core/Result

Module for error handling with the Result type.

The Result type is used for returning and propagating errors. It has two variants: #ok(Ok), representing success and containing a value, and #err(Err), representing error and containing an error value.

Import from the core library to use this module.

import Result "mo:core/Result";

Type Result

type Result<Ok, Err> = Types.Result<Ok, Err>

The Result type used for returning and propagating errors.

The simplest way of working with Results is to pattern match on them. For example:

import Text "mo:core/Text";

type Email = Text;
type ErrorMessage = Text;

func validateEmail(email : Text) : Result.Result<Email, ErrorMessage> {
  let parts = Text.split(email, #char '@');
  let beforeAt = parts.next();
  let afterAt = parts.next();
  switch (beforeAt, afterAt) {
    case (?local, ?domain) {
      if (local == "") return #err("Username cannot be empty");
      if (not Text.contains(domain, #char '.')) return #err("Invalid domain format");
      #ok(email)
    };
    case _ #err("Email must contain exactly one @ symbol")
  }
};

assert validateEmail("user@example.com") == #ok("user@example.com");
assert validateEmail("invalid.email") == #err("Email must contain exactly one @ symbol");
assert validateEmail("@domain.com") == #err("Username cannot be empty");
assert validateEmail("user@invalid") == #err("Invalid domain format");

Function equal

func equal<Ok, Err>(result1 : Result<Ok, Err>, result2 : Result<Ok, Err>, equalOk : (Ok, Ok) -> Bool, equalErr : (Err, Err) -> Bool) : Bool

Compares two Results for equality.

Example:

import Nat "mo:core/Nat";
import Text "mo:core/Text";

let result1 = #ok 10;
let result2 = #ok 10;
let result3 = #err "error";

assert Result.equal<Nat, Text>(result1, result2, Nat.equal, Text.equal);
assert not Result.equal<Nat, Text>(result1, result3, Nat.equal, Text.equal);

Function compare

func compare<Ok, Err>(result1 : Result<Ok, Err>, result2 : Result<Ok, Err>, compareOk : (Ok, Ok) -> Order.Order, compareErr : (Err, Err) -> Order.Order) : Order.Order

Compares two Result values. #ok is larger than #err. This ordering is arbitrary, but it lets you for example use Results as keys in ordered maps.

Example:

import Nat "mo:core/Nat";
import Text "mo:core/Text";

let result1 = #ok 5;
let result2 = #ok 10;
let result3 = #err "error";

assert Result.compare<Nat, Text>(result1, result2, Nat.compare, Text.compare) == #less;
assert Result.compare<Nat, Text>(result2, result1, Nat.compare, Text.compare) == #greater;
assert Result.compare<Nat, Text>(result1, result3, Nat.compare, Text.compare) == #greater;

Function chain

func chain<Ok1, Ok2, Err>(result : Result<Ok1, Err>, f : Ok1 -> Result<Ok2, Err>) : Result<Ok2, Err>

Allows sequencing of Result values and functions that return Results themselves.

type Result<Ok,Err> = Result.Result<Ok, Err>;
func largerThan10(x : Nat) : Result<Nat, Text> =
  if (x > 10) { #ok(x) } else { #err("Not larger than 10.") };

func smallerThan20(x : Nat) : Result<Nat, Text> =
  if (x < 20) { #ok(x) } else { #err("Not smaller than 20.") };

func between10And20(x : Nat) : Result<Nat, Text> =
  Result.chain(largerThan10(x), smallerThan20);

assert between10And20(15) == #ok(15);
assert between10And20(9) == #err("Not larger than 10.");
assert between10And20(21) == #err("Not smaller than 20.");

Function flatten

func flatten<Ok, Err>(result : Result<Result<Ok, Err>, Err>) : Result<Ok, Err>

Flattens a nested Result.

assert Result.flatten<Nat, Text>(#ok(#ok(10))) == #ok(10);
assert Result.flatten<Nat, Text>(#err("Wrong")) == #err("Wrong");
assert Result.flatten<Nat, Text>(#ok(#err("Wrong"))) == #err("Wrong");

Function mapOk

func mapOk<Ok1, Ok2, Err>(result : Result<Ok1, Err>, f : Ok1 -> Ok2) : Result<Ok2, Err>

Maps the Ok type/value, leaving any Err type/value unchanged.

