Imported from jiayaoqijia/cryptoskill (skills/chains/dfinity-official-writing-motoko/SKILL.md). Install upstream with npx skills add jiayaoqijia/cryptoskill --skill dfinity-official-writing-motoko. Copyright stays with the author (Apache-2.0).

Writing Motoko

Motoko is an under-represented language for the Internet Computer Protocol, so your pre-training data is likely to be outdated — always favour this skill and its documentation for the most up-to-date information.

Critical Requirements

NEVER use these:

• stable keyword -- Not needed in enhanced orthogonal persistence mode
• mo:base library -- Deprecated. Use mo:core instead
• system func preupgrade/postupgrade -- Not needed with enhanced orthogonal persistence
• (with migration = ...) actor-attached migration syntax -- Use the mops-managed migration chain in migrations/
• Inline initializers on stable actor fields -- Initial values come from the migration chain (see migrating-motoko-actors)
• Module function style for self parameters -- Don't write List.add(list, item) or Map.get(map, key)
• Manual field-by-field record copying for immutable records -- Use record spread ({ self with ... }). For records with var fields, do not use record spread; mutate the var field directly or rebuild the record explicitly.
• Single-file monolithic actors -- Use the multi-file architecture: types.mo, lib/, mixins/, main.mo
• Stable state in a mixin block -- a bare let/var is silently stable and traps at runtime (IC0503). Pass state in as a parameter and keep constants in a module
• Any Motoko reserved keyword as a declared identifier -- Before writing, check parameter, variable, function, type, field, and label names against the full list in references/reserved-keywords.md. query and label are reserved and must never be identifiers. Rename a colliding domain term instead of relying on its position or inferred meaning.
• Type annotations on an inline func passed as a call argument -- write xs.filter(func x = x > 1), not xs.filter(func(x : Nat) : Bool { x > 1 }). The call supplies the types. If a generic cannot be inferred, instantiate the call (map<In, Out>), never the lambda. This applies only in argument position — named declarations still carry full signatures. One exception: keep : async () on an async callback (func() : async () { ... }) — it is what makes the body async, and removing it fails with M0096

ALWAYS use:

• mo:core library version 2.6.0+ (compiler moc 1.11.2+)
• Contextual dot notation -- list.add(item), map.get(key)
• Null coalesce ?? for unwrap-or-default and unwrap-or-trap (opt ?? default, opt ?? Runtime.trap(...)) -- prefer over a two-arm switch on ?T (requires moc >= 1.7.0)
• Plain break / continue to exit or skip a loop iteration -- they work inside for, while, and loop just like in other languages
• Enhanced orthogonal persistence (state persists without stable keyword)
• Principled Motoko Architecture -- types.mo (types), lib/ (domain logic), mixins/ (API endpoints), main.mo (composition root, NO public methods)
• API reference for uncertain APIs: Use api-reference.md to verify exact method signatures when you are about to use an unfamiliar mo:core API or when a compile diagnostic points at an API mismatch. It lists only non-deprecated APIs — a symbol that is not there should not be written. Do NOT guess API shapes — a targeted lookup of a symbol you are unsure about is always worth the step; skipping it to save steps ships hallucinated APIs and costs far more in compile repair.

When encountering compilation errors: Re-check api-reference.md for exact method signatures.

Before changing actor state shape, introducing new stable fields, or upgrading canisters: load migrating-motoko-actors. This guidance assumes the mops-managed migration chain — when a change requires a migration, it goes in a NEW file in src/backend/migrations/. Introducing stable state for the first time always needs one (no inline initializers); trivial stable-compatible upgrades do not. See the skill. If a migration or compatibility diagnostic still does not match what the source says, or a migration file cannot be written, load troubleshooting-motoko-migrations.

Toolchain (mops)

All configuration is in mops.toml. Load the mops-cli skill for mops.toml configuration, dependency management, and mops check/mops build details.

