add go examples

05-typestate-go-structs/go.mod

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+module typestate-structs
+
+go 1.26.5

05-typestate-go-structs/main.go

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+package main
+
+type LockedCredentialManager struct {
+	entries map[string]string
+}
+
+type UnlockedCredentialManager struct {
+	entries map[string]string
+}
+
+func NewCredentialManager() LockedCredentialManager {
+	return LockedCredentialManager{
+		entries: make(map[string]string),
+	}
+}
+
+func (m LockedCredentialManager) Unlock(masterSecret string) UnlockedCredentialManager {
+	return UnlockedCredentialManager{
+		entries: m.entries,
+	}
+}
+
+func (m UnlockedCredentialManager) Set(key string, secret string) {
+	panic("todo")
+}
+
+func (m UnlockedCredentialManager) Get(key string) (string, bool) {
+	panic("todo")
+}
+
+func (m UnlockedCredentialManager) Lock() LockedCredentialManager {
+	return LockedCredentialManager{
+		entries: m.entries,
+	}
+}
+
+func main() {
+	pw := NewCredentialManager()
+
+	unlocked := pw.Unlock("pssst")
+	unlocked.Set("foo", "bar")
+	_, _ = unlocked.Get("foo")
+
+	unlocked.Lock()
+}

06-typestate-go-interfaces/go.mod

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+module typestate-interfaces
+
+go 1.26.5

06-typestate-go-interfaces/main.go

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+package main
+
+import "errors"
+
+type LockedCredentialManager interface {
+	Unlock(masterKey string) (UnlockedCredentialManager, error)
+}
+
+type UnlockedCredentialManager interface {
+	Lock() (LockedCredentialManager, error)
+	Set(key string, secret string) error
+	Get(key string) (string, error)
+}
+
+type credentialManager struct {
+	unlocked bool
+	entries  map[string]string
+}
+
+func NewCredentialManager() LockedCredentialManager {
+	return &credentialManager{
+		entries: make(map[string]string),
+	}
+}
+
+func (t *credentialManager) Unlock(masterKey string) (UnlockedCredentialManager, error) {
+	if t.unlocked {
+		return nil, errors.New("already unlocked")
+	}
+	t.unlocked = true
+	return t, nil
+}
+
+func (t *credentialManager) Lock() (LockedCredentialManager, error) {
+	if !t.unlocked {
+		return nil, errors.New("already locked")
+	}
+	t.unlocked = false
+	return t, nil
+}
+
+func (t *credentialManager) Set(key string, secret string) error {
+	if !t.unlocked {
+		return errors.New("locked")
+	}
+	panic("todo")
+}
+
+func (t *credentialManager) Get(key string) (string, error) {
+	if !t.unlocked {
+		return "", errors.New("locked")
+	}
+	panic("todo")
+}

README.md

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+# Type Level Programming
+
+## Sources
+
+- https://www.emoses.org/posts/resolver-resolved-pattern/
+- https://farazdagi.com/posts/2024-04-07-typestate-pattern/
+- https://youtu.be/_ccDqRTx-JU

go.work

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+go 1.26.5
+
+use (
+	./05-typestate-go-structs
+	./06-typestate-go-interfaces
+)