Imported from mikeboe/research-helper (.opencode/skills/go/SKILL.md). Install upstream with npx skills add mikeboe/research-helper --skill go. Copyright stays with the author.

Go Development Instructions

Follow idiomatic Go practices and community standards when writing Go code. These instructions are based on Effective Go, Go Code Review Comments, and Google's Go Style Guide.

General Instructions

• Write simple, clear, and idiomatic Go code
• Favor clarity and simplicity over cleverness
• Follow the principle of least surprise
• Keep the happy path left-aligned (minimize indentation)
• Return early to reduce nesting
• Prefer early return over if-else chains; use if condition { return } pattern to avoid else blocks
• Make the zero value useful
• Write self-documenting code with clear, descriptive names
• Document exported types, functions, methods, and packages
• Use Go modules for dependency management
• Leverage the Go standard library instead of reinventing the wheel (e.g., use strings.Builder for string concatenation, filepath.Join for path construction)
• Prefer standard library solutions over custom implementations when functionality exists
• Write comments in English by default; translate only upon user request
• Avoid using emoji in code and comments

Naming Conventions

Packages

• Use lowercase, single-word package names
• Avoid underscores, hyphens, or mixedCaps
• Choose names that describe what the package provides, not what it contains
• Avoid generic names like util, common, or base
• Package names should be singular, not plural

Package Declaration Rules (CRITICAL):

• NEVER duplicate package declarations - each Go file must have exactly ONE package line
• When editing an existing .go file:
  • PRESERVE the existing package declaration - do not add another one
  • If you need to replace the entire file content, start with the existing package name
• When creating a new .go file:
  • BEFORE writing any code, check what package name other .go files in the same directory use
  • Use the SAME package name as existing files in that directory
  • If it's a new directory, use the directory name as the package name
  • Write exactly one package <name> line at the very top of the file
• When using file creation or replacement tools:
  • ALWAYS verify the target file doesn't already have a package declaration before adding one
  • If replacing file content, include only ONE package declaration in the new content
  • NEVER create files with multiple package lines or duplicate declarations

Variables and Functions

• Use mixedCaps or MixedCaps (camelCase) rather than underscores
• Keep names short but descriptive
• Use single-letter variables only for very short scopes (like loop indices)
• Exported names start with a capital letter
• Unexported names start with a lowercase letter
• Avoid stuttering (e.g., avoid http.HTTPServer, prefer http.Server)

Interfaces

• Name interfaces with -er suffix when possible (e.g., Reader, Writer, Formatter)
• Single-method interfaces should be named after the method (e.g., Read → Reader)
• Keep interfaces small and focused

Constants

• Use MixedCaps for exported constants
• Use mixedCaps for unexported constants
• Group related constants using const blocks
• Consider using typed constants for better type safety

Code Style and Formatting

Formatting

• Always use gofmt to format code
• Use goimports to manage imports automatically
• Keep line length reasonable (no hard limit, but consider readability)
• Add blank lines to separate logical groups of code

Comments

• Strive for self-documenting code; prefer clear variable names, function names, and code structure over comments
• Write comments only when necessary to explain complex logic, business rules, or non-obvious behavior
• Write comments in complete sentences in English by default
• Translate comments to other languages only upon specific user request
• Start sentences with the name of the thing being described
• Package comments should start with "Package [name]"
• Use line comments (//) for most comments
• Use block comments (/* */) sparingly, mainly for package documentation
• Document why, not what, unless the what is complex
• Avoid emoji in comments and code

Error Handling

• Check errors immediately after the function call
• Don't ignore errors using _ unless you have a good reason (document why)
• Wrap errors with context using fmt.Errorf with %w verb
• Create custom error types when you need to check for specific errors
• Place error returns as the last return value
• Name error variables err
• Keep error messages lowercase and don't end with punctuation

Architecture and Project Structure

Package Organization

• Follow standard Go project layout conventions
• Keep main packages in cmd/ directory
• Put reusable packages in pkg/ or internal/
• Use internal/ for packages that shouldn't be imported by external projects
• Group related functionality into packages
• Avoid circular dependencies

Dependency Management

• Use Go modules (go.mod and go.sum)
• Keep dependencies minimal
• Regularly update dependencies for security patches
• Use go mod tidy to clean up unused dependencies
• Vendor dependencies only when necessary

Type Safety and Language Features

Type Definitions

• Define types to add meaning and type safety
• Use struct tags for JSON, XML, database mappings
• Prefer explicit type conversions
• Use type assertions carefully and check the second return value
• Prefer generics over unconstrained types; when an unconstrained type is truly needed, use the predeclared alias any instead of interface{} (Go 1.18+)

