diff --git a/content/20200826151514-variables.md b/content/20200826151514-variables.md
index 64b3055..443044d 100644
--- a/content/20200826151514-variables.md
+++ b/content/20200826151514-variables.md
@@ -6,7 +6,8 @@ title: Golang variables
 # Basics
 
 The \`var\` statement declares a list of variables. Type is last. A
-\`var\` statement can be at package or function level.
+\`var\` statement can be at package or function level. Use var when you
+to set a variable type but don't want to set the variable value yet.
 
 ``` go
 package main
@@ -21,6 +22,33 @@ func main() {
 }
 ```
 
+This also works with [interfaces](20200831171822-interfaces).
+
+``` go
+package main
+
+import "fmt"
+
+type printSomething interface {
+    print()
+}
+
+type agent struct {
+}
+
+func (d *agent) print() {
+    fmt.Println("tralala")
+}
+
+var (
+    a = &agent{}
+)
+
+func main() {
+    a.print()
+}
+```
+
 # Initializers
 
 A var declaration can include initializers, one per variable. If an
diff --git a/content/20200828181259-structs.md b/content/20200828181259-structs.md
index a82f8a1..3a86534 100644
--- a/content/20200828181259-structs.md
+++ b/content/20200828181259-structs.md
@@ -68,3 +68,118 @@ func main() {
     fmt.Println(v1, p, v2, v3)
 }
 ```
+
+# Struct tags
+
+[Struct tags](https://go.dev/wiki/Well-known-struct-tags) allow other
+modules accessing struct members to format member values.
+
+## Control encoding
+
+``` go
+package main
+
+import (
+    "encoding/json"
+    "fmt"
+    "log"
+    "os"
+    "time"
+)
+
+type User struct {
+    Name          string    `json:"name"`
+    Password      string    `json:"password"`
+    PreferredFish []string  `json:"preferredFish"`
+    CreatedAt     time.Time `json:"createdAt"`
+}
+
+func main() {
+    u := &User{
+        Name:      "Sammy the Shark",
+        Password:  "fisharegreat",
+        CreatedAt: time.Now(),
+    }
+
+    out, err := json.MarshalIndent(u, "", "  ")
+    if err != nil {
+        log.Println(err)
+        os.Exit(1)
+    }
+
+    fmt.Println(string(out))
+}
+```
+
+## Remove empty fields
+
+``` go
+package main
+
+import (
+    "encoding/json"
+    "fmt"
+    "log"
+    "os"
+    "time"
+)
+
+type User struct {
+    Name          string    `json:"name"`
+    Password      string    `json:"password"`
+    PreferredFish []string  `json:"preferredFish,omitempty"`
+    CreatedAt     time.Time `json:"createdAt"`
+}
+
+func main() {
+    u := &User{
+        Name:      "Sammy the Shark",
+        Password:  "fisharegreat",
+        CreatedAt: time.Now(),
+    }
+
+    out, err := json.MarshalIndent(u, "", "  ")
+    if err != nil {
+        log.Println(err)
+        os.Exit(1)
+    }
+
+    fmt.Println(string(out))
+}
+```
+
+## Ignore private fields
+
+``` go
+package main
+
+import (
+    "encoding/json"
+    "fmt"
+    "log"
+    "os"
+    "time"
+)
+
+type User struct {
+    Name      string    `json:"name"`
+    Password  string    `json:"-"`
+    CreatedAt time.Time `json:"createdAt"`
+}
+
+func main() {
+    u := &User{
+        Name:      "Sammy the Shark",
+        Password:  "fisharegreat",
+        CreatedAt: time.Now(),
+    }
+
+    out, err := json.MarshalIndent(u, "", "  ")
+    if err != nil {
+        log.Println(err)
+        os.Exit(1)
+    }
+
+    fmt.Println(string(out))
+}
+```