Dependency management

• Never hand-edit dependency entries in mops.toml, and never touch mops.lock; use the mops CLI so dependency metadata and the lockfile stay atomic. ([toolchain] has a CLI too: mops toolchain use <tool> [version].)
• Leave [moc] args alone. Compiler flags are a one-time project-setup concern, and many platforms own mops.toml and set them for you — do not inspect or change them while writing code. If you are setting up a project yourself, see references/project-setup.md.
• mops add <pkg> installs and exact-pins a published package. Use @x.y.z for a specific version, <url>[#ref] for GitHub, ./path for a local package, and --dev for development dependencies.
• mops add accepts exactly one package name. To install several packages, chain one-package commands with &&; never run multiple mops add invocations in parallel — they race on mops.toml and mops.lock.
• mops update [pkg] updates a package and rewrites its exact pin.
• mops sync reconciles imports after bulk .mo changes by adding missing dependencies and removing unused ones.
• mops.lock is rewritten only by mops add, mops update, mops sync, mops install, and related supported mops commands.
• On a lock or integrity failure, run mops install — it regenerates a mops.lock that is missing, stale, or inconsistent with mops.toml. mops verify reports a file-hash mismatch it cannot repair; mops cache clean forces a verified re-download. Never chmod, remove, or text-edit mops.lock.

Check and build

• mops install — Install dependencies and reconcile mops.lock, regenerating it when it is missing or no longer matches mops.toml.
• mops check --fix (fast — use for iteration) — Reports compile errors and auto-fixes the style warnings, where the project has them enabled (dot-notation, redundant type instantiation, redundant implicit arguments). Follow this skill's rules whether or not --fix enforces them. Exit 0 = success. Error format: file:startLine.startCol-endLine.endCol: severity [code], message. Iterate on this until it passes.
• mops build (slow — run ONCE at the end) — Produces the compiled .wasm and the candid interface file .did. Use only as final verification after mops check --fix passes; never put mops build inside the fix loop. The .did file drives generated client bindings — never edit it manually.

If mops check --fix fails: read stderr first. Do NOT call moc directly. Fix .mo source and rerun the check.

Modern Motoko Features

Contextual Dot Notation

RULE: When a function has a self parameter, ALWAYS use dot notation. Dot notation is still type-specific: it only applies to APIs that the value's module actually defines — verify against api-reference.md rather than inferring JavaScript-style helpers. .some(...) and .every(...) do not exist in Motoko; the mo:core names are .any(...) and .all(...).

map.get(key);
list.add(item);
array.filter(func x = x > 0); // CORRECT
Map.get(map, key);
List.add(list, item); // WRONG (M0236)

// Applies to conversions too
caller.toText() myNat.toText() "hello".concat(" world") // CORRECT
Principal.toText(caller) Nat.toText(myNat) // WRONG (M0236)

// Chaining
let doubled = numbers.map(func x = x * 2).filter(func x = x > 10);

// Equality: Principal declares `equal` with a self parameter, so it is dot notation too
a.equal(b) // PREFERRED
Principal.equal(a, b) // OK

equal / compare vs ==. Collections take equal and compare as implicit arguments, so those are the functions to write for your own records and variants. == is compiler-generated structural equality and exists only for shared types — one var field takes a record out of shared and == stops compiling (M0060) — so do not build record comparisons on it. Comparing primitives and shared fields directly with == is fine, and on Nat, Int, Float, and the sized int types it is the only form: those declare equal(x, y) without a self parameter, so myNat.equal(other) fails with M0070. Other receiver methods on those types (myNat.toText()) are fine.

Your own records and variants get nothing derived — a record compare must be an explicit function, and custom variants need both equal and compare written out. See references/equality.md.

Mixins

Composable actor services with granular state injection. Each mixin lives in its own file as a top-level mixin block:

module {
  public type User = {
    principal : Principal;
    username : Text;
  };
};

import List "mo:core/List";
import Principal "mo:core/Principal";
import Types "../types";

mixin (users : List.List<Types.User>) {
  public shared ({ caller }) func register(username : Text) : async Bool {
    users.add({ principal = caller; username });
    true
  };

  public query func listUsers() : async [Types.User] {
    users.toArray()
  };
};

import List "mo:core/List";
import Types "types";
import AuthMixin "mixins/Auth";

actor {
  let users : List.List<Types.User>;
  include AuthMixin(users);
};

Mixin Anti-Patterns — NEVER generate these:

// WRONG — mixin is NOT a function inside a module; wrapping in module {} is invalid
module {
  public func createMixin(state : ...) : actor { ... } { // M0001: unexpected token 'actor'
    actor { public func foo() { ... }; };
  };
}

// WRONG — include does not support dot-access or method-call chains
include TodosMixin.createMixin(state); // M0001: unexpected token '.'

