Motoko Doc Strings

Motoko doc strings

Purpose

Add triple-slash (///) doc comments to every public declaration in Motoko source files so that mo-doc renders meaningful documentation pages. This skill captures the conventions used in mo:core (caffeinelabs/motoko-core) and the lessons learned applying them across the motoko-bitcoin library.

When to Use

• The user asks to "add doc strings", "document the public API", or "generate docs" for a Motoko package.
• Reviewing or polishing a library before release / publication on MOPS.
• After adding new public symbols and noticing missing entries in the generated docs/.

What Counts as a "Public Object"

Document every declaration that ends up in the rendered docs. In a typical module { ... } file these are:

• public type ...
• public let ...
• public func ...
• public class ...
• The module itself (module { ... } or module Name { ... })
• For a public class, every public let, public var, and public func member inside the class body.
• For a nested public module, recurse and apply the same rules.

Private declarations (func, let, class without public) and helper types should NOT receive /// comments — they don't appear in mo-doc output and the noise hurts readability.

Comprehensiveness by Location

How thorough a doc string needs to be depends on where the file lives:

• Inside src/internal/ (some packages use src/private/ for the same role — apply the same rule) — doc strings can be brief. These modules are implementation details that end users are not expected to call directly, so a short one-liner stating what a declaration does is usually enough. Trap/error notes can be omitted unless the behavior is surprising to another maintainer.
• Anywhere else under src/ (i.e. outside src/internal/) — doc strings MUST be comprehensive. These are the public API surface that users will call, so they need every detail: argument units and formats, size/range constraints, return-value semantics, full failure behavior (Traps / Errors paragraphs), and runnable examples where helpful. Apply the full "User-Perspective Read-Through" checklist at the end of this skill to every declaration here.

Doc String Format

mo-doc parses lines that start with /// as Markdown documentation attached to the next declaration.

/// Brief one-line description ending in a period.
///
/// Optional longer paragraph(s) describing semantics, edge cases,
/// and return values. Refer to parameters by `name` in backticks.
///
/// Example:
/// ```motoko include=import
/// let result = Module.func(arg);
/// ```
public func func(arg : T) : U { ... };

Conventions used in mo:core:

• First line is a short imperative summary ("Returns ...", "Computes ...", "Decodes ...") ending with a period.
• One blank /// line separates paragraphs.
• Wrap parameter names, types, and short code in single backticks.
• Code blocks use ```motoko fences. Use the ```motoko include=import annotation when the snippet relies on the module's import header (defined elsewhere with ```motoko name=import).
• For module-level docs, define a named import block once at the top so all example snippets can reference it:

  /// ```motoko name=import
  /// import Hash "mo:bitcoin/Hash";
  /// ```
  module { ... }
  

• The name=import / include=import annotations are NOT consumed by mo-doc itself — they are directives for an external doctest runner (used by mo:core's CI and by Docusaurus-based doc sites) that prepends the named snippet before compiling each example. mo-doc passes them through verbatim into the rendered code-fence info string. If the project does not run doctests, you can omit them and just use plain ```motoko fences.

Where to put the module-level doc string

The module-level /// block must sit at the very top of the file, before the import statements, with one blank line separating it from the imports and one blank line between the imports and the module { ... } line. This matches the layout used in dfinity/motoko-core and is the only placement that mo-doc actually attaches to the module page — a doc block placed between the imports and module { ... } is silently ignored and the rendered module page will have no description.

All .mo files MUST have a module-level /// doc string at the very top of the file (before the import statements). This applies to every Motoko source file, not just modules with public APIs — having a doc block at the top ensures mo-doc renders a meaningful module page and helps maintainers quickly understand the file's purpose.

Special case for Mixins: This rule is especially important for files using the mixin keyword. Even though mo-doc currently skips files with the mixin keyword due to syntax differences, the doc string MUST still be placed at the very top of the file to ensure compatibility with future tool updates and maintainer readability.

