lens

sensorlenses.ts

import { array } from 'fp-ts/lib/Array'
import { fromTraversable, Lens, Prism, Iso } from 'monocle-ts'

interface Model {
  sensors: Array<Sensor>
}

interface Sensor {
  ip: string
  temp: number
}

// describes a focus into a Model's sensors property
const sensorsLens = Lens.fromProp<Model, 'sensors'>('sensors')

// describes a focus into a Sensor's temp property
const sensorValueLens = Lens.fromProp<Sensor, 'temp'>('temp')

// describes an array traversal of Sensor's
const sensorTraversal = fromTraversable(array)<Sensor>()

// describes a metric x temperature isomorphism
const tempToMetric = new Iso<number, number>(temp => temp * 1.8 + 32, metric => (metric - 32) / 1.8)

// composite Traversal to access all the given Sensor values
const composedTraversal = sensorsLens.composeTraversal(sensorTraversal).composeLens(sensorValueLens)

// refinement for hot sensors
const isHot = Prism.fromPredicate<number>(x => x > 30)

// our test data
const data: Model = {
  sensors: [
    { ip: '192.168.0.1', temp: 1 },
    { ip: '192.168.0.2', temp: 5 },
    { ip: '192.168.0.3', temp: 42 },
    { ip: '192.168.0.2', temp: 6 },
    { ip: '192.168.0.3', temp: 555 }
  ]
}

// get all values
const allValues = composedTraversal.asFold().getAll(data)

console.log(allValues) // [ 1, 5, 42, 6, 555 ]

// get only hot sensor values
const hotSensors = composedTraversal
  .composePrism(isHot)
  .asFold()
  .getAll(data)

console.log(hotSensors) // [ 42, 555 ]

// get only hot sensor values AS metric
const hotSensorsMetrics = composedTraversal
  .composePrism(isHot)
  .asFold()
  .composeIso(tempToMetric)
  .getAll(data)

console.log(hotSensorsMetrics) // [ 107.60000000000001, 1031 ]