sigfmt — Linter for Go Function Signatures

sigfmt is a powerful linter plugin for golangci-lint designed for automatic checking and formatting of function, method, and field function signatures in Go.

🎯 Motivation

The Problem

The standard gofmt formatter provides basic formatting but leaves freedom of choice when breaking long function signatures across multiple lines. This often leads to inconsistency, poor readability, and polluted git history.

1. Polluted git history: Unnecessary changes appear in diffs when different developers format code differently.

- func Save(data []byte,
-     path string) error {
+ func Save(data []byte, path string) error {

2. Bloated interfaces and structs: Without parameter packing, interfaces and structs take up excessive vertical space.

// Takes 16 lines for just three simple methods!
type UserService interface {
    Create(
        name string,
        email string,
        age int,
    ) error
    // ...
}

The Solution

sigfmt solves these problems by enforcing strict but reasonable rules:

• Compactness: If a signature fits on one line (considering the limit), it must be on one line.
• Structure: If a signature is long, it should be formatted to use vertical space most efficiently.
• Automation: The linter points out problems and suggests automatic fixes.

🚀 Features

The linter analyzes:

• Function declarations (func Foo(...))
• Type methods (func (s *S) Bar(...))
• Anonymous functions / Literals (var f = func(...))
• Methods in interfaces (type I interface { Method(...) })
• Struct fields with function type (type S struct { Callback func(...) })
• Generics (Go 1.18+): Correct handling of type parameters [T any].

🛠 Installation

sigfmt is a plugin for golangci-lint. You need to build a custom version of golangci-lint that includes this plugin.

Option 1: Automatic (Recommended)

1. Create a .custom-gcl.yml file in your project root:

   version: v1.64.6 # Use your desired golangci-lint version
   plugins:
     - module: 'github.com/vsfedorenko/sigfmt'
       version: v0.1.0 # Replace with the latest version

2. Run the command to build the custom binary:

   golangci-lint custom

   This will create a custom-gcl binary in your directory.

Option 2: Manual

1. Add a blank import of the module inside cmd/golangci-lint/plugins.go (or equivalent):

   import _ "github.com/vsfedorenko/sigfmt"

2. Run go mod tidy and make build to produce your custom golangci-lint.

⚙️ Configuration

Configure the linter in your .golangci.yml file.

Parameters:

• max-line-len (int): Maximum allowed line length. Default: 120.
• tab-width (int): Tab width for visual calculation. Default: 8.
• pack-struct-fields (bool): Enable packing of struct fields. Default: true.
• pack-interface-methods (bool): Enable packing of interface methods. Default: true.
• param-groups (list of lists): Define groups of parameter types that should be kept together on the same line.

Example .golangci.yml:

linters-settings:
  custom:
    sigfmt:
      type: "module"
      description: "Advanced function signature formatter"
      settings:
        max-line-len: 120
        tab-width: 8
        pack-struct-fields: true
        pack-interface-methods: true
        param-groups:
          - ["context.Context", "*sql.Tx"] # Group ctx and tx together
          - ["context.Context"]            # Ensure ctx is on its own line (if no tx)

💡 Usage

To run the linter, use your custom binary:

# Run analysis
./custom-gcl run

# Automatically fix issues
./custom-gcl run --fix

🧠 How It Works

The linter analyzes function signatures using the Go AST (Abstract Syntax Tree) and applies a two-stage formatting strategy:

1. Collapse Stage

Goal: Maximize compactness for short signatures.

The linter calculates the visual width of a signature if it were on a single line. If it fits within max-line-len (default 120), the signature must be collapsed.

Example:

- func Sum(
-     a int,
-     b int,
- ) int { ... }
+ func Sum(a int, b int) int { ... }

Diagnostic Message: "Signature can be formatted more compactly" Suggested Fix Message: "Format signature"

This applies to:

• Function declarations
• Type methods (with receivers)
• Anonymous functions / closures
• Interface methods
• Struct fields with function types

2. Reformat / Packing Stage

Goal: Optimize vertical space for long signatures.

If a signature doesn't fit on one line, the linter applies context-aware packing strategies. The diagnostic message for this stage is the same as for the collapse stage: "Signature can be formatted more compactly", with the fix message "Format signature".

