message: comp proof of concept
diff: +
  +
  +
  +type ExecutionContext = { getRuntimeCtx(c: Computation<any>): RuntimeContext, subscribe(o: (p: string, compId: number) => void): void }
  +
  +type RuntimeContext = { yield(name: string): Promise<void> | void, createChildContext(): RuntimeContext }
  +
  +type Computation<T> = { (ctx?: RuntimeContext): Promise<T>, _id: number }
  +
  +function create<T>(c: (ctx: RuntimeContext) => Promise<T>): Computation<T> { return 0 as any; }
  +
  +type MultistepFn<P, T> = (params: P, step: (s: number) => Promise<void> | void, ctx: RuntimeContext) => Promise<T>
  +type ComputationProvider<P, T> = (params: P) => Computation<T>
  +function MultistepComputation<P, T>(name: string, steps: string[], f: MultistepFn<P, T>): ComputationProvider<P, T> {
  + return params => create(async ctx => f(params, n => ctx.yield(steps[n]), ctx));
  +}
  +
  +
  +// if total count is specified, could automatically provide percentage
  +type UniformlyChunkedFn<S> = (chunkSize: number, state: S, totalCount?: number) => number
  +type UniformlyChunkedProvider<S> = (ctx: RuntimeContext, state: S) => Promise<S>
  +function UniformlyChunked<S>(label: string, initialChunk: number, f: UniformlyChunkedFn<S>): UniformlyChunkedProvider<S> {
  + // TODO: track average time required for single element and then determine chunk size based on that.
  + return 0 as any;
  +}
  +
  +const uniformPart = UniformlyChunked('Reading lines', 1000000, (size, state: { str: string, position: number, lines: string[] }) => {
  + state.position += size;
  + state.lines.push('');
  + return 0 /* number of lines read */;
  +});
  +
  +function readLines(str: string): Computation<string[]> {
  + return create(async ctx => {
  + const state = { str, position: 0, lines: [] };
  + await uniformPart(ctx, state);
  + return state.lines;
  + });
  +}
  +
  +const prependHiToLines = MultistepComputation('Hi prepend', ['Parse input', 'Prepend Hi'], async (p: string, step, ctx) => {
  + await step(0);
  + const lines = await readLines(p)(ctx);
  + await step(1);
  + const ret = lines.map(l => 'Hi ' + l);
  + return ret;
  +});
  +
  +
  +(async function() {
  + const r = await prependHiToLines('1\n2')();
  + console.log(r)
  +}())
  