Deliver an error type.

The error type was provided to remove all the mutable error variables.

errors.go

@@ -0,0 +1,31 @@
+package cachego
+
+import "fmt"
+
+type err string
+
+// Error returns the string error value.
+func (e err) Error() string {
+	return string(e)
+}
+
+const (
+	// ErrCacheExpired returns an error when the cache key was expired.
+	ErrCacheExpired = err("cache expired")
+
+	// ErrFlush returns an error when flush fails.
+	ErrFlush = err("unable to flush")
+
+	// ErrSave returns an error when save fails.
+	ErrSave = err("unable to save")
+
+	// ErrDelete returns an error when deletion fails.
+	ErrDelete = err("unable to delete")
+
+	// ErrDecode returns an errors when decode fails.
+	ErrDecode = err("unable to decode")
+)
+
+func Wrap(err, additionalErr error) error {
+	return fmt.Errorf("%s: %w", additionalErr, err)
+}

errors_test.go

@@ -0,0 +1,25 @@
+package cachego
+
+import (
+	"errors"
+	"fmt"
+	"testing"
+)
+
+func TestError(t *testing.T) {
+	expect := "failed"
+	er := err(expect)
+
+	if r := fmt.Sprint(er); r != expect {
+		t.Errorf("invalid string: expect %s, got %s", expect, r)
+	}
+}
+
+func TestWrap(t *testing.T) {
+	additionalErr := errors.New("failed")
+	err := Wrap(ErrSave, additionalErr)
+
+	if !errors.Is(err, ErrSave) {
+		t.Errorf("wrap failed: expected true")
+	}
+}