lang: Fix panic in lang/types/ValueOf() for Struct

Replace use of reflect.Value.Len() with NumField() which is intended to
return the number of fields in reflected Struct value.

Len should only be used for Array, Chan, Map, Slice and String types.

Add some trivial sanity check tests for ValueOf() for the simple and
complex container types.

Signed-off-by: Joe Groocock <me@frebib.net>

lang/types/value.go

@@ -143,7 +143,7 @@ func ValueOf(v reflect.Value) (Value, error) {
 		if err != nil {
 			return nil, errwrap.Wrapf(err, "can't determine type of %+v", value)
 		}
-		l := value.Len() // number of struct fields according to value
+		l := value.NumField() // number of struct fields according to value
 
 		if l != len(t.Ord) {
 			// programming error?

lang/types/value_test.go

@@ -21,6 +21,7 @@ package types
 
 import (
 	"fmt"
+	"reflect"
 	"sort"
 	"testing"
 )
@@ -590,5 +591,51 @@ func TestStruct2(t *testing.T) {
 }
 
 func TestValueOf0(t *testing.T) {
-	// TODO: implement testing of the ValueOf function
+	testCases := map[Value]interface{}{
+		&BoolValue{V: true}:  true,
+		&StrValue{V: "abc"}:  "abc",
+		&IntValue{V: 4}:      4,
+		&IntValue{V: -4}:     -4,
+		&FloatValue{V: 9.87}: 9.87,
+		&ListValue{
+			T: NewType("[]int"),
+			V: []Value{
+				&IntValue{V: 1},
+				&IntValue{V: 3},
+				&IntValue{V: 5},
+			},
+		}: []int64{1, 3, 5},
+		&MapValue{
+			T: NewType("map{str: int}"),
+			V: map[Value]Value{
+				&StrValue{V: "a"}: &IntValue{V: 1},
+				&StrValue{V: "b"}: &IntValue{V: 2},
+				&StrValue{V: "c"}: &IntValue{V: 3},
+			},
+		}: map[string]int{"a": 1, "b": 2, "c": 3}, // go map ordering is alphabetically sorted
+		&StructValue{
+			T: NewType("struct{num int; name str}"),
+			V: map[string]Value{
+				"num":  &IntValue{V: 42},
+				"name": &StrValue{V: "mgmt"},
+			},
+		}: struct {
+			num  int
+			name string
+		}{42, "mgmt"},
+		// TODO: implement ValueOf tests for TypeFunc
+	}
+
+	for value, gotyp := range testCases {
+		// get reflect.Value, then call ValueOf() for types.Value
+		val, err := ValueOf(reflect.ValueOf(gotyp))
+		if err != nil {
+			t.Errorf("function ValueOf(%+v) returned err %s", gotyp, err)
+		}
+		// use string representation comparison as maps are non-deterministic in order
+		// and cmp doesn't work as the pointers differ
+		if val.String() != value.String() {
+			t.Errorf("function ValueOf(%+v) gave %+v and doesn't match expected %+v", gotyp, val, value)
+		}
+	}
 }