Expression parser using combinators

expression.ts

import {
  any,
  char,
  delimited,
  int,
  many0,
  map,
  multispace0,
  nat,
  parse,
  Parser,
  token,
  tuple,
  tuple3,
} from "https://raw.githubusercontent.com/reu/parser-combinators-lecture/master/combinators.ts";

type Operator = "+" | "-" | "*" | "/" | "**";

type Expression =
  | { type: "NUMBER"; value: number }
  | {
    type: "BINARY_OPERATION";
    operator: Operator;
    left: Expression;
    right: Expression;
  };

const ws = <A>(p: Parser<A>): Parser<A> =>
  delimited(multispace0, p, multispace0);

const number: Parser<Expression> = map(
  any([
    map(tuple3(nat, char("."), nat), ([n, _, d]) => n + d * 10 ** -1),
    map(tuple3(int, char("."), nat), ([n, _, d]) => n - d * 10 ** -1),
    int,
  ]),
  (value) => ({ type: "NUMBER", value }),
);

const operand: Parser<Expression> = (input) =>
  any([
    delimited(ws(char("(")), expression, ws(char(")"))),
    number,
  ])(input);

const operation = (
  operators: Array<Operator>,
  precedent: Parser<Expression>,
): Parser<Expression> =>
  map(
    tuple(
      precedent,
      many0(tuple(ws(any(operators.map(token))), precedent)),
    ),
    ([left, right]) =>
      right.reduce((left, [operator, right]) => ({
        type: "BINARY_OPERATION",
        operator: operator as Operator,
        left,
        right,
      }), left),
  );

const operations = (operations: Array<Array<Operator>>): Parser<Expression> =>
  operations.reduce((expr, opers) => operation(opers, expr), operand);

export const expression = operations([
  ["**"],
  ["*", "/"],
  ["+", "-"],
]);

export const evaluate = parse(map(
  expression,
  function evaluate(expression: Expression): number {
    switch (expression.type) {
      case "NUMBER":
        return expression.value;
      case "BINARY_OPERATION": {
        switch (expression.operator) {
          case "+":
            return evaluate(expression.left) + evaluate(expression.right);
          case "-":
            return evaluate(expression.left) - evaluate(expression.right);
          case "*":
            return evaluate(expression.left) * evaluate(expression.right);
          case "/":
            return evaluate(expression.left) / evaluate(expression.right);
          case "**":
            return evaluate(expression.left) ** evaluate(expression.right);
        }
      }
    }
  },
));

expression_test.ts

import { assertEquals } from "https://deno.land/std@0.177.0/testing/asserts.ts";
import {
  evaluate,
  expression,
} from "https://gist.githubusercontent.com/reu/63b2937190eb7d5a6b70854f30d87b96/raw/expression.ts";

Deno.test(function expressionTest() {
  assertEquals(evaluate("1"), 1);
  assertEquals(expression("1"), [{ type: "NUMBER", value: 1 }, ""]);

  assertEquals(evaluate("-1"), -1);
  assertEquals(expression("-1"), [{ type: "NUMBER", value: -1 }, ""]);

  assertEquals(evaluate("1.5"), 1.5);
  assertEquals(expression("1.5"), [{ type: "NUMBER", value: 1.5 }, ""]);

  assertEquals(evaluate("-1.5"), -1.5);
  assertEquals(expression("-1.5"), [{ type: "NUMBER", value: -1.5 }, ""]);

  assertEquals(evaluate("1+2"), 3);
  assertEquals(expression("1+2"), [{
    type: "BINARY_OPERATION",
    operator: "+",
    left: { type: "NUMBER", value: 1 },
    right: { type: "NUMBER", value: 2 },
  }, ""]);

  assertEquals(evaluate("1*2"), 2);
  assertEquals(expression("1*2"), [{
    type: "BINARY_OPERATION",
    operator: "*",
    left: { type: "NUMBER", value: 1 },
    right: { type: "NUMBER", value: 2 },
  }, ""]);

  assertEquals(evaluate("4*2+3"), 11);
  assertEquals(expression("4*2+3"), [{
    type: "BINARY_OPERATION",
    operator: "+",
    left: {
      type: "BINARY_OPERATION",
      operator: "*",
      left: { type: "NUMBER", value: 4 },
      right: { type: "NUMBER", value: 2 },
    },
    right: { type: "NUMBER", value: 3 },
  }, ""]);

  assertEquals(evaluate("4 * ( 2 + 3)"), 20);
  assertEquals(expression("4*(2+3)"), [{
    type: "BINARY_OPERATION",
    operator: "*",
    left: { type: "NUMBER", value: 4 },
    right: {
      type: "BINARY_OPERATION",
      operator: "+",
      left: { type: "NUMBER", value: 2 },
      right: { type: "NUMBER", value: 3 },
    },
  }, ""]);

  assertEquals(evaluate("4*5**2+3"), 103);
  assertEquals(expression("4*5**2+3"), [{
    type: "BINARY_OPERATION",
    operator: "+",
    left: {
      type: "BINARY_OPERATION",
      operator: "*",
      left: { type: "NUMBER", value: 4 },
      right: {
        type: "BINARY_OPERATION",
        operator: "**",
        left: {
          type: "NUMBER",
          value: 5,
        },
        right: {
          type: "NUMBER",
          value: 2,
        },
      },
    },
    right: { type: "NUMBER", value: 3 },
  }, ""]);
});