Go API Documentation

API Documentation

Instructions for AI coding agents on adding godoc-compatible documentation comments to Go source files.

[!NOTE] This skill is for documenting Go packages, types, functions, and methods following Go documentation conventions.

• 1. Benefits
• 2. Principles
• 3. Patterns
  • 3.1. Package Documentation
  • 3.2. Function Documentation
  • 3.3. Type Documentation
  • 3.4. Method Documentation
  • 3.5. Constant and Variable Documentation
  • 3.6. Examples
  • 3.7. Formula Documentation
• 4. Workflow
• 5. Commands
• 6. Style Guide
• 7. Templates
  • 7.1. Package Template
  • 7.2. Type Template
  • 7.3. Function Template
  • 7.4. Method Template
  • 7.5. Constant/Variable Template
  • 7.6. Example Template
  • 7.7. Formula Template
• 8. References

1. Benefits

• Discoverability

  Well-documented APIs enable developers to quickly understand and use packages without reading implementation details.

• Maintainability

  Documentation embedded in source code stays synchronized with implementation, reducing drift between code and documentation.

• Tooling Integration

  Documentation comments are automatically extracted by go doc and displayed in IDEs, editors, and pkg.go.dev.

• Standards Compliance

  Following Go documentation conventions ensures consistency across the Go ecosystem.

2. Principles

Effective API documentation follows these core principles aligned with Go conventions.

• Complete

  Document all exported identifiers (packages, types, functions, methods, constants, and variables). Unexported identifiers may have documentation for internal clarity.

• Contextual

  Documentation provides context about usage patterns, performance characteristics, and concurrency safety.

• Consistent

  Use a uniform style and format throughout following Go documentation conventions defined in this skill.

• Concise

  Use clear, brief descriptions. The first sentence should be a complete sentence that starts with the name being documented.

• Concrete

  Provide specific details about behavior, edge cases, and error conditions rather than vague statements.

• Accurate

  Documentation must match the actual behavior. Update documentation whenever the implementation changes.

• Actionable

  Include usage examples, preconditions, postconditions, and error handling to help developers use the API correctly.

3. Patterns

3.1. Package Documentation

Package documentation appears in a comment immediately before the package clause or in a separate doc.go file.

• Format

  Package documentation starts with Package <name> followed by a description.

• Content

  Describes the package's purpose, main concepts, and typical usage patterns.

• Location

  Can be in any file in the package (typically the main file) or in a dedicated doc.go file.

3.2. Function Documentation

Function documentation describes what the function does and its contract.

• Format

  Starts with the function name followed by a description.

• Parameters

  Parameters are described in the main description when needed, not as separate annotations.

• Return Values

  Return values are described in the main description, including error conditions.

• Errors

  Document what errors can be returned and under what conditions.

• Panics

  Document any conditions that cause the function to panic.

3.3. Type Documentation

Type documentation describes the purpose and usage of a type (structs, interfaces, aliases).

• Format

  Starts with the type name followed by a description.

• Content

  Explains what the type represents, its responsibilities, and any invariants.

• Related Types

  May reference related types and their relationships.

3.4. Method Documentation

Method documentation is similar to function documentation.

• Format

  Starts with the method name followed by a description.

• Receiver

  The receiver type is mentioned when relevant to understanding the method.

• Context

  Describes how the method relates to the type's overall functionality.

3.5. Constant and Variable Documentation

Constants and variables are documented with their purpose and usage.

• Format

  Starts with the identifier name followed by a description.

• Grouped Declarations

  Groups of related constants/variables can share a single comment before the group.

3.6. Examples

Examples demonstrate typical usage patterns.

• Format

  Example functions follow the naming convention Example, Example_suffix, or ExampleType_Method.

• Output

  Examples can include // Output: comments to specify expected output for testing.

• Documentation

  Examples appear in godoc as executable code samples.

• Completeness

  Examples should be self-contained and runnable.

3.7. Formula Documentation

Mathematical formulas and equations for algorithms and technical documentation.

