fix(785): get to work when return is nil (#786)

* fix(785): `get` to work when return is `nil`

* improve testing

Author: tarasyarema

builtin/lib.go

@@ -374,6 +374,10 @@ func get(params ...any) (out any, err error) {
 	i := params[1]
 	v := reflect.ValueOf(from)
 
+	if from == nil {
+		return nil, nil
+	}
+
 	if v.Kind() == reflect.Invalid {
 		panic(fmt.Sprintf("cannot fetch %v from %T", i, from))
 	}

expr_test.go

@@ -2724,3 +2724,24 @@ func TestExpr_wierd_cases(t *testing.T) {
 	require.Error(t, err)
 	require.Contains(t, err.Error(), "unknown name A")
 }
+
+func TestIssue785_get_nil(t *testing.T) {
+	exprStrs := []string{
+		`get(nil, "a")`,
+		`get({}, "a")`,
+		`get(nil, "a")`,
+		`get({}, "a")`,
+		`({} | get("a") | get("b"))`,
+	}
+
+	for _, exprStr := range exprStrs {
+		t.Run("get returns nil", func(t *testing.T) {
+			env := map[string]interface{}{}
+
+			result, err := expr.Eval(exprStr, env)
+			require.NoError(t, err)
+
+			require.Equal(t, nil, result)
+		})
+	}
+}

test/issues/785/issue_test.go

@@ -0,0 +1,38 @@
+package issue_test
+
+import (
+	"testing"
+
+	"github.com/expr-lang/expr"
+	"github.com/expr-lang/expr/internal/testify/require"
+)
+
+func TestIssue785(t *testing.T) {
+	emptyMap := map[string]any{}
+
+	env := map[string]interface{}{
+		"empty_map": emptyMap,
+	}
+
+	{
+		code := `get(empty_map, "non_existing_key") | get("some_key") | get("another_key") | get("yet_another_key") | get("last_key")`
+
+		program, err := expr.Compile(code, expr.Env(env))
+		require.NoError(t, err)
+
+		output, err := expr.Run(program, env)
+		require.NoError(t, err)
+		require.Equal(t, nil, output)
+	}
+
+	{
+		code := `{} | get("non_existing_key") | get("some_key") | get("another_key") | get("yet_another_key") | get("last_key")`
+
+		program, err := expr.Compile(code, expr.Env(env))
+		require.NoError(t, err)
+
+		output, err := expr.Run(program, env)
+		require.NoError(t, err)
+		require.Equal(t, nil, output)
+	}
+}