Pointers vs Values

• Use pointer receivers for large structs or when you need to modify the receiver
• Use value receivers for small structs and when immutability is desired
• Use pointer parameters when you need to modify the argument or for large structs
• Use value parameters for small structs and when you want to prevent modification
• Be consistent within a type's method set
• Consider the zero value when choosing pointer vs value receivers

Interfaces and Composition

• Accept interfaces, return concrete types
• Keep interfaces small (1-3 methods is ideal)
• Use embedding for composition
• Define interfaces close to where they're used, not where they're implemented
• Don't export interfaces unless necessary

Concurrency

Goroutines

• Be cautious about creating goroutines in libraries; prefer letting the caller control concurrency
• If you must create goroutines in libraries, provide clear documentation and cleanup mechanisms
• Always know how a goroutine will exit
• Use sync.WaitGroup or channels to wait for goroutines
• Avoid goroutine leaks by ensuring cleanup

Channels

• Use channels to communicate between goroutines
• Don't communicate by sharing memory; share memory by communicating
• Close channels from the sender side, not the receiver
• Use buffered channels when you know the capacity
• Use select for non-blocking operations

Synchronization

• Use sync.Mutex for protecting shared state
• Keep critical sections small
• Use sync.RWMutex when you have many readers
• Choose between channels and mutexes based on the use case: use channels for communication, mutexes for protecting state
• Use sync.Once for one-time initialization
• WaitGroup usage by Go version:
  • If go >= 1.25 in go.mod, use the new WaitGroup.Go method (documentation):

    var wg sync.WaitGroup
    wg.Go(task1)
    wg.Go(task2)
    wg.Wait()

  • If go < 1.25, use the classic Add/Done pattern

Error Handling Patterns

Creating Errors

• Use errors.New for simple static errors
• Use fmt.Errorf for dynamic errors
• Create custom error types for domain-specific errors
• Export error variables for sentinel errors
• Use errors.Is and errors.As for error checking

Error Propagation

• Add context when propagating errors up the stack
• Don't log and return errors (choose one)
• Handle errors at the appropriate level
• Consider using structured errors for better debugging

API Design

HTTP Handlers

• Use http.HandlerFunc for simple handlers
• Implement http.Handler for handlers that need state
• Use middleware for cross-cutting concerns
• Set appropriate status codes and headers
• Handle errors gracefully and return appropriate error responses
• Router usage by Go version:
  • If go >= 1.22, prefer the enhanced net/http ServeMux with pattern-based routing and method matching
  • If go < 1.22, use the classic ServeMux and handle methods/paths manually (or use a third-party router when justified)

JSON APIs

• Use struct tags to control JSON marshaling
• Validate input data
• Use pointers for optional fields
• Consider using json.RawMessage for delayed parsing
• Handle JSON errors appropriately

HTTP Clients

• Keep the client struct focused on configuration and dependencies only (e.g., base URL, *http.Client, auth, default headers). It must not store per-request state
• Do not store or cache *http.Request inside the client struct, and do not persist request-specific state across calls; instead, construct a fresh request per method invocation
• Methods should accept context.Context and input parameters, assemble the *http.Request locally (or via a short-lived builder/helper created per call), then call c.httpClient.Do(req)
• If request-building logic is reused, factor it into unexported helper functions or a per-call builder type; never keep http.Request (URL params, body, headers) as fields on the long-lived client
• Ensure the underlying *http.Client is configured (timeouts, transport) and is safe for concurrent use; avoid mutating Transport after first use
• Always set headers on the request instance you’re sending, and close response bodies (defer resp.Body.Close()), handling errors appropriately

Performance Optimization

Memory Management

• Minimize allocations in hot paths
• Reuse objects when possible (consider sync.Pool)
• Use value receivers for small structs
• Preallocate slices when size is known
• Avoid unnecessary string conversions

I/O: Readers and Buffers

• Most io.Reader streams are consumable once; reading advances state. Do not assume a reader can be re-read without special handling

• If you must read data multiple times, buffer it once and recreate readers on demand:

  • Use io.ReadAll (or a limited read) to obtain []byte, then create fresh readers via bytes.NewReader(buf) or bytes.NewBuffer(buf) for each reuse
  • For strings, use strings.NewReader(s); you can Seek(0, io.SeekStart) on *bytes.Reader to rewind

• For HTTP requests, do not reuse a consumed req.Body. Instead:

