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
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