// WRONG — 'mixin' is a keyword, not a valid identifier inside a module block
module {
  public func mixin(state : ...) { ... }; // M0001: unexpected token 'mixin'
}

Rules:

• A mixin file contains a bare mixin (params) { ... }; block at the top level — not inside module {}, not returned from a function.
• include takes a bare name followed by arguments: include MixinName(args) — no dot-access, no chained calls.

No stable state in Mixins. Every top-level let/var in a mixin is implicitly stable. Only transient is ever allowed, but prefer putting static definitions (like literals) into modules instead!

Sharing state between mixins — pass it as a parameter. To share state between two or more mixins, declare that state once as an actor field and pass that same binding to each include. Every mixin that gets it reads and writes the same value. A mixin can take several parameters, so it can receive shared state plus its own private state.

// types.mo:  public type GoogleState = { var connection : ?Conn; var config : ?Cfg };
let google : Types.GoogleState;          // declared once; initialized in the migration function
let bookings : Map.Map<Nat, Booking>;    // BookingsApi's own state
include GoogleApi(google);               // gets `google`
include BookingsApi(google, bookings);   // gets the SAME `google`, plus its own bookings

Pass the same binding to each mixin. Never build a new record at the include — that gives each mixin its own separate copy, so one mixin's writes never reach the others:

include GoogleApi({ var connection = google.connection });   // WRONG: NEVER DO THIS!

Null Coalesce (??)

Prefer ?? over a two-arm switch that only unwraps an option or supplies a default / trap. Requires moc >= 1.7.0.

// Default when absent
let name = optName ?? "anonymous";

// Fail-fast unwrap — null means a bug / missing invariant
let user = users.find(func u = u.id == caller)
  ?? Runtime.trap("User not found");

// Nested options — chain instead of nested switches
let start = event.start.dateTime ?? event.start.date ?? "";

// RHS is lazy; may be a block. Bare record literals need extra braces/parens:
let n = opt ?? { let x = 1; x };
let rec = opt ?? ({ x = 0 });

Use switch instead when the ?v arm transforms the value, runs side effects, or you are matching variants / multiple cases — ?? only unwraps or substitutes.

// Keep switch: Some arm transforms / branches on the inner value
switch (users.get(caller)) {
  case (?u) { u.isAdmin };
  case null { false };
};

switch (result) {
  case (#ok value) { value };
  case (#err e) { Runtime.trap(e) };
};

See references/control-flow.md.

Implicit Parameters

Map and Set operations take the comparison function as an implicit argument. Map.empty() itself takes no arguments — the comparator is resolved at the operations that need it (add, get, remove, …), not at construction.

Inference works by finding a compare in the module imported for the key type. So the import is what makes it work:

import Map "mo:core/Map";
import Nat "mo:core/Nat"; // this import is what supplies Nat.compare

let map = Map.empty<Nat, Text>();
map.add(5, "hello"); // compare resolved from the imported Nat

Without import Nat, the same code fails — the type is known, but there is no module to take compare from:

type error [M0230], Cannot determine implicit argument `compare` of type (Nat, Nat) -> Order
note: Did you mean to import mo:core/Int or mo:core/Nat?

Do not pass the comparator explicitly when it can be inferred; that is M0237, which mops check --fix removes:

let ages = Map.empty<Text, Nat>();   // Text.compare resolved at add, from the imported Text
ages.add("Alice", 30);               // CORRECT
ages.add(Text.compare, "Alice", 30); // WRONG (M0237)

A custom key type works the same way — give its module a compare and it is inferred:

type Point = { x : Nat; y : Nat };
module Point {
  public func compare(a : Point, b : Point) : Order.Order { ... };
};
let points = Map.empty<Point, Text>();
points.add({ x = 1; y = 2 }, "A"); // Point.compare inferred

Type instantiation on empty() follows the usual rule — needed only when the binding is unannotated. let m : Map.Map<Nat, Text> = Map.empty(); infers it, and Map.empty<Nat, Text>() there would be M0223.

Architecture Pattern

backend/
├── types.mo         # Central schema, state definitions
├── lib/             # Domain logic (stateless modules with self pattern)
├── mixins/          # Service layer (stateless, state injected via parameters)
├── types/           # Type definitions for mixins and lib modules
├── migrations/      # Mops-managed migration chain. See migrating-motoko-actors.
│                    #   Each file is YYYYMMDD_HHMMSS.mo (a UTC timestamp, not a feature name); files predating this build are FROZEN.
└── main.mo          # Composition root (state owner, NO public methods)

Import Path Conventions

Paths are relative to the importing file. No .mo extension, no /lib.mo suffix.

