lens

A Racket package for creating and composing pure functional lenses.

raco pkg install lens (require lens)

A lens is a value that can be used to focus on a small subpiece of some larger structure. Given a lens and a value of that larger structure, two values can be dervied: a view value, which is the subpiece, and a context function, which accepts a new view value and returns a new structure with the old view replaced by the new view. Think of them as composable, pure functional getters and setters. See the full documentation for more detail.

> (lens-view first-lens '(1 2 3))
1
> (lens-set first-lens '(1 2 3) 'a)
'(a 2 3)
> (lens-transform first-lens '(1 2 3) number->string)
'("1" 2 3)
> (define first-of-b-key-lens (lens-compose first-lens (hash-ref-lens 'b)))
> (define a-hash (hash 'a '(1 2 3) 'b '(10 20 30) 'c '(100 200 300)))
> (lens-view first-of-b-key-lens a-hash)
10
> (lens-set first-of-b-key-lens a-hash 'foo)
#hash((a . (1 2 3)) (b . (foo 20 30)) (c . (100 200 300)))

Contributions

This project uses Github issues organized by a Waffle board to track what's being worked on. Check the board to see if there's any features, bugs, etc. that interest you, or create a new Github issue to inquire about something you'd like to see changed.