Theseus

Welcome to the hub for Theseus, an effect system library for Haskell. Theseus is an effect system that supports algebraic effects (including Coroutine), many higher order effects, and is designed so that semantics do not change when effects are reordered.

We’ll explore how, much like the Ship of Theseus, programs can be deconstructed and rebuilt without changing what they are.

Guides

Theseus contains several annotated source files transformed into docs using Docco. Those are the best place to learn more about Theseus.

For getting started, there are a couple good places:

• The tutorial is the best starting place. If you’re new to effect systems, it should bring you up to speed with a couple of examples. If you’re only new to Theseus, it’ll cover some of what makes Theseus unique.
• The File example showcases Theseus’ main feature. If you’re experienced with effect systems and want to know what makes Theseus special, look there.

The builtin effects are useful to walk through as well since they cover the range of Theseus’ complexity.

• The Reader implementation covers a simple higher order effect.
• The State implementation covers a simple first order effect that changes the return type.
• The Writer implementation is a higher order effect that also changes the return type and depends on a finalizer.
• The Error implementation introduces Theseus’ control flow manipulation features.
• The Coroutine effect gives another example of unique control flow.
• The Choice implementation shows the most complicated control flow.

Finally there are the inner workings. These might be helpful while you’re looking into the more complicated effects.

• The Eff definition explains how it’s all defined.
• The interpreters show how consuming works.

Shoutouts

There are some other amazing effect system libraries in Haskell exploring this space. A non-exhaustive list of ones I’m familiar with are:

• Fused-effects and Polysemy use a freer monad and Weave abstraction. They support a large number of higher order effects, but sacrifice some of the richer algebraic effects (like Coroutine) in the process.
• Eff approaches it with delimited continuations. Eff finds great success supporting the algebraic effects, but runs into more problems with the higher order effects.
• Bluefin, Effectful, and Cleff use a ReaderT IO based approach. They sacrifice even more of the algebraic effects (NonDet) for speed and ecosystem interoperability.
• Heftia’s approach supports the full gamut of algebraic and higher order effects. In exchange for all that support, it limits when richer algebraic effects can run.

Theseus, compared to those, is most similar to the free/freer effect systems. It changes some of the algebraic effect semantics (the biggest being that Choice is more breadth first than depth first), but in exchange has higher order effects and algebraic effects that play well with each other.