It's Kinda FRP-y

Last updated: 2020-01-26

I put together a helper package called kinda-ferpy for Racket that models dependency relationships like this:

(require kinda-ferpy)

(define x (stateful-cell 1))
(define y (stateful-cell 1))
(define sum (stateful-cell (+ (x) (y))))

(displayln (sum)) ; 2
(y 8)
(displayln (sum)) ; 9

stateful-cell encapsulates a Racket value such that it will always be up to date according to its dependencies. It’s a super boiled-down form of functional reactive programming, except signals, events, and dependency relationships are implicitly defined by how you write code. It’s not a full functional-reactive programming interface, just an expressive kind of one with some reasonable trade-offs. I borrowed the approach from MaiaVictor’s PureState library, which had a refreshing rant for those deep in thought about how to manage state.

For a less trivial example, here’s a progression from a Markdown file to an HTML document, plus a few ways to use it. % is an alias of stateful-cell, so I’ll shorten it here.

(require kinda-ferpy markdown)

(define md-path
  (% (build-path "index.md")))

(define html-path
  (% (path-replace-extension (md-path)
                             #".html")))

(define content-xexpr
  (% (parse-markdown (md-path))))

(define page-xexpr
  (% `(html (head (title "My Page"))
            (body ,(content-xexpr)))))

(define html-string
  (% (string-append "<!DOCTYPE html>"
                    (xexpr->html (page-xexpr)))))

(define html-link
  (% (string-append "/"
                    (path->string ,(html-path)))))

This hooks up a file system path to a Markdown file and expresses different output formats regarding an HTML document.

(html-string) evaluates to the content as a string, and (html-link) evaluates to a string that you can use as an href attribute. If I write (md-path (build-path "other.md")), then all of the values update to reflect other.md.

I will be using this package to upgrade unlike-assets, creating a better build system for Racket. If you are interested in this kind of work and want to have both support and a say over future development, please consider purchasing a subscription.