Go Mapper

Go Mapper

Generate pure mapper functions for GO modular architecture.

When to Use

• Map HTTP request DTOs to use case inputs
• Map persistence models to HTTP response DTOs
• Map between any two struct representations across layers
• Convert a slice of structs to a slice of another struct
• Any struct-to-struct transformation that lives in a module

Location

All mapper files live in internal/modules/<module>/mapper/.

One file per domain concept: <name>_mapper.go (e.g., user_mapper.go).

Function Signature Pattern

Every mapper function follows the To prefix convention. It accepts one or more inputs and returns exactly one output, optionally with an error.

func ToXxx(input InputType) OutputType
func ToXxx(input InputType) (OutputType, error)
func ToXxx(a TypeA, b TypeB) OutputType

Single output only. Error is the only allowed second return value.

File Structure

1. Package declaration and imports 2. Public mapper functions 3. Private helper functions (shared logic between public functions)

Examples

Basic mapper

package mapper

import (
	"github.com/cristiano-pacheco/pingo/internal/modules/user/dto"
	"github.com/cristiano-pacheco/pingo/internal/modules/user/model"
	"github.com/cristiano-pacheco/pingo/internal/modules/user/usecase"
)

func ToCreateUserInput(req dto.CreateUserRequest) usecase.CreateUserInput {
	return usecase.CreateUserInput{
		Name:  req.Name,
		Email: req.Email,
	}
}

func ToUserResponse(u model.UserModel) dto.UserResponse {
	return dto.UserResponse{
		ID:        u.ID,
		Name:      u.Name,
		Email:     u.Email,
		CreatedAt: u.CreatedAt,
	}
}

func ToUserListResponse(models []model.UserModel) []dto.UserResponse {
	responses := make([]dto.UserResponse, len(models))
	for i, u := range models {
		responses[i] = ToUserResponse(u)
	}
	return responses
}

Mapper with multiple inputs

package mapper

import (
	"github.com/cristiano-pacheco/pingo/internal/modules/order/dto"
	"github.com/cristiano-pacheco/pingo/internal/modules/order/model"
)

func ToOrderResponse(order model.OrderModel, items []model.OrderItemModel) dto.OrderResponse {
	return dto.OrderResponse{
		ID:    order.ID,
		Total: order.Total,
		Items: toOrderItemResponses(items),
	}
}

func toOrderItemResponses(items []model.OrderItemModel) []dto.OrderItemResponse {
	responses := make([]dto.OrderItemResponse, len(items))
	for i, item := range items {
		responses[i] = dto.OrderItemResponse{
			ID:       item.ID,
			Name:     item.ProductName,
			Quantity: item.Quantity,
			Price:    item.Price,
		}
	}
	return responses
}

Mapper with error return

Use when transformation can fail (e.g., parsing, validation during mapping).

package mapper

import (
	"github.com/cristiano-pacheco/pingo/internal/modules/product/dto"
	"github.com/cristiano-pacheco/pingo/internal/modules/product/errs"
	"github.com/cristiano-pacheco/pingo/internal/modules/product/model"
)

func ToProductModel(req dto.CreateProductRequest) (model.ProductModel, error) {
	price, err := parsePrice(req.Price)
	if err != nil {
		return model.ProductModel{}, errs.ErrInvalidPrice
	}
	return model.ProductModel{
		Name:  req.Name,
		Price: price,
	}, nil
}

Slice mapper pattern

When mapping a single item, add a corresponding list function if the type appears in collections.

func ToArticleResponse(a model.ArticleModel) dto.ArticleResponse {
	return dto.ArticleResponse{
		ID:     a.ID,
		Title:  a.Title,
		Author: toAuthorResponse(a),
	}
}

func ToArticleListResponse(models []model.ArticleModel) []dto.ArticleResponse {
	responses := make([]dto.ArticleResponse, len(models))
	for i, a := range models {
		responses[i] = ToArticleResponse(a)
	}
	return responses
}

func toAuthorResponse(a model.ArticleModel) dto.AuthorResponse {
	return dto.AuthorResponse{
		ID:   a.AuthorID,
		Name: a.AuthorName,
	}
}

Naming

• File: <name>_mapper.go in mapper/ package
• Public functions: ToXxx — where Xxx is the output type name (e.g., ToUserResponse, ToCreateUserInput)
• Private helpers: toXxx — lowercase prefix for shared sub-mapping logic
• Slice variants: ToXxxList or ToXxxListResponse

Rules

1. Functions only — no structs, no interfaces, no constructors, no port files 2. `To` prefix — every public function starts with To 3. Single output — one return value, or one return value + error 4. No `context.Context` — mappers are pure transformations, no I/O 5. No side effects — no logging, no database calls, no external dependencies 6. Private helpers — extract shared sub-mapping logic into private functions 7. No comments on functions — function names are self-documenting via the To convention 8. Slice helpers — add a list variant when the mapped type appears in collections 9. Error return only when transformation can fail — prefer no-error signatures; use error only for parsing or conversion failures

Workflow

1. Create mapper file in mapper/<name>_mapper.go 2. Run make lint to verify code quality 3. Run make nilaway for static analysis