Concise overview of Motoko

This is terse, slide-like introduction to Motoko and its features.

(For a gentler introduction, visit the other sections on this site.)

Motivation and Goals

A simple, useful language for the Internet Computer (IC)

Familiar syntax
Safe by default
Incorporating actor model for canister smart contracts
Seamless integration of Internet Computer features
Making most of present and future WebAssembly

Key Design Points

Object-oriented, functional & imperative
Objects as records of functions
async/await for sequential programming of asynchronous messaging
Structural typing with simple generics and subtyping
Safe arithmetic (both unbounded and checked)
Non-nullable types by default
Garbage collected (no manual memory management)
JavaScript-like syntax but statically typed & sane

Inspirations: Java, JavaScript, C#, Swift, Pony, ML, Haskell

Semantics

call-by-value (like Java, C, JS, ML; unlike Haskell)
declarations are locally mutually recursive
parametric, bounded polymorphism
subtyping as zero-cost subsumption, not coercion
no dynamic casts
no inheritance

Implementation(s)

implemented in OCaml (leverages wasm libary)
simple reference interpreter
less simple compiler to WebAssembly
- multipass with typed IR in each pass.
- uniform representation, unboxed arithmetic
- copying GC, compacting GC, or generational GC (select which with compiler flag)
- GC invoked after messages (for now)
polymorphism by erasure

The language

Expressions

Identifiers:
x, foo_bar, test123, List, Map
Parentheses ( … ) for grouping
Braces { … } for scoping (and records)
; for sequencing
Type annotations (to help type inference):
(42 : Int)
(zero cost)

Libraries

import Debug "mo:base/Debug";
import Int "mo:base/Int";

(import MyLib "src/MyLib" imports a library from the local file system.)

Specific bindings can be imported from the module using object patterns

import { push; nil } = "mo:base/List";

Libraries continued

import Debug "mo:base/Debug";
import Int "mo:base/Int";
import Trie "mo:base/Trie";

type Users = Trie.Trie<Text, Nat>; // reference types

Debug.print(Int.toText(7)); // reference functions/values

Blocks and declarations

type Delta = Nat;
func print() {
  Debug.print(Int.toText(counter));
};
let d : Delta = 42;
var counter = 1;
counter := counter + d;
print();

Semicolon after each declaration!
Mutually recursive
Mutable variables marked explicitly

Control flow

The usual suspects…

do { … }
if b …
if b … else …
switch e { case pat1 e1; …; case _ en }
while b …
loop …
loop … while b
for (pat in e) …
return, return e
label l e, break l e
do ? { … e! … }
async e, await e (restricted)
throw, try … catch x { … } (restricted)

Primitive types

Unbounded integers

Int

{ …, -2, 1, 0, 1, 2, … }

Inferred by default for negative literals.

Literals: 13, 0xf4, -20, +1, 1_000_000

Unbounded naturals

Nat

{ 0, 1, 2, … }

Non-negative, trap on underflow.

Inferred by default for non-negative literals

Literals: 13, 0xf4, 1_000_000

Nat <: Int

Nat is a subtype of Int

(you can supply a Nat wherever an Int is expected)

Bounded numbers (trapping)

Nat8, Nat16, Nat32, Nat64, Int8, Int16, Int32, Int64

Trap on over- and underflow; wrap-around and bit-manipulating operations available separately

Needs type annotations (somewhere)

Literals: 13, 0xf4, -20, 1_000_000

Floating point numbers

Float

IEEE 754 double precision (64 bit) semantics, normalized NaN

Inferred for fractional literals

Literals: 0, -10, 2.71, -0.3e+15, 3.141_592_653_589_793_12

Numeric operations

No surprises here

- x
a + b
a % b
a & b
a << b
…

a +% b, a -% b, … for wrapping, modular arithmetic (where appropriate)

Characters and Text

Char, Text

Unicode! Character = Unicode scalar value; no random access on text

'x', '\u{6a}', '☃',
"boo", "foo \u{62}ar ☃"
"Concat" # "enation"

