text/template: provide a mechanism for options

Add one option, which is the motivating example, a way to control
what happens when a map is indexed with a key that is not in the map.
Rather than do something specific for that case, we provide a simple
general option mechanism to avoid adding API if something else
comes up. This general approach also makes it easy for html/template
to track (and adapt, should that become important).

New method: Option(option string...). The option strings are key=value
pairs or just simple strings (no =).

New option:

 missingkey: Control the behavior during execution if a map is
 indexed with a key that is not present in the map.
	"missingkey=default" or "missingkey=invalid"
		The default behavior: Do nothing and continue execution.
		If printed, the result of the index operation is the string
		"<no value>".
	"missingkey=zero"
		The operation returns the zero value for the map type's element.
	"missingkey=error"
		Execution stops immediately with an error.

Fixes #6288.

Change-Id: Id811e2b99dc05aff324d517faac113ef3c25293a
Reviewed-on: https://go-review.googlesource.com/8462
Reviewed-by: Robert Griesemer <gri@golang.org>

1 files changed, 12 insertions, 1 deletions

diff --git a/src/text/template/exec.go b/src/text/template/exec.go
index 613a778188..e6e1287993 100644
--- a/src/text/template/exec.go
+++ b/src/text/template/exec.go
@@ -519,7 +519,18 @@ func (s *state) evalField(dot reflect.Value, fieldName string, node parse.Node,
 			if hasArgs {
 				s.errorf("%s is not a method but has arguments", fieldName)
 			}
-			return receiver.MapIndex(nameVal)
+			result := receiver.MapIndex(nameVal)
+			if !result.IsValid() {
+				switch s.tmpl.option.missingKey {
+				case mapInvalid:
+					// Just use the invalid value.
+				case mapZeroValue:
+					result = reflect.Zero(receiver.Type().Elem())
+				case mapError:
+					s.errorf("map has no entry for key %q", fieldName)
+				}
+			}
+			return result
 		}
 	}
 	s.errorf("can't evaluate field %s in type %s", fieldName, typ)