Add -update flag to gen tests

Author: antonmedv

test/gen/gen_test.go

@@ -1,27 +1,66 @@
 package main
 
 import (
+	"bufio"
+	"flag"
 	"os"
 	"strings"
 	"testing"
 
-	"github.com/expr-lang/expr/internal/testify/require"
-
 	"github.com/expr-lang/expr"
+	"github.com/expr-lang/expr/internal/testify/require"
 )
 
+var updateFlag = flag.Bool("update", false, "Drop failing lines from examples.txt")
+
 func TestGenerated(t *testing.T) {
+	flag.Parse()
+
 	b, err := os.ReadFile("../../testdata/examples.txt")
 	require.NoError(t, err)
 
 	examples := strings.TrimSpace(string(b))
+	var validLines []string
+
 	for _, line := range strings.Split(examples, "\n") {
+		line := line
 		t.Run(line, func(t *testing.T) {
 			program, err := expr.Compile(line, expr.Env(env))
-			require.NoError(t, err)
+			if err != nil {
+				if !*updateFlag {
+					t.Errorf("Compilation failed: %v", err)
+				}
+				return
+			}
 
 			_, err = expr.Run(program, env)
-			require.NoError(t, err)
+			if err != nil {
+				if !*updateFlag {
+					t.Errorf("Execution failed: %v", err)
+				}
+				return
+			}
+
+			validLines = append(validLines, line)
 		})
 	}
+
+	if *updateFlag {
+		file, err := os.Create("../../testdata/examples.txt")
+		if err != nil {
+			t.Fatalf("Failed to update examples.txt: %v", err)
+		}
+		defer func(file *os.File) {
+			_ = file.Close()
+		}(file)
+
+		writer := bufio.NewWriter(file)
+		for _, line := range validLines {
+			_, err := writer.WriteString(line + "\n")
+			if err != nil {
+				t.Fatalf("Failed to write to examples.txt: %v", err)
+			}
+		}
+		_ = writer.Flush()
+	}
 }