Concepts Tour

This section will try to introduce you to the concepts of Effects, Abilities and Handlers as present in Fx.go.

No knowledge or previous experience with other effect sytems is expected. We will try to explain things by working out from simple concepts to more interesting ones.

Effects

An Effect ( Fx[S, V] read: V provided S ) is the description of a program that computes V, provided that the requirement S is present, so that the computation of V can be performed.

Since effects are descriptions of programs, they compute nothing nor produce side-effects until they are finally evaluated, once all their requirements are at met.

Some people also use the recipe analogy for effects: you first have a precise description of each step it takes to cook something, along with a list of the requirements for it (the ingredients and utencils you will use) and once you have them all, you can actually perform the recipe.

Abilities

In Fx[S, V], S is said to be the Ability (sometimes also referred as the set of Abilities, Capabilities, Effect Environment or Effect Requirements) that are needed for computing V.

Abilities describe the external resources that would be needed, as well as the side-effects that are possible while computing V.

Examples of such Abilities are:

• network abilities (eg, performing http requests)
• console abilities (eg, printing to the terminal or reading user input)
• non-deterministic abilities (eg, generating random numbers or coin-flips)
• resource handling (eg, disciplined acquire/use/release of shared/limited resources)
• exception handling (eg, interruption/resumption and finalizers)
• anything else that interacts with the world outside of the program.

Handlers

A Handler for the S ability is a particular interpretation of what S means.

Handlers are the only side-effectful portion of your programs. It is possible, and quite common, to have different handlers (interpretations) for the same Ability, and each Handler decides how/when to perform world-modifying side-effects.

For example, for an http-request ability you can have a test-handler that just mock responses to fixed values so that you can easily assert on known values on your tests. You could also have a live-handler that actually performs requests via the network for production runs.