From 8ee8cef4916f632484504c6afa2406c40abeb221 Mon Sep 17 00:00:00 2001
From: James Littlejohn <thessem@gmail.com>
Date: Fri, 21 Jul 2017 10:36:44 +1000
Subject: [PATCH 1/2] Return errors applying filters as Render errors

---
 expressions/builders.go    |  5 ++++-
 expressions/expressions.go |  2 ++
 expressions/filters.go     | 10 ++++++++++
 3 files changed, 16 insertions(+), 1 deletion(-)

diff --git a/expressions/builders.go b/expressions/builders.go
index cd79cdc..40c621f 100644
--- a/expressions/builders.go
+++ b/expressions/builders.go
@@ -22,7 +22,10 @@ func makeFilter(fn valueFn, name string, args []valueFn) valueFn {
 	return func(ctx Context) evaluator.Value {
 		result, err := ctx.ApplyFilter(name, fn, args)
 		if err != nil {
-			panic(err)
+			panic(FilterError{
+				FilterName: name,
+				Err:        err,
+			})
 		}
 		return evaluator.ValueOf(result)
 	}
diff --git a/expressions/expressions.go b/expressions/expressions.go
index 9f1669c..b70fcb6 100644
--- a/expressions/expressions.go
+++ b/expressions/expressions.go
@@ -53,6 +53,8 @@ func (e expression) Evaluate(ctx Context) (out interface{}, err error) {
 				err = e
 			case UndefinedFilter:
 				err = e
+			case FilterError:
+				err = e
 			default:
 				panic(r)
 			}
diff --git a/expressions/filters.go b/expressions/filters.go
index 60779b2..62a136c 100644
--- a/expressions/filters.go
+++ b/expressions/filters.go
@@ -21,6 +21,16 @@ func (e UndefinedFilter) Error() string {
 	return fmt.Sprintf("undefined filter %q", string(e))
 }
 
+// FilterError is the error returned by a filter when it is applied
+type FilterError struct {
+	FilterName string
+	Err        error
+}
+
+func (e FilterError) Error() string {
+	return fmt.Sprintf("error applying filter %q (%q)", e.FilterName, e.Err)
+}
+
 type valueFn func(Context) evaluator.Value
 
 // AddFilter adds a filter to the filter dictionary.

From 3d99b41252afd108c14583383f0fd6b02b21537c Mon Sep 17 00:00:00 2001
From: James Littlejohn <thessem@gmail.com>
Date: Fri, 21 Jul 2017 10:57:52 +1000
Subject: [PATCH 2/2] Add test

---
 expressions/expressions_test.go | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/expressions/expressions_test.go b/expressions/expressions_test.go
index 033ee49..f381c52 100644
--- a/expressions/expressions_test.go
+++ b/expressions/expressions_test.go
@@ -1,6 +1,7 @@
 package expressions
 
 import (
+	"errors"
 	"fmt"
 	"strings"
 	"testing"
@@ -134,6 +135,10 @@ func TestEvaluateString(t *testing.T) {
 
 	_, err = EvaluateString("1 | undefined_filter", ctx)
 	require.Error(t, err)
+
+	cfg.AddFilter("error", func(input interface{}) (string, error) { return "", errors.New("test error") })
+	_, err = EvaluateString("1 | error", ctx)
+	require.Error(t, err)
 }
 
 func TestClosure(t *testing.T) {