Catch unimplemented panics

expressions/evaluator.go

@@ -1,6 +1,20 @@
 package expressions
 
-func (e expression) Evaluate(ctx Context) (interface{}, error) {
+import "github.com/osteele/liquid/errors"
+
+func (e expression) Evaluate(ctx Context) (out interface{}, err error) {
+	defer func() {
+		if r := recover(); r != nil {
+			switch e := r.(type) {
+			case UnimplementedError:
+				err = e
+			case errors.UndefinedFilter:
+				err = e
+			default:
+				panic(r)
+			}
+		}
+	}()
 	return e.evaluator(ctx), nil
 }
 

expressions/filters.go

@@ -3,7 +3,6 @@ package expressions
 import (
 	"fmt"
 	"reflect"
-	"runtime/debug"
 
 	"github.com/osteele/liquid/errors"
 	"github.com/osteele/liquid/generics"
@@ -49,7 +48,7 @@ func makeFilter(f valueFn, name string, param valueFn) valueFn {
 				case generics.GenericError:
 					panic(InterpreterError(e.Error()))
 				default:
-					fmt.Println(string(debug.Stack()))
+					// fmt.Println(string(debug.Stack()))
 					panic(e)
 				}
 			}

expressions/parser.go

@@ -21,6 +21,12 @@ type ParseError string
 
 func (e ParseError) Error() string { return string(e) }
 
+type UnimplementedError string
+
+func (e UnimplementedError) Error() string {
+	return fmt.Sprintf("unimplemented %s", string(e))
+}
+
 // Expression is a parsed expression.
 type Expression interface {
 	// Evaluate evaluates an expression in a context.
@@ -38,6 +44,8 @@ func Parse(source string) (expr Expression, err error) {
 			switch e := r.(type) {
 			case ParseError:
 				err = e
+			case UnimplementedError:
+				err = e
 			case errors.UndefinedFilter:
 				err = e
 			default:

filters/filters.go

@@ -24,13 +24,13 @@ func DefineStandardFilters() {
 	// dates
 	expressions.DefineFilter("date", func(in, format interface{}) interface{} {
 		if format != nil {
-			panic("date conversion format is not implemented")
+			panic(expressions.UnimplementedError("date conversion format"))
 		}
 		switch in := in.(type) {
 		case time.Time:
 			return in.Format("Mon, Jan 2, 06")
 		default:
-			panic("unimplemented date conversion")
+			panic(expressions.UnimplementedError("date conversion"))
 		}
 	})