nn/src/core.ts

import { withLog } from './debug';

let index = 0;

export abstract class Term {
  protected id = index++;
  protected cache: Term | null = null;

  public constructor(protected bindings: Record<string, Term>) { }

  public refs(_name: string): boolean {
    return false;
  };

  public bind(_name: string, _term: Term): Term {
    return this;
  }

  public reduce(): Term {
    return this;
  }

  public context(): string {
    return !Object.keys(this.bindings).length ? `<${this.id}>` : `<${this.id}>[bound ${Object.keys(this.bindings).join(', ')}] `;
  }

  public stringify(context?: boolean): string {
    return `${context ? this.context() : ''}${this.constructor.name}`;
  }

  public _(arg: RefTerm): Term {
    return c(this, arg);
  }
}

export class Undef extends Term {
  constructor(
    public from: string,
  ) { super({}); }

  override reduce(): Term {
    throw new Error(`${this.from} = undef`);
  }
}

export class Native extends Term {
  constructor(
    public value: any,
  ) { super({}); }

  public override stringify(): string {
    return `\`${JSON.stringify(this.value)}\``;
  }
}

export class Deferred extends Term {
  constructor(
    public func: (bindings: Record<string, Term>) => Term,
    bindings: Record<string, Term> = {},
  ) { super(bindings); }

  override refs(_name: string): boolean {
    return true;
  }

  public override bind(name: string, term: Term): Term {
    return this.bindings[name] || !this.refs(name)
      ? this
      : new Deferred(this.func, { ...this.bindings, [name]: term });
  }

  public override reduce(): Term {
    const term = this.cache
      ? withLog(
        `Executing ${this.stringify()}`,
        () => this.cache!,
        r => `got ${r.stringify()} (cached)`,
      )
      : this.cache = withLog(
        `Executing ${this.stringify()}`,
        () => this.func({ ...globals, ...this.bindings }),
        r => `got ${r.stringify()}`,
      );
    return Object.entries(this.bindings).reduce((t, b) => t.bind(...b), term).reduce();
  }

  public override stringify(context: boolean = true): string {
    return `${context ? this.context() : ''}\`deferred\``;
  }
}

export class Ref extends Term {
  constructor(
    public name: string,
    bindings: Record<string, Term> = {},
  ) { super(bindings); }

  override refs(name: string): boolean {
    return this.name === name;
  }

  public override bind(name: string, term: Term): Term {
    return this.bindings[name] || !this.refs(name)
      ? this
      : new Ref(this.name, { [name]: term });
  }

  public override reduce(): Term {
    if (this.cache) {
      return withLog(
        `Substituting ${this.stringify()}`,
        () => this.cache!,
        r => `with ${r.stringify()} (cached)`,
      );
    } else {
      return this.cache = withLog(
        `Substituting ${this.stringify()}`,
        () => (this.bindings[this.name] ?? globals[this.name] ?? new Undef(this.name)).reduce(),
        r => `with ${r.stringify()}`,
      ).reduce();
    }
  }

  public override stringify(context = true): string {
    return `${context ? this.context() : ''}${this.name}`;
  }
}

export class Lambda extends Term {
  constructor(
    public param: string,
    public body: Term,
    bindings: Record<string, Term> = {},
  ) { super(bindings); }

  override refs(name: string): boolean {
    return this.param !== name && this.body.refs(name);
  }

  override bind(name: string, term: Term): Term {
    return this.bindings[name] || !this.refs(name)
      ? this
      : new Lambda(this.param, this.body.bind(name, term), { ...this.bindings, [name]: term });
  }

  public override stringify(context = true): string {
    return this.body instanceof Lambda
      ? `${context ? this.context() : ''}\\ ${this.param} ${this.body.stringify(false).slice(2)}`
      : `${context ? this.context() : ''}\\ ${this.param} . ${this.body.stringify(false)}`;
  }
}

export class Call extends Term {
  constructor(
    public func: Term,
    public arg: Term,
    bindings: Record<string, Term> = {},
  ) { super(bindings); }

  override refs(name: string): boolean {
    return this.func.refs(name) || this.arg.refs(name);
  }

  override bind(name: string, term: Term): Term {
    return this.bindings[name] || !this.refs(name)
      ? this
      : new Call(this.func.bind(name, term), this.arg.bind(name, term), { ...this.bindings, [name]: term });
  }

  override reduce(): Term {
    if (this.cache) {
      return withLog(
        `Reducing ${this.stringify()}`,
        () => this.cache!,
        r => `to ${r.stringify()} (cached)`,
      );
    } else {
      return this.cache = withLog(
        `Reducing ${this.stringify()}`,
        () => {
          const func = this.func.reduce();
          if (func instanceof Lambda) {
            return withLog(
              `Applying ${func.stringify()} to ${this.arg.stringify()}`,
              () => func.body.bind(func.param, this.arg),
              r => `got ${r.stringify()}`,
            ).reduce();
          }
          return this;
        },
        r => `to ${r.stringify()}`,
      );
    }
  }

  public override stringify(context = true): string {
    return `${context ? this.context() : ''}${this.func instanceof Lambda ? `(${this.func.stringify(false)})` : this.func.stringify(false)} ${this.arg instanceof Ref || this.arg instanceof Native ? this.arg.stringify(false) : `(${this.arg.stringify(false)})`}`;
  }
}

const globals: Record<string, Term> = {};

type RefTerm = string | Term;

const termify = (t: RefTerm) => typeof t === 'string' ? r(t) : t;

export const _ = (v: any) => new Native(v);
export const r = (name: string): Ref => new Ref(name);
export const l = (param: string, body: RefTerm): Lambda => new Lambda(param, termify(body));
export const c = (func: RefTerm, arg: RefTerm, ...args: RefTerm[]): Call =>
  args.reduce<Call>((f: Call, a: RefTerm) => new Call(f, termify(a)), new Call(termify(func), termify(arg)));
export const d = (func: (bindings: Record<string, Term>) => Term) => new Deferred(func);

export const $ = <T>(t: Term): T => {
  const rt = t.reduce();
  return rt instanceof Native ? rt.value : rt.stringify();
};

let cm: string = 'core';

export const m = (name: string) => cm = name;

export const g = (name: string, term: RefTerm) => {
  name = `${cm}::${name}`;
  if (globals[name]) {
    throw new Error(`Duplicate global: "${name}"`);
  }
  globals[name] = termify(term);
  return new Ref(name);
};

export const undef = g('undef', new Undef('undef'));

export const id = g('id', l('f', 'f'));
export const cnst = g('cnst', l('x', l('_', 'x')));
export const pipe = g('pipe', l('f', l('g', l('x', c('f', c('g', 'x'))))));