tiny-pattern-ts/src/matcher-shared.ts

import type { IsLiteral, IsNever, ValueOf } from "type-fest";

// The primitive-union and tagged-union matchers differ in their universe, but the
// handler/fallback plumbing is identical; these are the shared pieces. The
// boundary is deliberate: `Handlers`, `Fallback` and `MustBePartial` stay with
// each matcher because they are built from its universe. See development/library.md.

// A handler: one universe member in, one return value out.
export type UnaryFn<T, R> = (shape: T) => R;

// The primitive universe a matcher can discriminate. The primitive-union matcher
// uses it directly; the tagged-union matcher uses it as the set of allowed
// discriminant (`Tag`) values. `boolean` is admitted as the pair `true | false`;
// see README § Caveats for the unsupported members.
export type Matchable = string | number | boolean | null | undefined;

// `boolean`, `null` and `undefined` cannot be property keys, so a mapped type
// over a universe that includes one keys each such member by its
// stringification. `Member` inverts that projection against the universe, so a
// handler callback still receives the *real* member (`true`, not `"true"`).
// Both matchers use the projection: the primitive-union matcher over its
// universe, the tagged-union matcher over a discriminant property's values. See
// README § Caveats for the limits.
export type PatternKey<T> = T extends boolean
    ? T extends true
        ? "true"
        : "false"
    : T extends null
      ? "null"
      : T extends undefined
        ? "undefined"
        : T;

// The member(s) of `T` whose `PatternKey` is `K`: the universe-keyed inverse of
// `PatternKey`. The key alone cannot recover the member (`"true"` and `true`
// share it), so the handler parameter is derived from `T` instead. For a
// supported (injective) universe the result is a single member.
export type Member<
    T extends Matchable,
    K extends PropertyKey,
> = T extends Matchable ? (PatternKey<T> extends K ? T : never) : never;

// The property key a `Matchable` member takes at runtime: booleans, `null` and
// `undefined` stringify, and a numeric literal becomes its decimal string.
export type Stringified<T> = T extends boolean
    ? T extends true
        ? "true"
        : "false"
    : T extends null
      ? "null"
      : T extends undefined
        ? "undefined"
        : T extends number
          ? `${T}`
          : never;

// The members of `T` that are also the stringification of another member, so
// `PatternKey` cannot invert them. `never` means the universe is injective.
export type Collisions<T> = Extract<T, Stringified<T>>;

// Why `T` is not a supported universe, or `never` when it is. A supported
// universe is a finite union of literals with no value/stringification
// collision: only then can `PatternKey` be inverted unambiguously.
export type UnsupportedReason<T extends Matchable> =
    IsLiteral<PatternKey<T>> extends false
        ? "broad types like string, number and template literals are not supported"
        : [Collisions<T>] extends [never]
          ? never
          : `a value and its stringification collide. Value is "${Collisions<T> & string}"`;

// The diagnostic for an unsupported universe. Extends `HandlerMap` so the
// implementation's `handlers: HandlerMap` stays assignable when the gate is
// intersected into a parameter; the property name is the message.
export type UnsupportedUniverse<Reason extends string> = HandlerMap &
    Readonly<Record<`unsupported universe: ${Reason}`, never>>;

// `unknown` for a supported universe (an intersection no-op), the diagnostic
// otherwise. Intersecting rather than branching keeps `R` inference intact.
export type UniverseGate<T extends Matchable> =
    IsNever<UnsupportedReason<T>> extends true
        ? unknown
        : UnsupportedUniverse<UnsupportedReason<T>>;

// `Extract` drops optional handlers (`undefined`) so `PatternReturns` also works
// when `P`'s constraint has optional keys.
export type PatternReturns<P> = ReturnType<
    Extract<ValueOf<P>, (...args: never[]) => unknown>
>;

// The diagnostic raised when a fallback is supplied for an already-exhaustive
// handler map. Each matcher's `MustBePartial` folds it into `Handled`'s
// constraint so the guard is checked after inference.
export interface RedundantFallback {
    readonly "every case is already handled, so the fallback is redundant": never;
}

// The runtime dispatch map the handler maps and fallback erase to.
export type HandlerMap = Record<
    string | number,
    UnaryFn<never, unknown> | undefined
>;