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

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))
 	}

test/fuzz/fuzz_corpus.txt

@@ -7221,6 +7221,11 @@ get(true ? i64 : greet, i32)
 get(true ? score : true, half)?.half
 get(true ? true : i, f64)
 get({"foo": foo, "bar": false}, type(i))
+get(nil, "a")
+get({}, "a")
+get(nil, "a") == nil
+get({}, "a") == nil
+({} | get("a") | get("b")) == nil
 greet
 greet != greet
 greet != greet != ok

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)
+	}
+}