
#<stream>
> (sequence->list
   (lens-view (map-lens (ref-lens 'key))
              (list (hash 'key 1 'a 2) (hash 'key 2 'b 3))))
'(1 2)
> (sequence->list
   (lens-set (map-lens (ref-lens 'key))
             (list (hash 'key 1 'a 2) (hash 'key 2 'b 3))
             (list 100 200)))
'(#hash((a . 2) (key . 100)) #hash((b . 3) (key . 200)))
> (sequence->list
   (lens-transform (map-lens (ref-lens 'key))
                   (list (hash 'key 1 'a 2) (hash 'key 2 'b 3))
                   (λ (lst) (map (λ (x) (* 10 x)) lst))))
'(#hash((a . 2) (key . 10)) #hash((b . 3) (key . 20)))

value
first-lens : lens?

A lens for the first element of a sequence?.

value
rest-lens : lens?

A lens for all but the first element of a sequence?.

procedure
(take-lens n) → lens?
n : exact-nonnegative-integer?

Constructs a lens for the first n elements of a sequence?.

procedure
(drop-lens n) → lens?
n : exact-nonnegative-integer?

Constructs a lens for the rest of a sequence? after the first n elements.

procedure
(subsequence-lens start end) → lens?
start : exact-nonnegative-integer?
end : exact-nonnegative-integer?

A lens for a subsequence from start to end. The range behavior corresponds to subsequence.

procedure
(subsequence*-lens start len) → lens?
start : exact-nonnegative-integer?
len : exact-nonnegative-integer?

A lens for a subsequence starting at start with length len elements. The range behavior corresponds to subsequence*.

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