tiny tiny tracery-like

minitrace.js

const matcher = /\[([^\[\]]+)\]/g

function randomInArray(arr) { return arr[Math.floor(Math.random() * arr.length)] }

export default function generate(concepts, concept, maxCycles = 10, seen = {}) {
  if(!concepts[concept]) {
    return `{error: unknown concept "${concept}"}`
  }
  return randomInArray(concepts[concept])
    .replace(matcher, (_, nextConcept) => {
      if(seen[nextConcept] > maxCycles) {
        return '{error: max cycles exceeded}'
      }
      const nextSeen = Object.assign({}, seen)
      nextSeen[nextConcept] = nextSeen[nextConcept] + 1 || 1
      return generate(concepts, nextConcept, maxCycles, nextSeen)
    })
}

usage.js

const concepts = {
  beings: ['dogs', 'cats', 'humans', 'robots', 'animes'],
  state: ['harmony', 'misery', 'terror'],
  howThingsAre: [
    '[beings] and [beings], living together in [state]!'
  ],
  recursive: [
    '[beings], but also [recursive]',
    '[beings]'
  ]
}

generate(concepts, 'howThingsAre') // => animes and robots, living together in misery!
generate(concepts, 'recursive') // => cats, but also humans, but also animes, but also cats, but also robots

// You can also use .bind() to partially apply a generator with a set of concepts...
const gen = generate.bind(undefined, concepts)

// ...and get a function that can be used as a shorthand for that set!  
gen('howThingsAre') // => dogs and animes, living together in harmony!
gen('beings')

// You can do the same thing for a single concept, and get a handy zero-argument getter.
const getStateOfAffairs = generate.bind(undefined, concepts, 'howThingsAre')
getStateOfAffairs()