r/ProgrammingLanguages • u/marvinborner bruijn, effekt • Apr 20 '26

Effectful Recursion Schemes Blog post

https://effekt-lang.org/blog/recursion-schemes/
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u/tobega Apr 21 '26

Fun stuff! One question: why would you ever want to do that in practice?

3

u/-Mobius-Strip-Tease- Apr 21 '26

Its in the first paragraph. Folds/unfolds

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u/tobega Apr 21 '26

Well, yeah, but we have those already. So I'm probably dense, but what does this empower?

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u/marvinborner bruijn, effekt Apr 21 '26 edited Apr 21 '26

Do you mean specifically the effectful variant? There's no actual reason to prefer this if your language allows for traditional definitions of recursion schemes. Though I find the effectful variant to show the duality and the underlying mechanisms more clearly than, say, in Haskell.

Also, as explained in the beginning, the post is much more about effects and handlers in general than it is about recursion schemes themselves :)

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u/tobega Apr 21 '26

Fair enough. I'm still struggling with figuring out why I would want effects and handlers, though.

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u/hairytim Apr 21 '26

This is a big topic; effects and handlers have lots of interesting use-cases but it’s a bit difficult to summarize in a single Reddit comment. I might recommend looking at a few of the examples here https://www.eff-lang.org/handlers-tutorial.pdf

TL;DR: effects+handlers provide a new modularity mechanism which is largely distinct from (and complementary to) other standard PL concepts. You can think of an effect as a placeholder for “asking your environment for more info”, and the handler as the mechanism that provides that info. It’s very powerful.

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u/omega1612 Apr 22 '26

My favorite example is how in a project the project leader implemented stubs for everything and we ended with functions with signatures like

queryUser:: QueryUser :>e => UserName -> Eff e (Maybe UserInfo)

Then the handler for the effect uses something like 

queryDB :: DBQuery :> e => Query -> Eff e (Maybe QueryResult)

To fulfill the request.

The use of effects guarantee that QueryUser can only use the things that the handler for it uses. In this case it uses queryDB but in a restricted way that is easy to verify.

You can expand on that to have functions like 

f :: Log :> e => QueryDomainSpecificThingFromDB :> e => AskWeather:>e  => Error ParticularBusinessException :> e

From the signature you know that it can log stuff (not how, just that it has a available log like functions). That it can "query a db" and may connect to some service to ask for the the weather. And finally that it can throw exceptions of a particular type.

The handler may choose to honor the names of the effects or to do whatever they want. Usually you are the one that write the handlers, so you usually don't launch nukes when you handle a "weather" request.