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This tutorial explains how Unison handles 'effectful' computations, like storing state or performing I/O, using abilities. It assumes you haven't come across abilities before, and covers everything from the ground up.

This is an unofficial tutorial, written before the one on unisonweb.org/docs. The approach taken here is slow and methodical. Your first stop should be the official tutorial, if you haven't seen it already.

This doc is a Unison transcript - the source is here.

Terminology note: other languages with ability systems typically call them 'effect handlers' or 'algebraic effects', but many of the ideas are the same.

Introducing abilities

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package zero

import izumi.reflect.Tag
import Kyo.*
import scala.collection.View.FlatMap

type Id[T]    = T
type Const[T] = [U] =>> T
type MX[T]    = Any

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// Adapted from http://lukajcb.github.io/blog/functional/2018/01/03/optimizing-tagless-final.html

import { Applicative, Applicative1 } from 'fp-ts/lib/Applicative'
import { Apply, Apply1, Apply2C, applySecond, liftA4 } from 'fp-ts/lib/Apply'
import * as array from 'fp-ts/lib/Array'
import * as const_ from 'fp-ts/lib/Const'
import { HKT, Type, Type2, URIS, URIS2 } from 'fp-ts/lib/HKT'
import { IO, io, URI as IOURI } from 'fp-ts/lib/IO'
import { Option, some } from 'fp-ts/lib/Option'
import { getProductSemigroup, Semigroup } from 'fp-ts/lib/Semigroup'

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import { log } from 'fp-ts/lib/Console'
import { none, Option, some } from 'fp-ts/lib/Option'
import { randomInt } from 'fp-ts/lib/Random'
import { fromIO, Task, task } from 'fp-ts/lib/Task'
import { createInterface } from 'readline'

//
// helpers
//

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import { log } from 'fp-ts/lib/Console'
import { Type, URIS } from 'fp-ts/lib/HKT'
import { none, Option, some } from 'fp-ts/lib/Option'
import { randomInt } from 'fp-ts/lib/Random'
import { fromIO, Task, task, URI as TaskURI } from 'fp-ts/lib/Task'
import { createInterface } from 'readline'

//
// helpers
//