implement ErrRequired

Author: MarkRosemaker

required.go

@@ -0,0 +1,23 @@
+package errpath
+
+import "strings"
+
+var _ pathWriter = (*ErrRequired)(nil)
+
+// ErrRequired signals that a required value is missing.
+type ErrRequired struct{}
+
+// Error fulfills the error interface.
+// Without a previous error path, it simply says "a value is required".
+// Naturally, this is not very helpful, so it makes sense to wrap `ErrRequired` in another error such as `ErrField` so the user knows that the field was required.
+func (e *ErrRequired) Error() string {
+	b := &strings.Builder{}
+	b.WriteString("a value")
+	e.writePath(b)
+	return b.String()
+}
+
+// writePath appends `" is required"` to the path so that the user knows the given path was required
+func (e *ErrRequired) writePath(b *strings.Builder) {
+	b.WriteString(" is required")
+}

required_test.go

@@ -0,0 +1,14 @@
+package errpath_test
+
+import (
+	"testing"
+
+	"github.com/MarkRosemaker/errpath"
+)
+
+func TestErrRequired(t *testing.T) {
+	err := &errpath.ErrRequired{}
+	if want := `a value is required`; err.Error() != want {
+		t.Fatalf("want: %s, got: %v", want, err)
+	}
+}