Challenge 1 - Reflection

Write a tool that produces JSON from a struct with reflect.

Challenge: a mini JSON encoder

In this challenge you will write a small encoder that reads struct fields by their tags using reflection. Rules: the field name comes from the json tag; if the tag is "-" the field is skipped; without a tag the field name is used.

type User struct {
    Ad    string `json:"ad"`
    Sifre string `json:"-"`
    Yas   int
}
// expected: ad=Ada Yas=36

Plan

Steps: get reflect.TypeOf and reflect.ValueOf → NumField loop → read the tag for each field → continue on "-" → f.Name if empty → append a name=value piece to a slice → join with strings.Join. On unexported (lower-case) fields Interface() panics; skip them too with f.IsExported().

Exercise Write the encoder

Write encode(v any) string and print the result for the User{"Ada", "1234", 36} value above. The output must be ad=Ada Yas=36.

Show example solution

package main

import (
    "fmt"
    "reflect"
    "strings"
)

type User struct {
    Ad    string `json:"ad"`
    Sifre string `json:"-"`
    Yas   int
}

func encode(v any) string {
    t, val := reflect.TypeOf(v), reflect.ValueOf(v)
    var parts []string
    for i := 0; i < t.NumField(); i++ {
        f := t.Field(i)
        tag := f.Tag.Get("json")
        if tag == "-" || !f.IsExported() {
            continue
        }
        if tag == "" {
            tag = f.Name
        }
        parts = append(parts, fmt.Sprintf("%s=%v", tag, val.Field(i).Interface()))
    }
    return strings.Join(parts, " ")
}

func main() {
    fmt.Println(encode(User{"Ada", "1234", 36}))
}

Test yourself

Where do you get the name of a field with no tag?
- Tag.Get("json")
- Value.String()
- Type.Name()
- StructField.Name
Show answer

StructField.Name — Tag.Get returns empty; in that case StructField.Name (the Go field name) is used. Type.Name() is the struct type's name.

Skip the field

if tag == "-" || !f.IsExported() {
    ____
}

Show answer

if tag == "-" || !f.IsExported() {
    continue
}

Rule and code

Etiket "-"

continue
No tag

f.Name
Unexported field

!f.IsExported()
Joining

strings.Join

Next lesson: Challenge 2 - Microservice

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