How do you validate HTTP request inputs in Go without a framework?

Input validation is a layered concern: structural validation (is the JSON well-formed?), field validation (are required fields present, within range?), and business validation (does the email already exist?). Go handles this without a framework using explicit checks and helper functions.

// Request and validation error types
type CreateUserRequest struct {
    Name  string `json:"name"`
    Email string `json:"email"`
    Age   int    `json:"age"`
}

type ValidationErrors map[string]string
func (ve ValidationErrors) Error() string {
    msgs := make([]string, 0, len(ve))
    for k, v := range ve { msgs = append(msgs, k+": "+v) }
    return strings.Join(msgs, ", ")
}

// Validator function
func validateCreateUser(req CreateUserRequest) error {
    errs := ValidationErrors{}

    if strings.TrimSpace(req.Name) == "" {
        errs["name"] = "is required"
    } else if len(req.Name) > 100 {
        errs["name"] = "must be 100 characters or fewer"
    }

    if req.Email == "" {
        errs["email"] = "is required"
    } else if !isValidEmail(req.Email) {
        errs["email"] = "is not a valid email address"
    }

    if req.Age < 0 || req.Age > 150 {
        errs["age"] = "must be between 0 and 150"
    }

    if len(errs) > 0 { return errs }
    return nil
}

// Handler
func createUserHandler(w http.ResponseWriter, r *http.Request) error {
    var req CreateUserRequest
    if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
        return &APIError{Status: 400, Code: "INVALID_JSON",
            Message: "request body must be valid JSON"}
    }

    if err := validateCreateUser(req); err != nil {
        var ve ValidationErrors
        if errors.As(err, &ve) {
            writeJSON(w, http.StatusUnprocessableEntity,
                map[string]any{"errors": ve})
            return nil
        }
        return err
    }

    user, err := userService.Create(r.Context(), req)
    if err != nil { return err }
    return writeJSON(w, http.StatusCreated, user)
}

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