Booleans

Bool

Literals: true, false

a or b
a and b
not b
if (b) e1 else e2

Functions

Function types

Simple functions:
```
Int.toText : Int -> Text
```
multiple arguments and return values
```
divRem : (Int, Int) -> (Int, Int)
```

can be generic/polymorphic

Option.unwrapOr : <T>(?T, default : T) -> T

first-class (can be passed around, stored)

map : <A, B>(f : A -> B, xs : [A]) -> [B]
let funcs : [<T>(T) -> T] = …

Function Declarations & Use

func add(x : Int, y : Int) : Int = x + y;

func applyNTimes<T>(n : Int, x : T, f : T -> ()) {
  if (n <= 0) return;
  f(x);
  applyNTimes(n-1, x, f);
};

applyNTimes<Text>(3, "Hello!", func(x) { Debug.print(x) } );

func() { … } short for func() : () = { … }
Parametric functions
Type instantiations may sometimes be omitted
Anonymous functions (a.k.a. lambdas)

Composite types

Tuples

(Bool, Float, Text)

immutable, heterogeneous, fixed size

let tuple = (true or false, 0.6 * 2.0, "foo" # "bar");

tuple.1;

let (_,_,t) = tuple;
t

Options

?Text

is either a value of that type, e.g. ?"hello", or null.

func display(x : ?Text) : Text {
  switch x {
    case (null) { "No value" };
    case (?y) { "Value: " # y };
  };
};

(display(null), display(?"Test"))

Option blocks

Switching on every option value can be inconvenient …

The option block, do ? { … }, allow you to safely access option values with a postfix null break ! expression.

Within do ? { … }, which returns an option, the expression e! immediately exits the block with null when the value of option e is null or continues with the option’s contents.

func add(x : ?Nat, y: ?Nat) : ?Nat {
  do ? { x! + y! };
};

(add(null, null), add (?1,null), add (?1,?2), add (null,?2));

Arrays (immutable)

[Text]

let days = [ "Mon", "Tue", "Wed", "Thu", "Fri", "Sat", "Sun" ];

assert(days.size() == 7);

assert(days[1] == "Tue");

// days[7] will trap (fixed size)

for (d in days.vals()) { Debug.print(d) };

Arrays (mutable)

[var Nat]

let counters = [var 1, 2, 3];

assert(counters.size() == 3);

counters[1] := counters[1] + 1;

// counters[3] will trap (fixed size)

counters;

Records

{first : Text; last : Text; salary : var Nat}

let employee = {first = "John"; last = "Doe"; var salary = 81_932};

Debug.print(
  employee.first # " " # employee.last # " earns " #
    Int.toText(employee.salary) # " pounds."
);

employee.salary += 79_496;

employee;

Objects

{first : Text; last : Text; get : () → Nat; add : Nat → ()}

object self {
  public let first = "John";
  public let last = "Doe";
  var salary : Nat = 81_932; // private by default
  public func get() : Nat = salary;
  public func add(bump : Nat) { salary += bump };
}

Classes

class Employee(fst : Text, lst : Text) {
  public let first = fst;
  public let last = lst;
  var salary : Nat = 0;
  public func get() : Nat = salary;
  public func add(bump : Nat) { salary += bump };
}

Classes are factories for constructing objects.
A class introduces a type and a function (for constructing instances).

