fix ConstExpr to handle custom functions returned error (#239)

Author: brenank

expr_test.go

@@ -1160,7 +1160,7 @@ func TestExpr_call_floatarg_func_with_int(t *testing.T) {
 	}
 }
 
-func TestConstExpr_error(t *testing.T) {
+func TestConstExpr_error_panic(t *testing.T) {
 	env := map[string]interface{}{
 		"divide": func(a, b int) int { return a / b },
 	}
@@ -1174,6 +1174,31 @@ func TestConstExpr_error(t *testing.T) {
 	require.Equal(t, "compile error: integer divide by zero (1:5)\n | 1 + divide(1, 0)\n | ....^", err.Error())
 }
 
+type divideError struct{ Message string }
+
+func (e divideError) Error() string {
+	return e.Message
+}
+func TestConstExpr_error_as_error(t *testing.T) {
+	env := map[string]interface{}{
+		"divide": func(a, b int) (int, error) {
+			if b == 0 {
+				return 0, divideError{"integer divide by zero"}
+			}
+			return a / b, nil
+		},
+	}
+
+	_, err := expr.Compile(
+		`1 + divide(1, 0)`,
+		expr.Env(env),
+		expr.ConstExpr("divide"),
+	)
+	require.Error(t, err)
+	require.Equal(t, "integer divide by zero", err.Error())
+	require.IsType(t, divideError{}, err)
+}
+
 func TestConstExpr_error_wrong_type(t *testing.T) {
 	env := map[string]interface{}{
 		"divide": 0,

optimizer/const_expr.go

@@ -2,12 +2,15 @@ package optimizer
 
 import (
 	"fmt"
-	. "github.com/antonmedv/expr/ast"
-	"github.com/antonmedv/expr/file"
 	"reflect"
 	"strings"
+
+	. "github.com/antonmedv/expr/ast"
+	"github.com/antonmedv/expr/file"
 )
 
+var errorType = reflect.TypeOf((*error)(nil)).Elem()
+
 type constExpr struct {
 	applied bool
 	err     error
@@ -70,7 +73,12 @@ func (c *constExpr) Exit(node *Node) {
 			}
 
 			out := fn.Call(in)
-			constNode := &ConstantNode{Value: out[0].Interface()}
+			value := out[0].Interface()
+			if len(out) == 2 && out[1].Type() == errorType && !out[1].IsNil() {
+				c.err = out[1].Interface().(error)
+				return
+			}
+			constNode := &ConstantNode{Value: value}
 			patch(constNode)
 		}
 	}