new SubFS that can tell you the full FS path

pkg/subfs/subfs.go

@@ -0,0 +1,28 @@
+package transpile
+
+import (
+	"io/fs"
+	"path"
+)
+
+func Sub(fsys fs.FS, dir string) (*SubFS, error) {
+	return &SubFS{fsys, dir}, nil
+}
+
+type SubFS struct {
+	fs.FS
+	dir string
+}
+
+func (f *SubFS) FullName(name string) string {
+	return path.Join(f.dir, name)
+}
+
+func (f *SubFS) Sub(dir string) (*SubFS, error) {
+	newFS, err := fs.Sub(f, dir)
+	newSubFS := SubFS{
+		FS:  newFS,
+		dir: f.FullName(dir),
+	}
+	return &newSubFS, err
+}

pkg/subfs/subfs_test.go

@@ -0,0 +1,26 @@
+package transpile
+
+import (
+	"io/fs"
+	"os"
+	"testing"
+)
+
+func TestSubFS(t *testing.T) {
+	testdata := os.DirFS("testdata")
+	if static, err := Sub(testdata, "static"); err != nil {
+		t.Error(err)
+	} else if buf, err := fs.ReadFile(static, "moo.txt"); err != nil {
+		t.Error(err)
+	} else if string(buf) != "moo.\n" {
+		t.Error("Wrong file contents")
+	} else if subdir, err := static.Sub("subdir"); err != nil {
+		t.Error(err)
+	} else if buf, err := fs.ReadFile(subdir, "moo2.txt"); err != nil {
+		t.Error(err)
+	} else if string(buf) != "moo too.\n" {
+		t.Error("Wrong file contents too")
+	} else if subdir.FullName("glue") != "static/subdir/glue" {
+		t.Error("Wrong full name")
+	}
+}

pkg/subfs/testdata/moo.txt

@@ -0,0 +1 @@
+moo.

pkg/subfs/testdata/subdir/moo2.txt

@@ -0,0 +1 @@
+moo too.