Go - lesson 4 | HackIndia Skills

Go - lesson 4 | HackIndia Skills | HackIndia

Example

Repository error

This example uses a map from your data lesson. The repository returns a typed value and a useful error.

The caller can check ErrProductNotFound without comparing text like "not found".

package store

import (
	"errors"
	"fmt"
)

type Product struct {
	ID    string
	Name  string
	Price int
}

var ErrProductNotFound = errors.New("product not found")

type ProductStore struct {
	items map[string]Product
}

func NewProductStore() ProductStore {
	return ProductStore{
		items: map[string]Product{
			"chai": {ID: "chai", Name: "Masala Chai", Price: 20},
		},
	}
}

func (s ProductStore) Get(id string) (Product, error) {
	p, ok := s.items[id]
	if !ok {
		return Product{}, fmt.Errorf("get product %q: %w", id, ErrProductNotFound)
	}
	return p, nil
}

Tip

Wrap, then check

Use fmt.Errorf with %w when you want to keep the original error inside a clearer message. This gives logs context and still lets code detect the real error.

Later, your HTTP handler can use errors.Is(err, ErrProductNotFound) and return 404 instead of 500.

if errors.Is(err, store.ErrProductNotFound) {
	// return HTTP 404 later
}

Example

Validate input

Validation is also a good place for useful errors. Keep the message short, but mention the field that failed.

For prices in rupees, reject zero or negative values before saving.

package store

import "fmt"

func ValidateProduct(p Product) error {
	if p.ID == "" {
		return fmt.Errorf("id is required")
	}
	if p.Price <= 0 {
		return fmt.Errorf("price must be positive: got %d", p.Price)
	}
	return nil
}

Return useful errors

One idea

Why this matters

Shape errors well

Repository error

Wrap, then check

Avoid string checks

Validate input

Do this now

Quick check