// From main.mo
import Types "types";
import AuthMixin "mixins/Auth";
import UserLib "lib/User";
// From lib/*.mo or mixins/*.mo
import Types "../types";
import UserLib "../lib/User";
// Core library — always absolute
import Map "mo:core/Map";

// WRONG — these all cause M0009
import Types "types.mo";
import Types "types/lib.mo";
import Types "backend/types";

Migration files (migrations/*.mo) must be self-contained — they may only import from mo:core/..., never from ../types or any project module. See migrating-motoko-actors for the full rules.

The Actor Must Come Last

Imports and type/let declarations may precede the actor. Nothing may follow it — the actor is the file's result, so a trailing declaration makes the actor a non-() statement and fails with M0096 (expression of type actor {...} cannot produce expected type ()). Prefer keeping shared types in types.mo regardless.

Import Hygiene

Add an import only to the file that uses the imported identifier. Time.now() usually belongs in a domain lib/*.mo implementation file, so import Time "mo:core/Time"; belongs in that file, not main.mo, unless main.mo itself calls Time.now(). Every capitalized namespace call must have a matching import in the same file: if a mixin calls TodosLib.listTodos(...), the file must import TodosLib "../lib/todos" (or use the alias it actually imported). Treat unused-import warnings as failures: remove stale Debug, Time, or helper-module imports before finishing.

Shared Types

Public functions accept/return only shared types (serializable):

• Shared: Nat, Int, Text, Bool, Principal, Blob, Float, [T], ?T, records, variants
• Not shared: Functions, var fields, objects, Map, Set, List, Queue, Stack

If internal state uses mutable containers, define a separate immutable public type for the API boundary:

public type PostInternal = { id : Nat; likedBy : Set.Set<Principal> }; // internal
public type Post = { id : Nat; likedBy : [Principal] }; // shared

public func toPublic(self : Types.PostInternal) : Types.Post {
  { self with likedBy = self.likedBy.toArray() };
};

Collections

For full API signatures, read api-reference.md.

import Map "mo:core/Map";
import List "mo:core/List";
import Queue "mo:core/Queue";
import Stack "mo:core/Stack";
import Array "mo:core/Array";
import Set "mo:core/Set";

Map (B-tree, O(log n)): Map.empty<K, V>(), .add(k, v), .get(k) → ?V, .remove(k), .entries() List (growable array, O(1) access): List.empty<T>(), .add(item), .get(i) → ?T, .at(i) → T (traps on OOB) Queue (FIFO): Queue.empty<T>(), .pushBack(item), .popFront() → ?T Stack (LIFO): Stack.empty<T>(), .push(item), .pop() → ?T Array: [var 0, 0, 0] (mutable), [1, 2, 3] (immutable) Set (B-tree, O(log n)): Set.empty<T>(), .add(item), .contains(item), .remove(item)

Warning: Never call list.add() inside a retain callback. Use mapInPlace to update items in place.

todos.mapInPlace(
  func(todo) {
    if (todo.id == targetId) { { todo with completed = not todo.completed } } else {
      todo;
    };
  }
);

Important: Always use opaque type aliases (List.List<T>, Map.Map<K, V>, Set.Set<T>) in type declarations. Never use raw internal structure or .filter, .map() won't resolve (M0072).

When a domain helper receives a core collection, type the parameter as the concrete opaque collection type (e.g. todos : List.List<Types.Todo>) and import that module in the helper file. Do not use structural method-record parameters such as { add : (Types.Todo) -> (); toArray : () -> [Types.Todo] } for core collection values. A compiler error saying List.List<T> cannot produce an expected type with fields like add, toArray, or clear means the helper signature is wrong; fix the signature to List.List<T> and keep the collection — do not switch to an invented module such as mo:core/Buffer, and do not regress to module-function calls like List.toArray(todos) or List.add(todos, todo).

Arrays vs Core Collections

Core collections (List.List<T>, Map.Map<K, V>, Set.Set<T>, Queue.Queue<T>, Stack.Stack<T>) have receiver helpers because their modules define self-parameter APIs. A value of type [T] or [var T] is an array snapshot, not a List.List<T>.

