Typed mutable slots for an effectful program: create a slot with a value, then read and write it in later steps. A slot's key carries its value type, so a key made for a number can only be read as a number. module.f.mjs holds the operations and the synchronous interpreter, and types.ts the Key<T> and MemOp types.

import { history, historyStep, mapStep } from '../module.f.mjs'
import { run } from '../mock/module.f.mjs'
import { create, memoryInitial, memoryOperationMap, read, write } from './module.f.mjs'

const created = history(create(0))
const written = historyStep(created, key => write(key, 41))
const incremented = historyStep(written, (_, key) => mapStep(read(key), n => n + 1))
const counter = mapStep(incremented, ([n]) => n)

run(memoryOperationMap)(memoryInitial)(counter)
// [{ next: 1, values: { mem0: 41 } }, ['ok', 42]]

Each link is bound at one level, and historyStep carries the key forward so a later link can read it. The program stays pure: the slot lives in the runner's state, not in the program. memoryOperationMap is the interpreter for mock's run, and node/memory the one for the Node runner. A program that also does I/O unions the operation types, Effect<IoOp | MemOp, T, E>.