Added filesystem abstraction

fs/fs.go

@@ -0,0 +1,69 @@
+package fs
+
+import (
+	"fmt"
+	"io"
+	"os"
+	"path/filepath"
+)
+
+type FS interface {
+	isFS()
+}
+
+type DirEntry struct {
+	Name string
+	FS   FS
+}
+
+type Entries interface {
+	Get() []DirEntry
+}
+
+type Dir struct {
+	Entries
+}
+
+func (d Dir) isFS() {}
+
+type Contents interface {
+	Get() io.Reader
+}
+
+type File struct {
+	Contents
+}
+
+func (f File) isFS() {}
+
+func Realize(path string, f FS) error {
+	if f == nil {
+		return fmt.Errorf("cannot realize nil FS at %s", path)
+	}
+	switch t := f.(type) {
+	case File:
+		osf, err := os.Create(path)
+		if err != nil {
+			return fmt.Errorf("cannot create file %s: %w", path, err)
+		}
+		defer osf.Close()
+		_, err = io.Copy(osf, t.Get())
+		if err != nil {
+			return fmt.Errorf("cannot write file %s: %w", path, err)
+		}
+	case Dir:
+		err := os.Mkdir(path, 0666)
+		if err != nil {
+			return fmt.Errorf("cannot create dir %s: %w", path, err)
+		}
+		for _, child := range t.Get() {
+			err := Realize(filepath.Join(path, child.Name), child.FS)
+			if err != nil {
+				return err
+			}
+		}
+	default:
+		panic("unknown File type")
+	}
+	return nil
+}