feat: explanation as observer too

internal/explanation/tracker.go

@@ -0,0 +1,28 @@
+package explanation
+
+import (
+	"fmt"
+
+	"git.maximhutz.com/max/lambda/internal/executer"
+	"git.maximhutz.com/max/lambda/pkg/lambda"
+)
+
+type Tracker struct {
+	process *executer.Executor
+}
+
+func Track(process *executer.Executor) *Tracker {
+	tracker := &Tracker{process: process}
+	process.On("start", tracker.Start)
+	process.On("step", tracker.Step)
+
+	return tracker
+}
+
+func (t *Tracker) Start() {
+	fmt.Println(lambda.Stringify(*t.process.Expression))
+}
+
+func (t *Tracker) Step() {
+	fmt.Println("    =", lambda.Stringify(*t.process.Expression))
+}