notry-ts

An abstraction on top of try-catch making exceptions type safe.

Installation

npm install --save notry-ts

Overview

Define functions that accept a Quit parameter specifying how it can fail. Call with notry to handle errors and return a discriminated union type Did prepresenting the result.

Usage

Import the notry function and Quit type.

import { notry, type Quit } from "notry-ts";

quit

Define a function that could fail. Use Quit to specify possible error types and call quit to throw.

/**
 * Return a random number between 0 and max.
 * Could fail with "Bad max" or "Out of range".
 */
function randomUnder(
  quit: Quit<"Bad max" | "Out of range">,
  max: number
): number {
  if (max > 1) {
    quit("Bad max");
  }
  const val = Math.random();
  if (val > max) {
    quit("Out of range");
  }
  return val;
}

notry

Use notry to run a quitable function and return a Did. Note that if the function throws without using quit, it will not be handled by notry.

// call randomUnder(0.9) and log the result
const did = notry(randomUnder, 0.9);
if (did.ok) {
  // did.y: number
  console.log(did.y);
} else {
  // did.n: "Bad max" | "Out of range"
  console.error(did.n);
}

Did

If the function returns normally, did.ok is true and did.y holds the return value. Otherwise if quit is called, did.ok is false and did.n holds the value it was called with.

quit.if

Convenience function for quitting on some condition. May not be appropriate if type narrowing is required.

function randomUnder(
  quit: Quit<"Bad max" | "Out of range">,
  max: number
): number {
  quit.if(max > 1, "Bad max");
  const val = Math.random();
  quit.if(val > max, "Out of range");
  return val;
}

quit.catch

Convenience function to map external exceptions to quit space.

/**
 * Return a random number between 0 and json.max.
 * Could fail with "Bad config" or "Bad max" or "Out of range".
 */
function randomFromJson(
  quit: Quit<"Bad config" | "Bad max" | "Out of range">,
  json: string
): number {
  const obj = quit.catch(JSON.parse, json, "Bad config");
  if (
    typeof obj !== "object" ||
    !obj ||
    !("max" in obj) ||
    typeof obj.max !== "number"
  ) {
    quit("Bad config");
  }
  const { max } = obj;
  quit.if(max > 1, "Bad max");
  const val = Math.random();
  quit.if(val > max, "Out of range");
  return val;
}

// call randomFromJson('{"max":0.9}') and log the result
const did = notry(randomFromJson, '{"max":0.9}');
if (did.ok) {
  // did.y: number
  console.log(did.y);
} else {
  // did.n: "Bad config" | "Bad max" | "Out of range"
  console.error(did.n);
}

nested errors

Split randomFromJson into two functions, either of which could fail.

import { notry, type Of, type Quit } from "notry-ts";

/**
 * Return the value of json.max
 * Could fail with "Bad config".
 */
function maxFromConfig(quit: Quit<"Bad config">, json: string): number {
  const obj = quit.catch(JSON.parse, json, "Bad config") as unknown;
  if (
    typeof obj !== "object" ||
    !obj ||
    !("max" in obj) ||
    typeof obj.max !== "number"
  ) {
    quit("Bad config");
  }
  return obj.max;
}

/**
 * Return a random number between 0 and json.max.
 * Could fail with a maxFromConfig error or "Bad max" or "Out of range".
 */
function randomFromJson(
  quit: Quit<Of<typeof maxFromConfig> | "Bad max" | "Out of range">,
  json: string
): number {
  const max = maxFromConfig(quit, json);
  quit.if(max > 1, "Bad max");
  const val = Math.random();
  quit.if(val > max, "Out of range");
  return val;
}

const did = notry(randomFromJson, '{"max":0.9}');
if (did.ok) {
  console.log(did.y);
} else {
  console.error(did.n);
}

async

notry and quit.catch both support async functions.