frank VS effects-bibliography

For the curious, "do be do be do" is a seminal paper in the literature on algebraic effects that introduces _frank_, quirky little language that has algebraic effects but not handlers, at least in the traditional sense.

Traditionally, an effect handler is an interpreter for a stream of commands, conforming to a specific interface. Generally, handlers surface in languages as a sort of generalized try/catch mechanism, that receive a "callback" to resume the "exception" that produced the command. In frank, not so.

Frank is based around the idea of _operators_, which generalize functions with the capability of interpreting multiple streams of commands. A plain function can be seen, in fact, as the special case of an operator that interprets no commands.

Operators are organized around ports and pegs. Pegs are the set of side effects that a computation needs. Each port is an offer to extend that set for downstream callers. Instead of building up a union of effects that each function needs, Frank propogated ambient ability inwards. Operators can then be composed based on the ports and pegs they offer.

operator: X → [peg]Y

This works partially because operators are shallow handlers and not deep handlers. Handlers interpret commands: if the handler itself is in scope when interpreting a command, then the language is said to have deep handlers. Frank has shallow handlers, meaning that commands are interpreted in an environment without the handling operator present. Shallow handlers give greater control to the programmer with respect to how commands are interpreted.

(This is a bad explanation because you already need to know what I'm talking about to understand what I'm talking about, but oh well.)

My one critism of frank is that the effect model is kinda hard for the working programmer to understand. I can explain Koka effects as "exceptions plus multiple resumption". I don't really have a categorical phrase for frank, and that's its innovation. This isn't so much a criticism but a plea for the pedagogical ramp to this research to improve.

do be do be do.

If you're still curious, check out the compiler github repo:

https://github.com/frank-lang/frank

And if anything is wrong in the above explanation, please correct me, because we all benefit from Cunningham's Law in the end. Allow me to be the fool.

How does this compare to other effect-oriented languages like Koka, Frank, and Eff?

I've been doing some work with Koka lately, but I briefly looked into the other three (including Effekt) and it mostly came down to, 'Koka seems most active in development'[1] and 'Koka had the easiest to use documentation for me'[2].

[1] E.g. https://github.com/effekt-lang/effekt had its last commit back in June; https://github.com/frank-lang/frank last commit last year; but https://github.com/koka-lang/koka last update was Oct 15. Effekt seems semi-active, at least, compared to Frank. While stability is good, I wouldn't expect it in a language actively being used for research.

[2] Comparing https://koka-lang.github.io/koka/doc/book.html and https://effekt-lang.org/docs/ and https://www.eff-lang.org/learn/

Frank's effect handling. "A strict functional programming language with a bidirectional effect type system designed from the ground up around a novel variant of Plotkin and Pretnar's effect handler abstraction. ... Frank [is different from other PLs with effect type systems in that it is based on] generalising the basic mechanism of functional abstraction itself. A function is simply the special case of a Frank operator that interprets no commands. Moreover, Frank's operators can be multihandlers which simultaneously interpret commands from several sources at once, without disturbing the direct style of functional programming with values. Effect typing in Frank employs a novel form of effect polymorphism which avoid mentioning effect variables in source code. This is achieved by propagating an ambient ability inwards, rather than accumulating unions of potential effects outwards."

In addition to those 2 (which I think have phenomenal documentation), the language Eff has more theoretical introduction. There is also the Effect bibliography which has a lot of different effect system in different languages, as well as tutorials and academic papers on the subject.

I think it goes back to the neverending quest to find ways of representing computation that allows of ease of composition, changing implementation details, eliminating classes of errors by construction, etc. Monads have had some success in this arena, but they have notable issues with composition; monad transformers help, but can become unwieldy in their own ways.

An alternative are effects, hypothetically allowing for ease in building programs as separate but composeable components which can then be freely mixed in or swapped out. In practice I have found working with effect systems in Haskell via libraries stresses the type system so much you end up with scoped type variables and type applications everywhere. My understanding is that the theory behind using effects to structure computations comes from category theory's Lawvere theories (see e.g. Pretnar's 2010 dissertation on https://github.com/yallop/effects-bibliography). Lawvere theories give rise to many monads (see Bartosz Milewski's article on it -- https://bartoszmilewski.com/2017/08/26/lawvere-theories/), but with nicer compositional properties.

This is where languages like Effekt, Eff, Frank, and Koka come in -- by writing the entire language and type system to support the theories, a lot of the pain of expressing it in Haskell can be avoided.

Not really an answer but Jeremy Yallop maintains a bibliography on the theory and practice of computational effects.