/// One-line module summary.
///
/// Longer description.
///
/// ```motoko name=import
/// import Foo "mo:pkg/Foo";
/// ```

import Bar "mo:core/Bar";
import Baz "mo:core/Baz";

module {
  // ...
}

For named modules (module Name { ... }, e.g. src/bitcoin/Script.mo) the same rule applies: doc block at the top of the file, then a blank line, then imports, then a blank line, then module Name { ... }.

Where to put per-declaration comments

Place /// lines immediately above the declaration with no blank line between them. If there is an existing legacy // ... comment, put the /// block above the legacy comment (the legacy comment can stay as implementation notes).

/// Public API description goes here.
// Legacy implementation notes can stay below the doc string.
public func encode(input : [Nat8]) : Text { ... };

Common Pitfalls

1. apply_patch and literal \n

When inserting multi-line content, write actual newlines in the patch body (one + per real line). NEVER use the escape sequence \n inside inserted text — it will be written as the literal two characters and break the file.

2. Don't accidentally insert into the middle of a function

apply_patch matches on context. When the surrounding context is too short or appears multiple times, the inserted block may land inside a function body. After inserting docs, run a quick syntax check (e.g. mo-doc or moc --check) to catch this. A telltale sign is a syntax error [M0001], unexpected token 'import' message — that means a new module-level block was placed inside a loop/func body.

3. Class member documentation order

mo-doc renders class fields and methods in source order, but the class's own description block is shown after the member list when the class doc appears above the constructor. To keep the class summary at the top of the rendered class page, place the /// block on the line directly preceding public class Name(...).

4. Re-exported types

Re-exported types like public type Signature = Types.Signature; still need a one-line /// description so they are not rendered as "(no description)".

5. Skip private helpers and constants

Don't add /// to let, func, or type declarations that lack public. They never appear in the output and the comments add visual noise.

6. module Name { ... } named modules

Files like src/bitcoin/Script.mo use the form module Script { ... } instead of the bare module { ... }. The same top-of-file placement rule applies — the module-level /// block goes at the very start of the file (before the imports), not on the line directly preceding module Script {.

7. Preserve unfamiliar parameter syntax verbatim

If a function signature uses syntax you don't recognise — e.g. self : [var T] (Motoko's method-dispatch hook that enables x.func(...) call sites) or key : (implicit : T -> Nat32) (the implicit parameter feature) — do NOT rewrite it. These shapes are load-bearing: they affect how call sites in examples work (users.bucketSort<User>(...) only compiles because the first parameter is named self), and a doc pass that "cleans them up" can silently invalidate every example you just wrote.

8. Module doc must be at the top of the file

mo-doc only treats a /// block as the module description when it appears at the very beginning of the file, ahead of the import statements. A doc block placed between the imports and module { ... } compiles fine but produces an empty module description in the rendered HTML. If you find an existing project with module docs adjacent to module { ... }, relocate them to the top of the file (a small Python script that finds the trailing /// block before module and prepends it to the file works well for batch migration).

8. Documenting Record Types

For record types with multiple members, documenting each member with a per-line /// can make the code and the rendered documentation very verbose. Instead, prefer documenting all members in a single block above the type declaration. Use backticks for member names.

/// An HTTP request.
/// `method`: HTTP method (e.g., "GET", "POST").
/// `url`: Request URL.
/// `headers`: List of headers.
/// `body`: Request body.
public type Request = {
  method : Text;
  url : Text;
  headers : [(Text, Text)];
  body : Blob;
};

Workflow

1. Inventory public declarations:

   grep -RInE "public (type|func|class|let)" src
   

2. For each .mo file:

   • Add a module-level /// block (with a name=import example) at the beginning of the file, even before the block of import statements.
   • Add /// blocks above every public declaration inside the top-level modules.
   • Recurse into nested public declarations. For instance public members of public classes need doc strings. Public members of public modules need doc strings. And so on.

3. Re-scan to catch anything missed:

   awk '
     FNR==1{prev=""}
     {
       if ($0 ~ /^[ \t]*public[ \t]+(type|func|class|let)([ \t(]|$)/) {
         p=prev; gsub(/^[ \t]+|[ \t]+$/, "", p);
         if (p !~ /^\/{3}/) printf "%s:%d:%s\n", FILENAME, FNR, $0;
       }
       if ($0 !~ /^[ \t]*$/) prev=$0;
     }
   ' src/*.mo src/**/*.mo
   

   Empty output = all public declarations are preceded by a /// line.