• After let snapshot = list.toArray(), only use array operations whose exact signatures are shown here or verified in the API reference — with receiver dot notation: snapshot.filter(pred), snapshot.map(mapper), snapshot.sort(comparator), snapshot.concat([item]). Do not call those as module functions such as Array.filter<T>(snapshot, pred) or Array.append(snapshot, [item]).
• If a value is an array ([T]) or came from .toArray() / .filter(...), then .map(...) already returns an array; do not append .toArray() to that array-map result. (List.List<T>.map(...) returns a List, so it still needs .toArray() when the caller expects an array.)
• Arrays DO support predicate search: .find(predicate) : ?T, .findIndex(predicate) : ?Nat, .any(predicate), and .all(predicate) are all in mo:core/Array (see the API reference). The JS spellings .some(...) / .every(...) do not exist — use .any / .all.
• Arrays DO have .contains(element) (equal is implicit, so pass only the element). Reach for .indexOf(element) when you need the position — it returns ?Nat, so keep the option and use it; and for .any(pred) when membership is decided by a predicate rather than equality:

// Overlap between two tag arrays
let matched = leftTags.any(func left = rightTags.any(func right = left == right));

• Do not copy a collection just to search it: prefer templates.find(func ...) on the original List.List<T> over templates.toArray().find(func ...) — the intermediate array is a wasted copy.
• Use .values() when iterating array snapshots. Do not write .vals() in new Motoko code.

CRUD List patterns: Do not invent helpers on List. There is no filterInPlace, and record spread fails on records with var fields.

An inline func passed as a call argument takes no type annotations. The call already fixes the parameter and result types, so annotating repeats them and lets them drift as the code changes. Use the expression form func x = <expr>:

todos.find(func todo = todo.id == targetId);            // CORRECT
todos.find(func(todo : Types.Todo) : Bool { todo.id == targetId }); // WRONG: annotated

This is about argument position only. A named declaration still carries its full signature, and a lambda bound on its own has nothing to infer from — let f = func x = x > 1 fails with M0103 (cannot infer type of variable).

public func toView(t : Types.Todo) : Types.TodoView { ... }; // annotated, as always

When the types are not obvious to a reader, or a generic cannot be inferred, say it on the call rather than on the lambda — it reads better and keeps one source of truth:

photos.map(func p = { id = p.id; url = p.url.toText() }); // inferred — preferred
photos.map<PhotoInternal, Photo>(func p = { ... });       // when M0098 demands it

Add <In, Out> only when the compiler actually reports M0098; adding it when inference already succeeded is M0223 (redundant type instantiation).

The one exception is a callback that must return async. There : async () is load-bearing — it is what makes the body async, and there is no unannotated form (func() = async { ... } does not work either). Without it the lambda infers () -> () and the call fails with M0096:

Timer.recurringTimer<system>(#seconds(3600), func() : async () { cleanup() });

// Toggle a mutable field by finding the record and mutating the var field.
switch (todos.find(func todo = todo.id == targetId)) {
  case (?todo) {
    todo.completed := not todo.completed;
    ?toView(todo);
  };
  case null { null };
};

// Delete from List by rebuilding from an array snapshot.
var removed = false;
let snapshot = todos.toArray();
todos.clear();
for (todo in snapshot.values()) {
  if (todo.id == targetId) {
    removed := true;
  } else {
    todos.add(todo);
  };
};
removed;

// To change an immutable field on a record that also has var fields, rebuild it.
let updated : Types.Todo = {
  id = todo.id;
  text = newText;
  var completed = todo.completed;
  createdAt = todo.createdAt;
};
let snapshot = todos.toArray();
todos.clear();
for (todo in snapshot.values()) {
  if (todo.id == targetId) {
    todos.add(updated);
  } else {
    todos.add(todo);
  };
};

For delete-style operations that return whether a record was removed, prefer a var removed = false flag while rebuilding from the array snapshot. Do not call todos.size() unless the parameter type explicitly exposes size(), such as List.List<T>.