A. Regular Functions (Conservative)

Preserves logical parameter grouping. Uses minimal reformatting to respect developer intent.

  func ProcessData(
-     param1 string,
-     param2 string,
-     param3 int,
-     param4 bool,
  ) error {
+ func ProcessData(
+     param1 string, param2 string,
+     param3 int, param4 bool,
+ ) error {
      // ...
  }

B. Interfaces & Structs (Aggressive)

Packs multiple parameters per line to minimize vertical space bloat. Interface definitions often have many similar methods, so aggressive packing significantly improves readability.

  type Logger interface {
-     Log(
-         level Level,
-         msg string,
-         args ...interface{},
-     )
+     Log(level Level, msg string, args ...interface{})

-     Error(
-         msg string,
-         err error,
-     )
+     Error(msg string, err error)
  }

C. Parameter Groups (Advanced)

When configured with param-groups, the linter keeps semantically related parameters together on the same line:

param-groups:
  - ["context.Context", "*sql.Tx"]  # Always group ctx and tx
  - ["context.Context"]              # If no tx, keep ctx on its own line

- func Query(
-     ctx context.Context,
-     tx *sql.Tx,
-     sql string,
-     args ...interface{},
- ) error {
+ func Query(
+     ctx context.Context, tx *sql.Tx,
+     sql string, args ...interface{},
+ ) error {
      // ...
  }

Width Calculation

The linter calculates visual width considering:

• Tab expansion (tab-width, default 8)
• Receiver length (for methods)
• Type parameter length (for generics)
• Return type length
• Comment preservation

Example:

// Visual width = len("func Map[T any, R any](items []T, fn func(T) R) []R")
func Map[T any, R any](items []T, fn func(T) R) []R  // 54 chars (fits in 120)

📸 Examples Gallery

1. Basic Function Collapsing

Simple Functions:

- func ShortFunction(
-     a int,
-     b string,
- ) error {
+ func ShortFunction(a int, b string) error {
      return nil
  }

- func Sum(
-     nums ...int,
- ) int {
+ func Sum(nums ...int) int {
      total := 0
      for _, n := range nums {
          total += n
      }
      return total
  }

Multiple Return Values:

- func MultipleReturns(
-     x int,
-     y int,
- ) (int, error) {
+ func MultipleReturns(x int, y int) (int, error) {
      return x + y, nil
  }

- func NamedReturns(
-     a int,
-     b int,
- ) (sum int, err error) {
+ func NamedReturns(a int, b int) (sum int, err error) {
      return a + b, nil
  }

Mixed Parameters (shorthand notation):

- func MixedParams(
-     a, b int,
-     c string,
- ) error {
+ func MixedParams(a, b int, c string) error {
      return nil
  }

2. Methods

Type Methods:

  type Calculator struct{}

- func (c *Calculator) Add(
-     a int,
-     b int,
- ) int {
+ func (c *Calculator) Add(a int, b int) int {
      return a + b
  }

Anonymous Functions / Closures:

- var myFunc = func(
-     a int,
-     b int,
- ) int {
+ var myFunc = func(a int, b int) int {
      return a + b
  }

  group.Go(
-     func(
-         ctx context.Context,
-     ) error {
+     func(ctx context.Context) error {
          // ...
      },
  )

3. Interfaces (Aggressive Packing)

Simple Interface Methods:

  type MyInterface interface {
-     Method(
-         ctx context.Context,
-     ) error
+     Method(ctx context.Context) error

-     Get(
-         id string,
-     ) error
+     Get(id string) error

-     GetMultiple(
-         id string,
-     ) (string, error)
+     GetMultiple(id string) (string, error)
  }

Complex Interfaces (Packing Multiple Parameters Per Line):

  type ComplexInterface interface {
-     ProcessWithVeryLongNameAndManyParameters(
-         parameterOne string,
-         parameterTwo string,
-         parameterThree string,
-         parameterFour string,
-     ) error
+     ProcessWithVeryLongNameAndManyParameters(parameterOne string, parameterTwo string,
+         parameterThree string, parameterFour string) error

-     ProcessManyParams(
-         parameterOne string,
-         parameterTwo string,
-         parameterThree string,
-         parameterFour string,
-         parameterFive string,
-         parameterSix string,
-         parameterSeven string,
-         parameterEight string,
-     ) error
+     ProcessManyParams(parameterOne string, parameterTwo string, parameterThree string,
+         parameterFour string, parameterFive string, parameterSix string,
+         parameterSeven string, parameterEight string) error
  }

Service Interfaces (Real-World Example):

- type UserRepository interface {
-     Create(
-         ctx context.Context,
-         name string,
-         email string,
-     ) (*User, error)
-
-     Update(
-         ctx context.Context,
-         id int,
-         name string,
-         email string,
-     ) error
-
-     Delete(
-         ctx context.Context,
-         id int,
-     ) error
- }
+ type UserRepository interface {
+     Create(ctx context.Context, name string, email string) (*User, error)
+     Update(ctx context.Context, id int, name string, email string) error
+     Delete(ctx context.Context, id int) error
+ }

