fix Compound.String argument rendering

trealla/query_test.go

@@ -332,6 +332,8 @@ func Example() {
 
 	// start a new query
 	query := pl.Query(ctx, "member(X, [1, foo(bar), c]).")
+	// calling Close is not necessary if you iterate through the whole query, but it doesn't hurt
+	defer query.Close()
 
 	// iterate through answers
 	for query.Next(ctx) {

trealla/term.go

@@ -203,7 +203,12 @@ func (c Compound) String() string {
 		if i > 0 {
 			buf.WriteString(", ")
 		}
-		buf.WriteString(fmt.Sprintf("%v", arg))
+		text, err := marshal(arg)
+		if err != nil {
+			buf.WriteString(fmt.Sprintf("<invalid: %v>", err))
+			continue
+		}
+		buf.WriteString(text)
 	}
 	buf.WriteRune(')')
 	return buf.String()

trealla/term_test.go

@@ -5,7 +5,7 @@ import "testing"
 func TestCompound(t *testing.T) {
 	c0 := Compound{
 		Functor: "foo",
-		Args:    []Term{"bar", 4.2},
+		Args:    []Term{Atom("bar"), 4.2},
 	}
 	want := "foo(bar, 4.2)"
 	if c0.String() != want {