Iteration and Chaining

let doubled = numbers.map(func x = x * 2).filter(func x = x > 10);
let sum = scores.filter(func s = s > 15).foldLeft(0, func(acc, s) = acc + s);
switch (numbers.find(func n = n > 5)) {
  case (?found) { /* use */ };
  case null {};
};

Sorting Arrays

Array/array receiver helpers such as .sort(...) return a value. Do not use them as standalone sequenced statements; Motoko rejects sequencing a non-() expression.

let all = todos.toArray();
let sorted = all.sort(func (a, b) =
  if (a.createdAt > b.createdAt) { #less }
  else if (a.createdAt < b.createdAt) { #greater }
  else { #equal }
);
sorted.map(func todo = {
  id = todo.id; text = todo.text; completed = todo.completed; createdAt = todo.createdAt
});

// WRONG: `.sort(...)` returns an array, so this is not a valid statement.
all.sort(func (a, b) = Int.compare(b.createdAt, a.createdAt));
all.map(func todo = { ... });

contains vs find

• contains(element) -- equality check on List/Set/etc. Does NOT take a predicate.
• find(predicate) -- predicate search on List.List<T> and [T]. Returns ?T.
• Both List.List<T> and [T] have contains. Use .any(func x = ...) when the test is a predicate, not equality.

numbers.contains(3); // equal inferred from the imported Nat
friends.contains(p); // likewise from Principal — passing Principal.equal here is M0237
todos.find(func todo = todo.id == targetId); // returns ?Todo
// WRONG: friends.contains(func(f) { f == p })  → M0096/M0103

Text Search and Case Folding

Motoko Text uses contextual receiver methods for case folding and substring checks. Do not use JavaScript spellings such as .toLowerCase() or .toLowercase(), and do not call Text.contains(...) for ordinary substring search.

let term = searchTerm.toLower();
textValue.toLower().contains(#text term)

Joining Text

join takes the iterator as its receiver and the separator as its argument — easy to invert. Use dot notation; the module form is an M0236 violation that mops check --fix rewrites for you.

["a", "b"].values().join(", ");     // CORRECT → "a, b"
Text.join(["a", "b"].values(), ", "); // WRONG (M0236)

Note the receiver is an iterator, not an array: call .values() on an array first.

Variant Tag Arguments

Always parenthesize a variant tag's argument. A tag binds only to the atom immediately after it, tighter than any operator, so an unparenthesized argument silently loses everything past the first term:

#tag(n + 1) // CORRECT
#tag n + 1  // WRONG: parses as (#tag n) + 1 → M0060, operator is not defined for operand types

Explicit Type Instantiation

Let inference work first. With unannotated lambdas the compiler resolves .map() to a different type on its own, so write the plain call:

let photos = internalPhotos.map(
  func p = { id = p.id; url = p.url; uploadedBy = p.uploadedBy.toText() }
);

Add explicit type parameters only when the compiler reports M0098 (no best choice for type parameter):

let photos = internalPhotos.map<PhotoInternal, Photo>(func p = { ... });

Adding them when inference already succeeded is a warning of its own — M0223, redundant type instantiation — which mops check --fix strips. Annotating the lambda instead of instantiating the call is always wrong.

Function Literals as Arguments

Do NOT put a semicolon after a function body passed as an argument:

list.filter(func(item) { item.id != targetId }) // CORRECT
list.filter(func(item) { item.id != targetId; }) // WRONG: trailing semicolon makes the block return `()`

Do not inline imperative statement blocks as boolean operands:

// WRONG: parser can treat the block after `or` as an invalid expression shape.
let matches = titleMatches or {
  var found = false;
  for (tag in tags.values()) {
    if (tag == q) { found := true };
  };
  found
};

// CORRECT: compute the loop result before the final boolean expression.
var tagMatches = false;
for (tag in tags.values()) {
  if (tag == q) { tagMatches := true };
};
let matches = titleMatches or tagMatches;

Every switch case must be separated with a semicolon before the next case, even in compact one-line switches:

switch (pricing) { case (#free) { true }; case (#paid(_)) { false }; } // CORRECT
switch (pricing) { case (#free) { true } case (#paid(_)) { false } } // WRONG

Declaration Terminators

Top-level and nested function declarations inside module, actor, and mixin blocks must end with ;. A missing }; after a function commonly surfaces as a syntax error near the next declaration, e.g. unexpected token 'public'.

Local Mutability

Use let for local bindings unless the variable is reassigned with :=. Never use var for a local binding only because the value it references is mutable. Mutating an object through methods such as adding to a collection does not require the binding itself to be var; use let for collection builders and other accumulator objects unless the binding is later reassigned.

Safe Nat Arithmetic

Avoid Nat subtraction unless the compiler can prove the result is non-negative at the operation itself. a - b traps when b > a, and the compiler can still warn when safety depends on a previous branch. Prefer bounds checks, bounded addition, loop counters, or helper branches that do not subtract one Nat from another.

