On this pageImmutable Updates with evo

Immutability

Immutable Updates with evo

Foldkit provides evo for immutable model updates. It wraps Effect's Struct.evolve with stricter type checking — if you remove or rename a key from your Model, you’ll get type errors everywhere you try to update it.

import { evo } from 'foldkit/struct'

type Model = { count: number; lastUpdated: number }
const model: Model = { count: 0, lastUpdated: 0 }

// evo takes the model and an object of transform functions
const newModel = evo(model, {
  count: count => count + 1,
  lastUpdated: () => Date.now(),
})

// Invalid keys are caught at compile time
const badModel = evo(model, {
  counnt: count => count + 1, // ❌ Error: 'counnt' does not exist in Model
})

Each property in the transform object is a function that takes the current value and returns the new value. Properties not included remain unchanged.