Golang Style

---
name: golang-style
description:
  Use this skill BEFORE writing or editing any Go (.go) files. Triggers when
  about to create, modify, or add code to .go files. Enforces happy path coding,
  error wrapping, sentinel errors, and godoc-style comments.
---

# Go Coding Conventions

Follow these conventions strictly when writing Go code.

## Happy Path Coding

Structure code so the successful path flows straight down. Handle errors
immediately, then continue with main logic.

```go
// Correct: happy path flows down.
func ProcessUser(id string) (*User, error) {
    user, err := db.GetUser(id)
    if err != nil {
        return nil, fmt.Errorf("get user %s: %w", id, err)
    }

    if err := user.Validate(); err != nil {
        return nil, fmt.Errorf("validate user %s: %w", id, err)
    }

    return user, nil
}

// Wrong: main logic nested inside conditions.
func ProcessUser(id string) (*User, error) {
    user, err := db.GetUser(id)
    if err == nil {
        if err := user.Validate(); err == nil {
            return user, nil
        } else {
            return nil, err
        }
    }
    return nil, err
}
```

## Error Wrapping

Always wrap errors with context using `%w`. Include the operation and relevant
identifiers.

```go
// Correct: wrapped with context.
if err != nil {
    return fmt.Errorf("create order for customer %s: %w", customerID, err)
}

// Wrong: no context.
if err != nil {
    return err
}
```

## Sentinel Errors

Define package-level sentinel errors for expected error conditions. Use
`errors.Is()` to check.

```go
// Define at package level.
var (
    ErrNotFound     = errors.New("not found")
    ErrUnauthorized = errors.New("unauthorized")
    ErrInvalidInput = errors.New("invalid input")
)

// Return sentinel errors.
func GetUser(id string) (*User, error) {
    user := db.Find(id)
    if user == nil {
        return nil, ErrNotFound
    }
    return user, nil
}

// Check with errors.Is().
user, err := GetUser(id)
if errors.Is(err, ErrNotFound) {
    // Handle not found case.
}
```

## Comments

All comments end with a period.

```go
// ProcessOrder handles order creation and validation.
func ProcessOrder(o *Order) error {
    // Validate the order before processing.
    if err := o.Validate(); err != nil {
        return err
    }
    // Continue with order processing.
    return nil
}
```

## Naming Conventions

Never use Go's predeclared identifiers as variable or parameter names. These
include built-in functions and constants that can be shadowed but should not be.

```go
// Wrong: shadows built-in identifiers.
func process(new string, len int, make bool) error {
    copy := "data"
    return nil
}

// Correct: use descriptive names instead.
func process(name string, length int, shouldCreate bool) error {
    dataCopy := "data"
    return nil
}
```

Predeclared identifiers to avoid:

- Functions: `new`, `make`, `len`, `cap`, `append`, `copy`, `delete`, `close`,
  `panic`, `recover`, `print`, `println`, `complex`, `real`, `imag`, `clear`,
  `min`, `max`
- Constants: `true`, `false`, `iota`, `nil`
- Types: `error`, `bool`, `string`, `int`, `int8`, `int16`, `int32`, `int64`,
  `uint`, `uint8`, `uint16`, `uint32`, `uint64`, `uintptr`, `float32`,
  `float64`, `complex64`, `complex128`, `byte`, `rune`, `any`, `comparable`

## Line Length

Maximum line length is 120 characters. Break long lines at logical points.

```go
// Correct: break at logical points.
func ProcessOrderWithValidation(
    ctx context.Context,
    order *Order,
    validator OrderValidator,
) (*Result, error) {
    return nil, fmt.Errorf(
        "process order %s for customer %s: %w",
        order.ID,
        order.CustomerID,
        err,
    )
}
```

## Documentation Lookup

Use `go doc` to look up standard library and package documentation:

```bash
go doc fmt.Errorf
go doc errors.Is
go doc context
```

## Einride

If the project is under the Einride organization, always use the Makefiles in
the project which are generated by Sage (the `.sage` folder).