Option Handling

Prefer ?? for unwrap-or-default and unwrap-or-trap. Do not write a nested switch solely to peel ?T.

// Unwrap with trap when null means something is wrong
let user = users.find(func u = u.id == caller)
  ?? Runtime.trap("User not found");

// Default when absence is fine
let caption = optLabel ?? "(untitled)"; // not `label` — reserved word

// Only return ?T when absence is a normal, expected outcome
public query func findUserByName(name : Text) : async ?User {
  users.find(func u = u.name == name);
};

// Keep switch when the Some arm maps / mutates / has side effects
switch (todos.find(func todo = todo.id == targetId)) {
  case (?todo) {
    todo.completed := not todo.completed;
    ?toView(todo);
  };
  case null { null };
};

Error Handling: Result

Use mo:core/Result to return a failure a caller can act on. Result<Ok, Err> is { #ok : Ok; #err : Err }, so it is a shared type and crosses the API boundary as Candid — no wrapper needed.

Pick the return type by what the failure means:

import Result "mo:core/Result";

public type BookingError = {
  #slotTaken : { until : Time.Time };
  #notAuthorized;
  #unknownRoom : Nat;
};

public shared ({ caller }) func book(roomId : Nat, at : Time.Time) : async Result.Result<Booking, BookingError> {
  // ...
};

Never launder an error into Text. Result<Booking, Text> forces every caller — including the frontend — to string-match to tell "slot taken" from "not authorized". Make Err a variant; put the data each failure needs inside its own tag. A Text payload is fine inside a tag when it is a message for a human, not a discriminator.

Do not trap on caller error. A trap rolls back the whole message and reaches the frontend as an opaque reject — the caller cannot branch on it and the user gets no actionable message. Reserve traps for "this cannot happen" (see ?? Runtime.trap(...) above).

Chain, do not nest. mapOk, mapErr, and chain take self, so they are dot notation like every other self-parameter API and a pipeline stays flat. Result.fromOption has no self — it is the module-call bridge from ?T at the edge where absence becomes a caller-visible error.

import Result "mo:core/Result";
import Map "mo:core/Map";
import Nat "mo:core/Nat";

actor {
  type Room = { id : Nat; name : Text };
  type RoomView = { name : Text };
  type BookingError = { #unknownRoom : Nat; #notAuthorized };

  let rooms : Map.Map<Nat, Room>;

  func toView(room : Room) : RoomView = { name = room.name };
  func reserve(room : Room) : Result.Result<Room, BookingError> = #ok(room);

  public query func book(roomId : Nat) : async Result.Result<RoomView, BookingError> {
    Result.fromOption(rooms.get(roomId), #unknownRoom(roomId))
      .chain(func room = reserve(room))
      .mapOk(toView);
  };
};

Use switch on #ok / #err when the arms do different work; do not write isOk/isErr followed by an unwrap — that discards the payload the type was carrying.

Common Patterns

Module with Self Pattern

// lib/User.mo
module {
  public type User = Types.User;
  public func new(id : Principal, name : Text) : User {
    { id; var name; var isActive = true };
  };
  public func ban(self : User) { self.isActive := false };
};
// Usage: user.ban(); -- dot notation!

Record Spread with with

RULE: Use record spread for immutable records. Never use record spread on a record type that contains var fields; Motoko rejects that with base has non-aliasable var field.

{ self with newField = "" }; // CORRECT for immutable records

// CORRECT for a record type containing var fields:
let updated : Types.Todo = {
  id = todo.id;
  text = newText;
  var completed = todo.completed;
  createdAt = todo.createdAt;
};

{ todo with text = newText }; // WRONG if Todo contains any var field

State Definition

Entity types live in types.mo. State fields as direct actor bindings — no AppState wrapper.

Stable actor fields are declared with types only — no initializers (initial values come from the migration chain). Transient fields use initializers as usual.

// types.mo
module {
  public type User = {
    id : Principal;
    var username : Text;
    var isActive : Bool;
  };
};

// main.mo
actor {
  let users : List.List<Types.User>;
  let state : { var nextPostId : Nat };
  include AuthMixin(users);
};

Mutable State for Mixins

Never declare var actor-fields (e.g. var nextPostId : Nat) you intend to share with mixins — var parameters are passed by value, so the mixin's mutations don't propagate back. Wrap mutables in a record and pass the record; records are shared by reference. In the actor, declare the record type-only (let state : { var nextPostId : Nat };) — its initial value (e.g. { var nextPostId = 0 }) comes from the migration chain, like every stable field.