Variadic Interface Methods:

  type VariadicInterface interface {
-     Process(
-         items ...string,
-     ) error
+     Process(items ...string) error
  }

Handler Interfaces (Functional Parameters):

  type HandlerInterface interface {
-     Handle(
-         ctx context.Context,
-         handler func(string) error,
-     ) error
+     Handle(ctx context.Context, handler func(string) error) error

      HandleMultiple(ctx context.Context, handlers ...func(string) error) error
  }

4. Struct Fields with Function Types

Simple Handlers:

  type Handler struct {
-     Process func(
-         ctx context.Context,
-     ) error
+     Process func(ctx context.Context) error
  }

Multiple Function Fields:

  type MultiHandler struct {
-     OnStart func(
-         id string,
-     ) error
+     OnStart func(id string) error

      OnStop func() error

-     OnProcessWithVeryLongNameAndManyParameters func(
-         parameterOne string,
-         parameterTwo string,
-         parameterThree string,
-         parameterFour string,
-     ) error
+     OnProcessWithVeryLongNameAndManyParameters func(parameterOne string,
+         parameterTwo string, parameterThree string, parameterFour string) error

-     GetData func(
-         key string,
-     ) (string, error)
+     GetData func(key string) (string, error)
  }

Variadic and Named Returns:

  type VariadicHandler struct {
-     Process func(
-         items ...string,
-     ) error
+     Process func(items ...string) error
  }

  type NamedReturnsHandler struct {
-     Process func(
-         id string,
-     ) (result string, err error)
+     Process func(id string) (result string, err error)
  }

Higher-Order Functions:

  type HigherOrderHandler struct {
-     GetHandler func(
-         config string,
-     ) func(string) error
+     GetHandler func(config string) func(string) error
  }

  type CallbackHandler struct {
-     Process func(
-         callback func(string) error,
-     ) error
+     Process func(callback func(string) error) error

      ProcessMultiple func(callback func(string) error, fallback func() error) error
  }

5. Generics (Go 1.18+)

Basic Generics:

- func Generic[
-     T any,
- ](
-     val T,
- ) {
+ func Generic[T any](val T) {
      // ...
  }

Multiple Type Parameters:

- func Map[
-     T any,
-     R any,
- ](
-     items []T,
-     fn func(T) R,
- ) []R {
+ func Map[T any, R any](items []T, fn func(T) R) []R {
      result := make([]R, len(items))
      for i, item := range items {
          result[i] = fn(item)
      }
      return result
  }

Generic Interfaces:

  type GenericInterface[T any] interface {
-     Process(
-         item T,
-     ) error
+     Process(item T) error

      GetAll() []T
  }

  type MultiGenericInterface[K comparable, V any] interface {
-     Get(
-         key K,
-     ) (V, bool)
+     Get(key K) (V, bool)

-     Set(
-         key K,
-         value V,
-     ) error
+     Set(key K, value V) error

      Delete(key K)
  }

Generic Struct Fields:

  type GenericHandler[T any] struct {
-     Process func(
-         item T,
-     ) error
+     Process func(item T) error

      Transform func(item T) T
  }

6. Complex Type Definitions

Channel Types:

- func Stream(
-     ctx context.Context,
-     in <-chan Item,
-     out chan<- Result,
- ) error {
+ func Stream(ctx context.Context, in <-chan Item, out chan<- Result) error {
      // ...
  }

API Handlers:

- func CreateUser(
-     w http.ResponseWriter,
-     r *http.Request,
- ) {
+ func CreateUser(w http.ResponseWriter, r *http.Request) {
      // ...
  }

- func UpdateUser(w http.ResponseWriter, r *http.Request, id string,
-     name string, email string) {
+ func UpdateUser(w http.ResponseWriter, r *http.Request, id string, name string, email string) {
      // ...
  }

Complex Order Processing:

- func ProcessOrder(
-     ctx context.Context,
-     orderID string,
-     items []Item,
-     shippingAddress *Address,
-     paymentMethod PaymentMethod,
-     options ...Option,
- ) (
-     *Order,
-     error,
- ) {
+ func ProcessOrder(ctx context.Context, orderID string, items []Item, shippingAddress *Address, paymentMethod PaymentMethod, options ...Option) (*Order, error) {
      // ...
  }

7. Parameter Grouping (Advanced Feature)

When using param-groups configuration, related parameters are kept together for better semantic organization:

*Database Operations (context.Context + sql.Tx grouping):

  // Configuration: param-groups: [["context.Context", "*sql.Tx"]]

- func LongQueryFunctionWithManyArguments(
-     ctx context.Context,
-     tx *sql.Tx,
-     query string,
-     args ...interface{},
- ) error {
+ func LongQueryFunctionWithManyArguments(
+     ctx context.Context, tx *sql.Tx,
+     query string, args ...interface{}) error {
      return nil
  }

Repository Interface with Grouping:

  type Repository interface {
-     Create(
-         ctx context.Context,
-         tx *sql.Tx,
-         id int,
-         name string,
-     ) error
+     Create(ctx context.Context, tx *sql.Tx,
+         id int, name string) error

-     Update(
-         ctx context.Context,
-         data []byte,
-     ) error
+     Update(ctx context.Context, data []byte) error
  }

Handler Functions with Grouping:

  type Handler struct {
-     OnCreate func(
-         ctx context.Context,
-         tx *sql.Tx,
-         data string,
-     ) error
+     OnCreate func(ctx context.Context,
+         tx *sql.Tx, data string) error

-     OnUpdate func(
-         ctx context.Context,
-         id int,
-     ) error
+     OnUpdate func(ctx context.Context, id int) error
  }

8. Long Signatures (Reformatting Strategy)

When signatures don't fit on one line, sigfmt applies intelligent packing to maximize readability while minimizing vertical space:

  func ComplexCalculation(
-     inputMatrix [][]float64,
-     weights []float64,
-     bias float64,
-     activationFunc func(float64) float64,
-     learningRate float64,
-     epochs int,
-     dropoutRate float64,
  ) error {
+ func ComplexCalculation(
+     inputMatrix [][]float64, weights []float64, bias float64,
+     activationFunc func(float64) float64, learningRate float64,
+     epochs int, dropoutRate float64,
+ ) error {
      // ...
  }

9. Local Structs in Functions

  func ComplexCase() {
      type LocalStruct struct {
-         Handler func(
-             ctx context.Context,
-             id string,
-         ) error
+         Handler func(ctx context.Context, id string) error

          Simple func() error

-         VeryLongHandler func(
-             parameterWithVeryLongName string,
-             anotherParameterWithVeryLongName string,
-             yetAnotherParameterWithVeryLongName string,
-         ) error
+         VeryLongHandler func(parameterWithVeryLongName string,
+             anotherParameterWithVeryLongName string,
+             yetAnotherParameterWithVeryLongName string) error
      }
      _ = LocalStruct{}
  }

⚡ Performance

• Fast: Does not require type loading (register.LoadModeSyntax).
• Efficient: Uses minimal memory (~20-30MB for 10k LOC) and works only with AST.
• Parallel: Supports parallel execution via golangci-lint.

🤔 FAQ

**Why not use gofmt or gofumpt?** They provide basic formatting but lack strict rules for line length and parameter packing. sigfmt` complements them.

Can parameter packing be disabled? Yes, use pack-struct-fields: false and pack-interface-methods: false.

How to handle special cases? Use //nolint:sigfmt to ignore specific functions.

💻 Development

The project uses Makefile for common development tasks:

• make test: Run all tests.
• make test-race: Run tests with the race detector.
• make test-coverage: Generate a test coverage report.
• make test-update-golden: Update golden files (use when diagnostic messages or expected fixes change).
• make fmt: Format Go code.
• make lint: Run linters.
• make check: Run all checks (test, fmt, lint).
• make build-example: Build the example custom golangci-lint binary.
• make run-example: Run the linter on the example project.
• make clean: Clean up build artifacts.

Updating Diagnostic Messages

When updating diagnostic messages or changing the expected output of the linter, follow these steps:

1. Update constants in linter.go (or internal/format/formatter.go for messages).
2. Run tests (make test). They will likely fail, indicating where the messages need to be updated.
3. Update diagnostic messages in test files: find testdata/src -name "*.go" -exec sed -i '' 's/OLD_MESSAGE/NEW_MESSAGE/g' {} \; (replace OLD_MESSAGE and NEW_MESSAGE accordingly).
4. Update golden files: make test-update-golden
5. Verify all tests pass: make test

❤️ Community & Acknowledgements

This project is made with ❤️ for the open-source community by Vadim Fedorenko.

🤝 Contributing

Contributions are welcome!

1. Fork the repository.
2. Create a branch.
3. Ensure tests pass (make test).
4. Submit a Pull Request.

📝 License

MIT License

🔗 Useful Links

• golangci-lint documentation
• Developing custom linters