Conal Elliott

While working on Eros, I encountered a function programming pattern I hadn’t known. I was struck by the simplicity and power of this pattern, and I wondered why I hadn’t run into it before. I call this idea “semantic editor combinators”, because it’s a composable way to create transformations on rich values. I’m writing this post in order to share this simple idea, which is perhaps “almost obvious”, but not quite, due to two interfering habits:

• thinking of function composition as binary instead of unary, and
• seeing the functions first and second as about arrows, and therefore esoteric.

What I enjoy most about these (semantic) editor combinators is that their use is type-directed and so doesn’t require much imagination. When I have the type of a complex value, and I want to edit some piece buried inside, I just read off the path in the containing type, on the way to the buried value.

I started writing this post last year and put it aside. Recent threads on the Reactive mailing list (including a dandy explanation by Peter Verswyvelen) and on David Sankel’s blog reminded me of my unfinished post, so I picked it up again.

Edits:

• 2008-11-29: added type of v6 example. Tweaked inO2 alignment.

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While working on Eros, I encountered a function programming pattern I hadn’t known. I was struck by the simplicity and power of this pattern, and I wondered why I hadn’t...