groot/riofs: add generic Get[T] to retrieve ROOT objects

groot/riofs/walk.go

@@ -229,6 +229,22 @@ func fileOf(d Directory) *File {
 	panic("impossible")
 }
 
+// Get retrieves the named key from the provided directory.
+func Get[T any](dir Directory, key string) (T, error) {
+	obj, err := Dir(dir).Get(key)
+	if err != nil {
+		var v T
+		return v, err
+	}
+
+	v, ok := obj.(T)
+	if !ok {
+		return v, fmt.Errorf("riofs: could not convert %q (%T) to %T", key, obj, *new(T))
+	}
+
+	return v, nil
+}
+
 var (
 	_ Directory = (*recDir)(nil)
 )

groot/riofs/walk_example_test.go

@@ -10,6 +10,7 @@ import (
 	stdpath "path"
 	"strings"
 
+	"go-hep.org/x/hep/groot/rhist"
 	"go-hep.org/x/hep/groot/riofs"
 	"go-hep.org/x/hep/groot/root"
 )
@@ -56,3 +57,20 @@ func ExampleWalk() {
 	// dirs-6.14.00.root/dir1/dir11 (TDirectoryFile)
 	// dirs-6.14.00.root/dir1/dir11/h1 (TH1F)
 }
+
+func ExampleGet() {
+	f, err := riofs.Open("../testdata/dirs-6.14.00.root")
+	if err != nil {
+		log.Fatal(err)
+	}
+	defer f.Close()
+
+	h1, err := riofs.Get[rhist.H1](f, "dir1/dir11/h1")
+	if err != nil {
+		log.Fatal(err)
+	}
+	fmt.Printf("histo: %s (%s)", h1.Name(), h1.Class())
+
+	// Output:
+	// histo: h1 (TH1F)
+}

groot/riofs/walk_test.go

@@ -13,9 +13,55 @@ import (
 	"testing"
 
 	"go-hep.org/x/hep/groot/rbase"
+	"go-hep.org/x/hep/groot/rhist"
 	"go-hep.org/x/hep/groot/root"
 )
 
+func TestGet(t *testing.T) {
+	f, err := Open("../testdata/dirs-6.14.00.root")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer f.Close()
+
+	h1, err := Get[rhist.H1](f, "dir1/dir11/h1")
+	if err != nil {
+		t.Fatalf("could not get histo: %+v", err)
+	}
+	if h1 == nil {
+		t.Fatalf("invalid H1 value")
+	}
+
+	h1f, err := Get[*rhist.H1F](f, "dir1/dir11/h1")
+	if err != nil {
+		t.Fatalf("could not get histo: %+v", err)
+	}
+	if h1f == nil {
+		t.Fatalf("invalid H1F value")
+	}
+
+	_, err = Get[*rhist.H1D](f, "dir1/dir11/h1")
+	if err == nil {
+		t.Fatalf("expected an error")
+	}
+	if want := fmt.Errorf(`riofs: could not convert "dir1/dir11/h1" (*rhist.H1F) to *rhist.H1D`); err.Error() != want.Error() {
+		t.Fatalf("invalid error:\ngot= %+v\nwant=%+v", err, want)
+	}
+
+	_, err = Get[any](f, "dir1/dir11/h1")
+	if err != nil {
+		t.Fatalf("could not get histo: %+v", err)
+	}
+
+	_, err = Get[any](f, "dir1/dir11/h1_XXX")
+	if err == nil {
+		t.Fatalf("expected an error")
+	}
+	if want := fmt.Errorf(`riofs: dir11: could not find key "h1_XXX;9999"`); err.Error() != want.Error() {
+		t.Fatalf("invalid error:\ngot= %+v\nwant=%+v", err, want)
+	}
+
+}
 func TestDir(t *testing.T) {
 	f, err := Open("../testdata/dirs-6.14.00.root")
 	if err != nil {