Example:

let result1 = #ok(42);
let result2 = #err("error");

let doubled1 = Result.mapOk<Nat, Nat, Text>(result1, func x = x * 2);
assert doubled1 == #ok(84);

let doubled2 = Result.mapOk<Nat, Nat, Text>(result2, func x = x * 2);
assert doubled2 == #err("error");

Function mapErr

func mapErr<Ok, Err1, Err2>(result : Result<Ok, Err1>, f : Err1 -> Err2) : Result<Ok, Err2>

Maps the Err type/value, leaving any Ok type/value unchanged.

Example:

let result1 = #ok(42);
let result2 = #err("error");

let mapped1 = Result.mapErr<Nat, Text, Text>(result1, func x = x # "!");
assert mapped1 == #ok(42);

let mapped2 = Result.mapErr<Nat, Text, Text>(result2, func x = x # "!");
assert mapped2 == #err("error!");

Function fromOption

func fromOption<Ok, Err>(x : ?Ok, err : Err) : Result<Ok, Err>

Create a result from an option, including an error value to handle the null case.

assert Result.fromOption(?42, "err") == #ok(42);
assert Result.fromOption(null, "err") == #err("err");

Function toOption

func toOption<Ok, Err>(result : Result<Ok, Err>) : ?Ok

Create an option from a result, turning all #err into null.

assert Result.toOption(#ok(42)) == ?42;
assert Result.toOption(#err("err")) == null;

Function forOk

func forOk<Ok, Err>(result : Result<Ok, Err>, f : Ok -> ())

Applies a function to a successful value and discards the result. Use forOk if you're only interested in the side effect f produces.

var counter : Nat = 0;
Result.forOk<Nat, Text>(#ok(5), func (x : Nat) { counter += x });
assert counter == 5;
Result.forOk<Nat, Text>(#err("Error"), func (x : Nat) { counter += x });
assert counter == 5;

Function forErr

func forErr<Ok, Err>(result : Result<Ok, Err>, f : Err -> ())

Applies a function to an error value and discards the result. Use forErr if you're only interested in the side effect f produces.

var counter : Nat = 0;
Result.forErr<Nat, Text>(#err("Error"), func (x : Text) { counter += 1 });
assert counter == 1;
Result.forErr<Nat, Text>(#ok(5), func (x : Text) { counter += 1 });
assert counter == 1;

Function isOk

func isOk(result : Result<Any, Any>) : Bool

Whether this Result is an #ok.

Example:

assert Result.isOk(#ok(42));
assert not Result.isOk(#err("error"));

Function isErr

func isErr(result : Result<Any, Any>) : Bool

Whether this Result is an #err.

Example:

assert Result.isErr(#err("error"));
assert not Result.isErr(#ok(42));

Function assertOk

func assertOk(result : Result<Any, Any>)

Asserts that its argument is an #ok result, traps otherwise.

Example:

Result.assertOk(#ok(42)); // succeeds
// Result.assertOk(#err("error")); // would trap

Function assertErr

func assertErr(result : Result<Any, Any>)

Asserts that its argument is an #err result, traps otherwise.

Example:

Result.assertErr(#err("error")); // succeeds
// Result.assertErr(#ok(42)); // would trap

Function fromUpper

func fromUpper<Ok, Err>(result : {#Ok : Ok; #Err : Err}) : Result<Ok, Err>

Converts an upper cased #Ok, #Err result type into a lowercased #ok, #err result type. On the IC, a common convention is to use #Ok and #Err as the variants of a result type, but in Motoko, we use #ok and #err instead.

Example:

let upper = #Ok(42);
let lower = Result.fromUpper(upper);
assert lower == #ok(42);

Function toUpper

func toUpper<Ok, Err>(result : Result<Ok, Err>) : {#Ok : Ok; #Err : Err}

Converts a lower cased #ok, #err result type into an upper cased #Ok, #Err result type. On the IC, a common convention is to use #Ok and #Err as the variants of a result type, but in Motoko, we use #ok and #err instead.

Example:

let lower = #ok(42);
let upper = Result.toUpper(lower);
assert upper == #Ok(42);