free-trek-hs

A study of Free monads and their applications in testing service orchestration, with a Star Trek theme.

This is a Haskell port of the Scala-based study free-trek.

Prerequisites

Stack 1.3.2+ recommended

Running

Run stack test in the root directory.

Free Trek and Haskell

The implementation closely follows ideas from David Laing's and Arnaud Bailly's respective posts on Free DSLs and Cofree interpreters, with some modifications, and using types from the Haskell standard libraries where possible.

This version of Free Trek features a similar algebra structure as the Scala based study, but has a (extensible) Cofree based interpretation mechanism.

Notable differences to the Scala version include:

The Inject module contains the Sum/Coproduct composition of the system algebras. This code mostly already existed in Cats.
The trace interpreter uses the Sum representation of the types as the trace type. In Scala, a trait super type was used.
There is no need for a Execution module, since the MonadPlan m type is a constraint (instead of a separate type), and Haskell is clever enough to select a suitable concrete type automatically. The mtl package provides the RWS type.

For completeness, the setting of the free-trek study is presented below in adapted form.

The Problem

James T. Kirk has a problem. The USS Enterprise has recently been fitted with a new multitronic computer, and while her subsystems are tried and true, the Captain is concerned whether the new computer will correctly coordinate those systems.

After all, each expedition into space can feature a number of potentially dangerous encounters! Not only that, but the computer needs to keep track of the remaining dilithium and coordinate warp speed accordingly.

We have been given a spec of the Enterprise's systems by Montgomery Scott, found in the Domain module. Now, we only need to write a highly testable system that will satisfy the intrepid Captain!

USS Enterprise Operating Manual

Note! Before attempting to operate your Constitution-class starship, you must first turn the crank shaft to start the ignition!

Warp Core

The heart of the ship, producing the energy for the other systems.

Before making warp jumps, you need to convert dilithium crystals equal to the warp speed, or lower the warp factor (to zero if necessary)
When operating in asteroid fields, you should engage the antimatter reaction assembly

Warp Drive

The warp-capable drives that propel the starship in the vast void of space.

To perform a warp jump, tune the warp field coil to desired warp speed
When in empty space, you should take the opportunity to attenuate the plasma conduit
Trudging through nebulae, it is important to disentangle the electro-plasma system

Phasers

Powerful directed-energy beam weapons that operate on various frequencies.

When encountering hostile space crabs, focus for high frequency optronics, without forgetting to purge the prefire chamber
When operating near space-time anomalies (a common occurrence), it is wise to rewire the power cell matrix

Deflector Shields

Highly polarized energetic distortion field emitters that protect the starship.

When encountering hostile space crabs, go for parallel frequency modulation
While negotiating asteroid fields, it is prudent to use anti-parallel frequency modulation instead
Always decompress the graviton emitter when operating near space-time anomalies!

seance / free-trek-hs