[!NOTE] Go's godoc does not natively support LaTeX or MathML rendering. Use text-based representations, Unicode symbols, and preformatted blocks for mathematical expressions.

• Text Descriptions

  Describe formulas using clear mathematical language (e.g., "the square root of the sum of squares").

• Unicode Symbols

  Use Unicode mathematical symbols for common operations (×, ÷, ≤, ≥, √, ∑, π, Δ, etc.).

• ASCII Representations

  Use ASCII art for simple formulas (e.g., (a + b) / c, sqrt(x^2 + y^2)).

• Preformatted Blocks

  Use indented text for multi-line formulas and equations to preserve formatting.

• Variable Notation

  Clearly define variables and their meanings before presenting formulas.

• Examples

  Include concrete numerical examples to illustrate formula application.

• External References

  Link to external resources (Wikipedia, MathWorld) for complex mathematical concepts.

4. Workflow

1. Identify

   Identify exported functions, types, constants, and variables in pkg/ or internal/ that need documentation.

2. Review Existing

   Check if documentation exists and if it needs updating or improvement.

3. Add Documentation

   Add or update documentation comments following the templates and style guide.

4. Verify Format

   Ensure comments follow Go conventions:

   • Start with the identifier name
   • Form complete sentences
   • Use proper punctuation
   • Are placed immediately before the declaration

5. Validate

   Run go doc <package> or go doc <package>.<identifier> to verify documentation renders correctly.

6. Apply Templates

   Structure all documentation using the template patterns.

5. Commands

6. Style Guide

• License Header

  All source files must include // SPDX-License-Identifier: Apache-2.0 at the top.

• Comment Style

  Use // comment style, not /* */ for documentation comments.

• First Sentence

  The first sentence should be a complete, standalone summary that starts with the name being documented.

• Capitalization

  Start with the identifier name (capitalized as it appears in code), then continue with normal sentence capitalization.

• Punctuation

  Use proper punctuation. End sentences with periods.

• Present Tense

  Use present tense to describe what the code does (e.g., Percent calculates not Percent will calculate).

• Blank Lines

  No blank lines within a documentation comment block.

• Positioning

  Place the comment immediately before the declaration with no blank line in between.

• Code References

  Reference other identifiers without special markup (godoc will automatically link them).

• Links

  Use URLs directly in documentation; godoc will render them as links.

• Formatting

  Indented text is displayed as preformatted (code blocks).

• Paragraphs

  Separate paragraphs with blank comment lines (lines containing only //).

• Lists

  Use simple text lists; no special markup is needed.

• No Redundancy

  Avoid repeating the function signature in the documentation.

7. Templates

Use these templates for documenting Go code. Replace placeholders with actual values.

7.1. Package Template

Use this template for package documentation. Place in the main package file or in doc.go.

// SPDX-License-Identifier: Apache-2.0

// Package <name> provides <brief description of package purpose>.
//
// <Extended description of what the package does, main concepts,
// and typical usage patterns.>
//
// Example usage:
//
// <code example showing typical usage>
//
// <Additional notes about concurrency, performance, or other important
// considerations.>
package <name>

7.2. Type Template

Use this template for documenting types (structs, interfaces, type aliases).

// <TypeName> <describes what the type represents and its purpose>.
//
// <Extended description of responsibilities, invariants, and usage patterns.
// Include information about thread safety, lifecycle, or other important
// characteristics.>
//
// Example:
//
// <code example showing typical usage of the type>
type <TypeName> struct {
 // <FieldName> <describes the field's purpose>
 <FieldName> <type>
}

7.3. Function Template

Use this template for documenting functions.

// <FunctionName> <describes what the function does>.
//
// <Extended description including parameter meanings, return value
// descriptions, and any error conditions.>
//
// The function <describes behavior, preconditions, postconditions>.
//
// Returns <description of return value(s)>. If an error occurs,
// it returns <description of error conditions>.
//
// Example:
//
// <code example showing typical usage>
func <FunctionName>(<params>) (<returns>) {
 // implementation
}

7.4. Method Template

Use this template for documenting methods.