  • Keep the original payload as []byte and set req.Body = io.NopCloser(bytes.NewReader(buf)) before each send
  • Prefer configuring req.GetBody so the transport can recreate the body for redirects/retries: req.GetBody = func() (io.ReadCloser, error) { return io.NopCloser(bytes.NewReader(buf)), nil }

• To duplicate a stream while reading, use io.TeeReader (copy to a buffer while passing through) or write to multiple sinks with io.MultiWriter

• Reusing buffered readers: call (*bufio.Reader).Reset(r) to attach to a new underlying reader; do not expect it to “rewind” unless the source supports seeking

• For large payloads, avoid unbounded buffering; consider streaming, io.LimitReader, or on-disk temporary storage to control memory

• Use io.Pipe to stream without buffering the whole payload:

  • Write to *io.PipeWriter in a separate goroutine while the reader consumes
  • Always close the writer; use CloseWithError(err) on failures
  • io.Pipe is for streaming, not rewinding or making readers reusable

• Warning: When using io.Pipe (especially with multipart writers), all writes must be performed in strict, sequential order. Do not write concurrently or out of order—multipart boundaries and chunk order must be preserved. Out-of-order or parallel writes can corrupt the stream and result in errors.

• Streaming multipart/form-data with io.Pipe:

  • pr, pw := io.Pipe(); mw := multipart.NewWriter(pw); use pr as the HTTP request body
  • Set Content-Type to mw.FormDataContentType()
  • In a goroutine: write all parts to mw in the correct order; on error pw.CloseWithError(err); on success mw.Close() then pw.Close()
  • Do not store request/in-flight form state on a long-lived client; build per call
  • Streamed bodies are not rewindable; for retries/redirects, buffer small payloads or provide GetBody

Profiling

• Use built-in profiling tools (pprof)
• Benchmark critical code paths
• Profile before optimizing
• Focus on algorithmic improvements first
• Consider using testing.B for benchmarks

Testing

Test Organization

• Keep tests in the same package (white-box testing)
• Use _test package suffix for black-box testing
• Name test files with _test.go suffix
• Place test files next to the code they test

Writing Tests

• Use table-driven tests for multiple test cases
• Name tests descriptively using Test_functionName_scenario
• Use subtests with t.Run for better organization
• Test both success and error cases
• Consider using testify or similar libraries when they add value, but don't over-complicate simple tests

Test Helpers

• Mark helper functions with t.Helper()
• Create test fixtures for complex setup
• Use testing.TB interface for functions used in tests and benchmarks
• Clean up resources using t.Cleanup()

Security Best Practices

Input Validation

• Validate all external input
• Use strong typing to prevent invalid states
• Sanitize data before using in SQL queries
• Be careful with file paths from user input
• Validate and escape data for different contexts (HTML, SQL, shell)

Cryptography

• Use standard library crypto packages
• Don't implement your own cryptography
• Use crypto/rand for random number generation
• Store passwords using bcrypt, scrypt, or argon2 (consider golang.org/x/crypto for additional options)
• Use TLS for network communication

Documentation

Code Documentation

• Prioritize self-documenting code through clear naming and structure
• Document all exported symbols with clear, concise explanations
• Start documentation with the symbol name
• Write documentation in English by default
• Use examples in documentation when helpful
• Keep documentation close to code
• Update documentation when code changes
• Avoid emoji in documentation and comments

README and Documentation Files

• Include clear setup instructions
• Document dependencies and requirements
• Provide usage examples
• Document configuration options
• Include troubleshooting section

Tools and Development Workflow

Essential Tools

• go fmt: Format code
• go vet: Find suspicious constructs
• golangci-lint: Additional linting (golint is deprecated)
• go test: Run tests
• go mod: Manage dependencies
• go generate: Code generation

Development Practices

• Run tests before committing
• Use pre-commit hooks for formatting and linting
• Keep commits focused and atomic
• Write meaningful commit messages
• Review diffs before committing

Common Pitfalls to Avoid

• Not checking errors
• Ignoring race conditions
• Creating goroutine leaks
• Not using defer for cleanup
• Modifying maps concurrently
• Not understanding nil interfaces vs nil pointers
• Forgetting to close resources (files, connections)
• Using global variables unnecessarily
• Over-using unconstrained types (e.g., any); prefer specific types or generic type parameters with constraints. If an unconstrained type is required, use any rather than interface{}
• Not considering the zero value of types
• Creating duplicate package declarations - this is a compile error; always check existing files before adding package declarations