   Portability: use [ \t]+ (and explicit [ \t(] boundaries) instead of [[:space:]]+ / \b. BSD/macOS awk silently fails to match POSIX classes here — the script returns empty output and looks like a pass. The cost of this miss is a full iteration to diagnose.

   Known false positives (don't waste edits "fixing" these — mo-doc itself handles both cases correctly):

   • A legacy // ... comment between the /// block and the public ... line. The skill's format guidance explicitly allows this layout, but the audit awk only looks at the immediately previous non-blank line. Verify by reading the rendered HTML before assuming a real miss.
   • A public func / public let inside a /* ... */ block comment. The awk has no notion of block comments, so commented-out code shows up as missing. Cross-check by grep-ing the file for /* near the flagged line.

4. Generate docs:

   # mo-doc will NOT create nested output subdirectories on its own.
   # If src/ has subdirectories, mkdir -p the mirrored paths under
   # docs/ first or the run aborts on the first file it can't write.
   mkdir -p docs $(find src -mindepth 1 -type d | sed 's|^src|docs|')
   mo-doc --source src --output docs --format html
   

   No output = success. Two failure modes to watch for:

   • A Fatal error: exception Sys_error("docs/.../X.html: No such file or directory") means an output subdirectory is missing.
   • Any Skipping ... line indicates a syntax error that must be fixed (often a stray apply_patch corruption).

5. Spot-check the rendered output:

   • docs/index.html — every module should appear in the listing.
   • Each docs/<Module>.html — module description, types, functions, and class members should all show their text.

Tips for Writing Useful Descriptions

• Describe what the function does and what it returns, not how it is implemented.
• For low-level helpers (readBE32, writeLE64, etc.) a one-liner stating the byte order, width, and offset semantics is sufficient.
• For domain types (SighashType, WitnessProgram, OutPoint) name the spec or BIP that defines the format and link to it.
• Keep examples short and self-contained; prefer literal byte arrays over reading from external sources.

Files with many similar public lets (constants tables)

Each public let needs its own /// line — there is no syntax for a shared doc block. For files like FIPS round-constant tables (e.g. SHA256's K00..K63, SHA512's K00..K79), doing N individual Edit calls is slow and noisy. Read the file once and rewrite it whole with Write, emitting one /// <name> round constant K<NN>. line per entry. A module-level /// at the top can carry the shared context (spec section, purpose) so the per-entry lines stay terse.

Documenting Error and Trap Behavior (REQUIRED)

Every public function doc string MUST describe its full failure behavior. Readers cannot tell from a type signature alone whether a function traps, returns null, returns #err, or simply produces a wrong-but-defined result on bad input — the doc string is the only place this contract is recorded.

What to look for in the implementation

Scan the function body (and every helper it calls) for:

• Runtime.trap(...) / Debug.trap(...) / Prim.trap(...) calls.
• assert ...; statements (a failed assert traps).
• Pattern matches that are non-exhaustive in practice (e.g. a switch on ?T whose null branch traps, or a case (#err _) Runtime.trap ...).
• Implicit traps from the standard library: out-of-bounds array indexing (a[i] when i >= a.size()), Nat subtraction underflow, division by zero, Option.unwrap on null.
• Explicit error returns: ?T returning null, Result<T, E> returning #err, variant returns like { #ok; #err }.

What to write

For each failure mode, state in the doc:

1. The condition — what input or state triggers it, in user-facing terms ("when input contains a character outside the Base58 alphabet", not "when mapBase58[c] == 255").
2. The outcome — traps, returns null, returns #err(...), etc.
3. For Result/option returns, list every distinct error case separately when the variants carry meaning.

Use a dedicated Traps and/or Errors paragraph (or both) at the end of the doc, after the example and before the runtime/space notes.