// <MethodName> <describes what the method does>.
//
// <Extended description including how it relates to the receiver type,
// parameter meanings, return value descriptions, and any error conditions.>
//
// The method <describes behavior, preconditions, postconditions>.
//
// Returns <description of return value(s)>. If an error occurs,
// it returns <description of error conditions>.
func (r <ReceiverType>) <MethodName>(<params>) (<returns>) {
 // implementation
}

7.5. Constant/Variable Template

Use this template for documenting constants and variables.

// <IdentifierName> <describes the constant/variable and its purpose>.
const <IdentifierName> = <value>

// <GroupDescription> describes this group of related constants.
const (
 // <ConstName1> <describes this specific constant>
 <ConstName1> = <value1>

 // <ConstName2> <describes this specific constant>
 <ConstName2> = <value2>
)

7.6. Example Template

Use this template for creating example functions.

// Example demonstrates basic usage of <name>.
func Example() {
 // Setup
 <setup code>
 
 // Usage
 <example usage code>
 
 // Output:
 // <expected output>
}

// Example<Type>_<method> demonstrates usage of <Type>.<method>.
func Example<Type>_<method>() {
 // Example implementation
 <example code>
 
 // Output:
 // <expected output>
}

7.7. Formula Template

Use this template for documenting functions with mathematical formulas.

// <FunctionName> <describes what the function calculates>.
//
// The calculation follows the formula:
//
//	<formula in ASCII or indented text>
//
// Where:
//   - <var1> is <description>
//   - <var2> is <description>
//   - <result> is <description>
//
// For example, with <var1> = <value1> and <var2> = <value2>:
//
//	<result> = <calculated_example>
//
// The function returns <description of return value>.
// If an error occurs, it returns <description of error conditions>.
func <FunctionName>(<params>) (<returns>) {
 // implementation
}

Example with percentage calculation:

// Percent calculates the percentage of a value.
//
// The calculation follows the formula:
//
//	result = value × (percent / 100)
//
// Where:
//   - value is the base amount
//   - percent is the percentage in range [0, 100]
//   - result is the calculated percentage of the value
//
// For example, 25% of 200:
//
//	result = 200 × (25 / 100) = 50
//
// The function returns the calculated result as float64.
// Returns an error if percent is outside the range [0, 100].
func Percent[T constraints.Integer | constraints.Float](percent, value T) (float64, error) {
 // implementation
}

Example with Unicode symbols:

// Distance calculates the Euclidean distance between two points.
//
// The calculation follows the formula:
//
//	d = √((x₂ - x₁)² + (y₂ - y₁)²)
//
// Where:
//   - (x₁, y₁) is the first point
//   - (x₂, y₂) is the second point
//   - d is the distance (d ≥ 0)
//
// For example, distance between (0, 0) and (3, 4):
//
//	d = √((3 - 0)² + (4 - 0)²) = √(9 + 16) = √25 = 5
//
// The function returns the distance as a non-negative float64.
func Distance(x1, y1, x2, y2 float64) float64 {
 // implementation
}

Example with preformatted multi-line formula:

// Change calculates the percentage change between two values.
//
// The calculation follows the formula:
//
//	         (newValue - oldValue)
//	change = ───────────────────── × 100
//	              |oldValue|
//
// Where:
//   - oldValue is the original value
//   - newValue is the new value
//   - change is the percentage change (positive for increase, negative for decrease)
//
// For example, change from 50 to 75:
//
//	change = (75 - 50) / |50| × 100 = 50%
//
// The function returns the percentage change as float64.
// Returns an error if oldValue is zero (division by zero).
func Change[T constraints.Integer | constraints.Float](oldValue, newValue T) (float64, error) {
 // implementation
}

8. References

• Go Documentation Comments specification.
• Go Effective Go - Commentary guide.
• Go Godoc documentation.
• Go Godoc tool documentation.
• Go Go Code Review comments.