Just sugar for:

type Employee = {first : Text; last : Text; get : () -> Nat; add : Nat -> ()};

func Employee(fst : Text, lst : Text) : Employee = object { … }

Variants

{#sun; #mon; #tue; #wed; #thu; #fri; #sat}

type Day = {#sun; #mon; #tue; #wed; #thu; #fri; #sat};

func toText(d : Day) : Text {
  switch d {
     case (#sun) "Sunday";
     case (#mon) "Monday";
     case (#tue) "Tuesday";
     case (#wed) "Wednesday";
     case (#thu) "Thursday";
     case (#fri) "Friday";
     case (#sat) "Saturday";
   };
};

func sort(d : Day) : { #weekDay; #weekEnd } {
  switch d {
    case (#sun or #sat) #weekEnd;  // or pattern
    case _ #weekDay;  // wildcard pattern
  };
};

Recursive Types

type List = {
  #item : {head : Text; tail : List}; // variant with payload!
  #empty                     // ^^^^ recursion!
};

func reverse(l : List) : List {
  func rev(l : List, r : List) : List {
    switch l {
      case (#empty) { r };
      case (#item { head; tail }) { // nested patterns
        rev(tail, #item {head; tail = r})
      }
    }
  };
  rev(l, #empty);
};

let l = reverse(#item {head = "A"; tail = #item {head = "B"; tail = #empty}});

Generic types

type List<T> = {
  #item : {head : T; tail : List<T>};
  #empty
};

func reverse<T>(l : List<T>) : List<T> {
  func rev(l : List<T>, r : List<T>) : List<T> {
    switch l {
      case (#empty) { r };
      case (#item { head; tail }) { // a nested pattern
        rev(tail, #item {head; tail = r})
      }
    }
  };
  rev(l, #empty);
};

let s : List<Text> =
  reverse(#item {head = "A"; tail = #item {head = "B"; tail = #empty}});

let ns : List<Nat> =
  reverse(#item {head = 0; tail = #item {head = 1; tail = #empty}})

Packages and modules

Modules

// the type of base/Int.mo
module {
  type Int = Prim.Types.Int;
  toText : Int -> Text;
  abs : Int -> Nat;
  // ...
}

modules contain named types and values (like objects),
but are restricted to static content (pure, no state, …)

Module imports

import Debug "mo:base/Debug";  // import from package
import Int "mo:base/Int";
import MyLib "lib/MyLib";  // import from local file MyLib.mo

base package provides basic features as separate modules.

More libraries popping up!

MyLib.mo must contain a module or actor class, eg:

module {
  public type List<T> = …;

  public func reverse<T>(l : List<T>) : List<T> { … };
}

Platform features

Actor types

Like object types, but marked as actor:

type Broadcast = actor {
  register : Receiver -> ();
  send : Text -> async Nat;
};

type Receiver = actor {
  recv : query Text -> async Nat
};

sharable arguments and no or async result type.

register is a oneway IC method (unawaitable).
send is an IC update method
recv is IC query method

IC canister with Candid interface ≈ Motoko actor

sharable ≈ serializable

Sharable:

all primitive types
records, tuples, arrays, variants, options
with immutable sharable components
actor types
shared function type

Not sharable:

mutable things
local functions
objects (with methods)

A complete actor

import Array "mo:base/Array";

actor Broadcast {
  type Receiver = actor {recv : query Text -> async Nat};

  var r : [Receiver] = [];

  public func register(a : Receiver) {
    r := Array.append(r, [a]);
  };

  public func send(t : Text) : async Nat {
    var sum = 0;
    for (a in r.vals()) {
      sum += await a.recv(t);
    };
    return sum;
  };
}

a typical actor/canister main file

Async/await

async T

asychronous future or promise

introduced by async { … }
(implicit in async function declaration)

await e
suspends computation pending e’s result:
if the result is a value, continues with that value,
if the result is an Error, throws the error.

  public func send(t : Text) : async Nat {
    var sum = 0;
    for (a in r.vals()) {
      sum += await a.recv(t); // may return Nat or `throw` error
    };
    return sum;
  };

(Errors can be handled using try … catch …)

Concurrency Hazards

Functions that await are not atomic.
Suspension introduces concurrency hazards.

A bad implementation of send:

  var sum = 0; // shared state!
  public func send(t : Text) : async Nat {
    sum := 0;
    for (a in r.vals()) {
      sum += await a.recv(t);
    };
    return sum;
  };

(Concurrent sends will share and clobber sum.)

Beware of race conditions!