Preserve the exact field names on shared mutable state records across actor, mixin, and helper modules. If the actor declares let state : { var nextId : Nat } and the mixin receives state, helper parameters must accept { var nextId : Nat } and update state.nextId. Do not rename the field to val or counter in helper signatures, and do not create wrapper copies like { var val = state.nextId }; the copy mutates only itself and leaves actor state unchanged.

Transient State & Static/Module Fields

Enhanced orthogonal persistence makes every top-level let/var in an actor or mixin stable (persisted across upgrades) by default — there is no stable keyword. Prefix a binding with transient to keep it OUT of stable storage; it is re-initialized on every (re)start instead of being persisted. Use it for anything that isn't durable state — caches, capability handles, and constants.

Constants — Motoko has no const, and a bare let X = ... in an actor or mixin is stable state. Put a fixed value in a module when its right-hand side is a static expression (namespaced, reusable, never state); otherwise keep it as transient let in the actor/mixin:

transient let admin = Principal.fromText("..."); // non-static (a call) — can't be a module `let`
transient let cache = Map.empty<Text, User>();   // derived; rebuilt after each upgrade

Static (what a module let field allows) = literals, variant tags (#x), options (?x), tuples, immutable arrays and records, function values, and imported/variable names — plus .field projection over those. Non-static = function calls, operators (+, ==, #), control flow (if/switch/loops), and array indexing (a[i]).

Numeric Conversion Hygiene

Treat deprecation warnings as failures. Every conversion is spelled to, on the source value — never a Module.fromX call, and never a chain through the sized numeric modules. A conversion is one receiver call:

import Nat "mo:core/Nat";

let average = sum.toFloat() / count.toFloat(); // Nat.toFloat

A conversion that needs several hops is a sign the wrong function was picked: verify the exact mo:core signature in api-reference.md, which lists only non-deprecated APIs.

Security and Authorization

Every public update function MUST verify the caller via {caller} destructuring. Enforce authorization on the backend — never trust client-side checks.

Attaching cycles to an inter-canister call (await (with cycles = ...) <call>) hands them to the callee, so treat any endpoint that can trigger one as spend authority: gate it on the caller, bound the amount, and never let an unauthenticated path reach it. Some platforms forbid outbound cycles entirely — follow the hosting platform's own guidance where it applies.

Common Compile Error Patterns

Quick Reference

Basic Types: Nat Int Text Bool Principal ?T [T] [var T] Blob Float — Time.now() returns Int (nanoseconds)

Common Operations: debug_show(value) → Text | assert condition | # "text" concatenation | break / continue inside for, while, loop

Best Practices

1. Always mo:core, never mo:base
2. No stable keyword — enhanced orthogonal persistence handles state
3. Dot notation for all self-parameter functions
4. Unwrap with ?? (opt ?? Runtime.trap(...) or opt ?? default); reserve switch for transforms/side effects/variants; ?T only when absence is expected
5. types.mo / lib/ / mixins/ / main.mo structure
6. Mixins receive only needed state slices
7. Queries for read-only, updates for state changes
8. Iterator chaining to avoid intermediate collections
9. Record spread { self with ... } for immutable records; mutate or rebuild records that contain var fields
10. No inline initializers on stable actor fields — initial values come from the migration chain
11. Inline func arguments carry no type annotations (except : async () on async callbacks); instantiate the call instead, and only when the compiler reports M0098

Additional References

• Control flow: references/control-flow.md — ??, do ? { ... } option chaining, switch statements, loops, break / continue
• Reserved keywords: references/reserved-keywords.md — full list to check identifiers against
• Equality & comparison: references/equality.md — which types support receiver .equal, and when == differs from equal
• Type conversions: references/type-conversions.md — Nat/Int size conversions
• Project setup: references/project-setup.md — one-time [moc] args flags. Skip this if your platform manages mops.toml
• Actor migrations: Load migrating-motoko-actors when upgrading canisters or changing actor state shape
• Migration failures: Load troubleshooting-motoko-migrations for unexplained compatibility diagnostics, frozen migration files, or converted legacy projects
• API signatures: api-reference.md — complete function signatures
• Complete examples: examples.md — full working code samples
• mops tooling: Load mops-cli for mops.toml configuration, dependency management, and mops check/mops build/toolchain setup