proceed to learn go!

creditcards/creditcards.go

@@ -0,0 +1,19 @@
+package creditcards
+
+import "errors"
+
+
+type card struct {
+	number string
+}
+
+func New(number string) (card,error) {
+	if number == "" {
+		return card{}, errors.New("number must not be empty")
+	}
+	return card{number},nil
+}
+
+func (c *card) Number() string {
+	return c.number
+}

creditcards/creditcards_test.go

@@ -0,0 +1,26 @@
+package creditcards_test
+
+import (
+	"creditcards"
+	"testing"
+)
+
+func TestNew(t *testing.T) {
+	t.Parallel()
+	want := "1234567890"
+	cc, err := creditcards.New(want)
+	if err != nil {
+		t.Fatal(err)
+	}
+	got := cc.Number()
+	if want != got {
+		t.Errorf("want %q, got %q", want, got)
+	}
+}
+func TestNewInvalidReturnsError(t *testing.T) {
+    t.Parallel()
+    _, err := creditcards.New("")
+    if err == nil {
+        t.Fatal("want error for invalid card number, got nil")
+    }
+}

creditcards/go.mod

@@ -0,0 +1,3 @@
+module creditcards
+
+go 1.24.4