CRITICAL: Use the wording "Traps" (capitalised, present tense) for unrecoverable failures (e.g., assert, trap). This makes the failure modes highly visible in rendered docs and easily greppable.

/// Decodes a Base58-encoded string into the original byte array.
///
/// ```motoko include=import
/// let bytes = Base58.decode("StV1DL6CwTryKyV");
/// ```
///
/// Traps if `encoded` contains any character that is not in the Base58
/// alphabet (i.e. not in
/// `123456789ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnopqrstuvwxyz`).
public func decode(encoded : Text) : [Nat8] { ... };

For graceful errors:

/// Parses a WIF-encoded private key.
///
/// Returns `#err(msg)` when:
/// - the input is not valid Base58Check (bad checksum or alphabet),
/// - the decoded payload has an unexpected length (must be 33 or 34 bytes),
/// - the version byte does not match `network`.
public func decode(wif : Text, network : Network) : Result<PrivateKey, Text> { ... };

Pure / total functions

If a function genuinely cannot fail (e.g. Array.size, a pure arithmetic helper that uses fixed-width types and total operators), say so explicitly with a one-liner like "Never traps." or omit the failure section entirely — but only after auditing the body to confirm.

Worked examples from motoko-bitcoin

• Base58.decode — traps on any character outside the alphabet (not a Result, no graceful fallback).
• Base58Check.decode — returns ?[Nat8]; null on bad alphabet, bad length, or checksum mismatch. Document each.
• Bech32.decode — returns Result<_, Text> with distinct messages for invalid characters, mixed case, bad checksum, length out of range, invalid HRP. Mention each error category, not just "returns #err".
• Bip32.derivePath — traps on hardened derivation from a public key.
• Fp.inverse (and the / operator) — traps when the value is zero.
• Affine.add / Jacobi.add — document behaviour at the point at infinity and for equal/opposite inputs.
• Transaction serialization (Transaction.toBytes, TxInput.toBytes, etc.) — note any size limits that would cause Nat32/Nat64 conversion traps.

Audit workflow for an existing file

1. List every public func and public class member.
2. For each, read the body and follow the call graph one level into private helpers.
3. Note every trap, assert, Result/Option return, and any implicit trap source (subtraction, indexing, division).
4. Update the doc string to enumerate the conditions.
5. Re-run mo-doc and visually scan the rendered HTML for sections that still lack a "Traps" / "Errors" paragraph on a non-trivial function.

Final Step: User-Perspective Read-Through (REQUIRED)

After every public declaration has a doc string, do one more pass. Read each doc string from the perspective of a first-time user of the API who has not seen the implementation. For every doc, ask:

• What is the unit / format of each argument and the return value? (bytes vs. bits, big- vs. little-endian, satoshis vs. BTC, raw vs. DER-encoded, compressed vs. uncompressed, 0-based vs. 1-based, …)
• For Motoko array parameters, is [T] (immutable) vs. [var T] (mutable) clear? Examples that mix Array.tabulate (returns [T]) with VarArray.repeat (returns [var T]) routinely confuse newcomers — spell out which shape the function expects.
• What are the size or range constraints on each input?
• Which BIP / RFC / spec defines the format, and is it linked?
• For mutating methods, what state changes? Is the receiver still usable afterward?
• For functions that take a callback or proxy (e.g. an ECDSA signer), what is the expected input/output shape of the callback?
• For variant returns, what does each variant mean semantically (not just what tag it carries)?
• Are domain-specific terms ("witness program", "tap leaf", "sighash", "scriptPubKey") used without a one-line explanation or link?
• For constants (EMPTY_WITNESS, dustThreshold), what is the value and why does it have the value it has?
• For re-exported types, where is the actual definition (and is the link there)?
• If you removed the function name, would the description still be unambiguous? If two near-identical functions exist (e.g. toBytes vs. toBytesIgnoringWitness, encode vs. encodeUncompressed), is the difference between them spelled out?

If any question remains unanswered, extend the doc string to address it. Prefer one extra sentence in the doc over forcing the user to read the source. Do this pass file by file; it usually surfaces 2–5 missing facts per non-trivial module.