export function createSignal<T>(): Signal<T | undefined>;
export function createSignal<T>(value: T, options?: SignalOptions<T>): Signal<T>;
export function createSignal<T>(
  value?: T,
  options?: SignalOptions<T | undefined>
): Signal<T | undefined> {
  options = options ? Object.assign({}, signalOptions, options) : signalOptions;

  const s: SignalState<T | undefined> = {
    value,
    observers: null,
    observerSlots: null,
    comparator: options.equals || undefined
  };

  if ("_SOLID_DEV_" && !options.internal) {
    if (options.name) s.name = options.name;
    registerGraph(s);
  }

  const setter: Setter<T | undefined> = (value?: unknown) => {
    if (typeof value === "function") {
      if (Transition && Transition.running && Transition.sources.has(s)) value = value(s.tValue);
      else value = value(s.value);
    }
    return writeSignal(s, value);
  };

  return [readSignal.bind(s), setter];
}

返り値である Signal の型。Getter 関数、Setter 関数のタプルを返す。Getter 関数なことろが React の useState と異なる。

export type Accessor<T> = () => T;

export type Setter<T> = (undefined extends T ? () => undefined : {}) &
  (<U extends T>(value: (prev: T) => U) => U) &
  (<U extends T>(value: Exclude<U, Function>) => U) &
  (<U extends T>(value: Exclude<U, Function> | ((prev: T) => U)) => U);

export type Signal<T> = [get: Accessor<T>, set: Setter<T>];

SignalOptions 型

export interface BaseOptions {
  name?: string;
}

export interface EffectOptions extends BaseOptions {}

export interface MemoOptions<T> extends EffectOptions {
  equals?: false | ((prev: T, next: T) => boolean);
}

export interface SignalOptions<T> extends MemoOptions<T> {
  internal?: boolean;
}