Actor import

import Broadcast "canister:Broadcast";
/* or
import Broadcast "ic:r7inp-6aaaa-aaaaa-aaabq-cai";
*/
actor Self {

  var count = 0;

  public func go() {
    Broadcast.register(Self);
  };

  public query func recv(msg : Text) : async Nat {
    return count;
  }
}

(assumes there is a Candid file describing the interface of the import)

A Candid interface file

Broadcast's Candid file (produced by moc --idl Broadcast.mo compiler).

Broadcast.did:

type Receiver =
 service {
   recv: (text) -> (nat) query;
 };
service : {
  register: (Receiver) -> () oneway;
  send: (text) -> (nat);
}

A language independent interface definition.

Could just as easily describe a Rust implementation of Broadcast.

Principal and caller

import Principal "mo:base/Principal";

actor Self {

  public shared(context) func hello() : async Text {
    let myself : Principal = Principal.fromActor(Self);
    if (context.caller == myself) {
      "Talking to yourself is the first sign of madness";
    } else {
      "Hello, nice to see you";
    };
  };

}

Errors

import Principal "mo:base/Principal";
import Error "mo:base/Error";

actor Self {

  public shared(context) func hello() : async Text {
    let myself : Principal = Principal.fromActor(Self);
    if (context.caller == myself) {
      throw Error.reject("Talking to yourself is the first sign of madness");
    } else {
      "Hello, nice to see you";
    };
  };

};

async {
  let t = try Self.hello() catch (e) { Error.message(e); }
};

Similar to exceptions in other languages,
but only available in async contexts, e.g. shared functions; async blocks

Stable variables

If we upgrade the Broadcast actor, all current registrations are lost.
To preserve them, declare the state variable r as stable.

import Array "mo:base/Array";

actor Broadcast {

  type Receiver = actor {recv : query Text -> async Nat};

  stable var r : [Receiver] = []; // declare r `stable`

  public func register(a : Receiver) { … }
  public func send(t : Text) : async Nat { … }

  // optional pre-upgrade action
  system func preupgrade() { Debug.print("saving receivers"); }

  // optional post-upgrade action
  system func postupgrade() {  Debug.print("restoring receivers"); }
}

stable variables must have stable types (see manual)
system hooks can’t send messages

Type system

Structural

/*
type List = {
  #item : {head : Text; tail : List};
  #empty
};

func reverse(l : List) : List { //... };
*/
type Stack = {
   #empty;
   #item : {tail : Stack; head : Text};
};

let stack : Stack = #empty;

let revStack = reverse(stack); // works though reverse defined on List (not Stack)

Type definitions
do not create types,
but name existing types

Despite their different names, Stack and List are equivalent types.

Subtyping (Variants)

WeekDay <: Day

type WeekDay = {#mon; #tue; #wed; #thu; #fri};

type Day = {#sun; #mon; #tue; #wed; #thu; #fri; #sat};

func toText(d : Day) : Text {
  switch d
   { case (#sun) "Sunday";
     case (#mon) "Monday";
     //...
   };
};

let mon : WeekDay = #mon;
let t = toText(mon); // also works, since WeekDay <: Day

t1 <: t2: t1 can be used wherever t2 is expected

Employee <: Person

type Employee = {first : Text; last : Text; var salary : Nat};
type Person = {first : Text; last : Text};

func toText(p : Person) : Text {
  p.last # "," # p.first;
};

let employee : Employee =
  { first = "John"; last = "Doe"; var salary = 161_401};

let t = toText(employee); // also works, since Employee <: Person

Fin

Not covered

Polymorphic functions with type bounds
User defined iterator objects, supporting for loops.
Actor classes
debug_show for conversion of almost any value to text.
debug e expressions for debug-only compilation
do ? { … e! … } blocks for handling/propagating option values.
assert e expressions for conditional traps
tools:
- mo_doc (generates doc from doc comments),
- vessel (package manager)
- mo_ide (LSP language server for VSCode, emacs etc)

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