import { Expr, FieldName, ProductPattern, Tag, VariableName } from "../expr";
import { ThrownRuntimeError } from "./error";

export type Value =
  | { tag: "string", value: string }
  | { tag: "number", value: number }
  | { tag: "tag", tag_name: Tag  }
  | { tag: "tagged", tag_name: Tag, value: Value }
  | { tag: "tuple", values: Value[] }
  | { tag: "record", fields: Map<FieldName, Value> }
  | { tag: "closure", closure: Closure }

export type ValueTag =
  | "string"
  | "number"
  | "tag"
  | "tagged"
  | "tuple"
  | "record"
  | "closure"

// Used as a Stack of frames. Basically a linked list.
export type Env =
  | { tag: "nil" }
  | { tag: "frame", frame: EnvFrame, parent: Env }

export type EnvFrame = Map<VariableName, Value>;

export type Closure = {
  env: Env,
  parameters: ProductPattern[],
  body: Expr,
}

export namespace Value {
  export const string = (value: string): Value => ({ tag: "string", value });
  export const number = (value: number): Value => ({ tag: "number", value });
  export const tag = (tag_name: Tag): Value => ({ tag: "tag", tag_name });
  export const tagged = (tag_name: Tag, value: Value): Value => ({ tag: "tagged", tag_name, value });
  export const tuple = (values: Value[]): Value => ({ tag: "tuple", values });
  export const record = (fields: Map<FieldName, Value>): Value => ({ tag: "record", fields });
  export const closure = (closure: Closure): Value => ({ tag: "closure", closure });
}

export namespace Env {
  export function nil(): Env {
    return { tag: "nil" };
  }
  
  export function push_frame(env: Env, frame: EnvFrame): Env {
    return { tag: "frame", frame, parent: env };
  }

  // may throw `ThrownRuntimeError`
  export function lookup(env: Env, var_name: VariableName): Value {
    let cur = env;
    while (cur.tag !== "nil") {
      if (cur.frame.has(var_name)) {
        return cur.frame.get(var_name)!;
      }
      cur = cur.parent;
    }
    throw ThrownRuntimeError.error({
      tag: "VariableLookupFailure",
      name: var_name,
    });
  }

  export function frame_insert_mut(frame: EnvFrame, var_name: VariableName, value: Value) {
     frame.set(var_name, value);
  }
}