exp/template: support field syntax on maps
While using exp/template in practice, the syntax for
indexing values using the "index" action was found to be
very inconvenient for frequent use when handling dynamic
data from maps such as the ones used with json and yaml,
that use a type like map[string]interface{}.
For these kinds of maps, the default handling of fields as
{{.Field}} makes the task of handling the several references
significantly more pleasant and elegant, and is equivalent
to what's currently done in the "template" package and in
other external packages (e.g. mustache).
Even with this change, the index action is still relevant
as it allows indexing maps in other scenarios where keys
wouldn't be valid field names.
R=golang-dev, r, gustavo
CC=golang-dev
https://golang.org/cl/4898043
diff --git a/src/pkg/exp/template/doc.go b/src/pkg/exp/template/doc.go
index 46a3804..f65946a 100644
--- a/src/pkg/exp/template/doc.go
+++ b/src/pkg/exp/template/doc.go
@@ -101,6 +101,17 @@
.Field1.Field2
Fields can also be evaluated on variables, including chaining:
$x.Field1.Field2
+ - The name of a key of the data, which must be a map, preceded
+ by a period, such as
+ .Key
+ The result is the map element value indexed by the key.
+ Key invocations may be chained and combined with fields to any
+ depth:
+ .Field1.Key1.Field2.Key2
+ Although the key must be an alphanumeric identifier, unlike with
+ field names they do not need to start with an upper case letter.
+ Keys can also be evaluated on variables, including chaining:
+ $x.key1.key2
- The name of a niladic method of the data, preceded by a period,
such as
.Method
@@ -109,9 +120,9 @@
any type) or two return values, the second of which is an os.Error.
If it has two and the returned error is non-nil, execution terminates
and an error is returned to the caller as the value of Execute.
- Method invocations may be chained and combined with fields
+ Method invocations may be chained and combined with fields and keys
to any depth:
- .Field1.Method1.Field2.Method2
+ .Field1.Key1.Method1.Field2.Key2.Method2
Methods can also be evaluated on variables, including chaining:
$x.Method1.Field
- The name of a niladic function, such as
diff --git a/src/pkg/exp/template/exec.go b/src/pkg/exp/template/exec.go
index 06e5d2b..08cb390 100644
--- a/src/pkg/exp/template/exec.go
+++ b/src/pkg/exp/template/exec.go
@@ -394,13 +394,14 @@
if method, ok := methodByName(ptr, fieldName); ok {
return s.evalCall(dot, method, fieldName, args, final)
}
+ hasArgs := len(args) > 1 || final.IsValid()
// It's not a method; is it a field of a struct?
receiver, isNil := indirect(receiver)
if receiver.Kind() == reflect.Struct {
tField, ok := receiver.Type().FieldByName(fieldName)
if ok {
field := receiver.FieldByIndex(tField.Index)
- if len(args) > 1 || final.IsValid() {
+ if hasArgs {
s.errorf("%s is not a method but has arguments", fieldName)
}
if tField.PkgPath == "" { // field is exported
@@ -408,6 +409,16 @@
}
}
}
+ // If it's a map, attempt to use the field name as a key.
+ if receiver.Kind() == reflect.Map {
+ nameVal := reflect.ValueOf(fieldName)
+ if nameVal.Type().AssignableTo(receiver.Type().Key()) {
+ if hasArgs {
+ s.errorf("%s is not a method but has arguments", fieldName)
+ }
+ return receiver.MapIndex(nameVal)
+ }
+ }
if isNil {
s.errorf("nil pointer evaluating %s.%s", typ, fieldName)
}
diff --git a/src/pkg/exp/template/exec_test.go b/src/pkg/exp/template/exec_test.go
index 8a610da..82f56e1 100644
--- a/src/pkg/exp/template/exec_test.go
+++ b/src/pkg/exp/template/exec_test.go
@@ -39,6 +39,8 @@
MSI map[string]int
MSIone map[string]int // one element, for deterministic output
MSIEmpty map[string]int
+ MXI map[interface{}]int
+ MII map[int]int
SMSI []map[string]int
// Empty interfaces; used to see if we can dig inside one.
Empty0 interface{} // nil
@@ -85,6 +87,8 @@
SB: []bool{true, false},
MSI: map[string]int{"one": 1, "two": 2, "three": 3},
MSIone: map[string]int{"one": 1},
+ MXI: map[interface{}]int{"one": 1},
+ MII: map[int]int{1: 1},
SMSI: []map[string]int{
{"one": 1, "two": 2},
{"eleven": 11, "twelve": 12},
@@ -211,6 +215,14 @@
{".X", "-{{.X}}-", "-x-", tVal, true},
{".U.V", "-{{.U.V}}-", "-v-", tVal, true},
+ // Fields on maps.
+ {"map .one", "{{.MSI.one}}", "1", tVal, true},
+ {"map .two", "{{.MSI.two}}", "2", tVal, true},
+ {"map .NO", "{{.MSI.NO}}", "<no value>", tVal, true},
+ {"map .one interface", "{{.MXI.one}}", "1", tVal, true},
+ {"map .WRONG args", "{{.MSI.one 1}}", "", tVal, false},
+ {"map .WRONG type", "{{.MII.one}}", "", tVal, false},
+
// Dots of all kinds to test basic evaluation.
{"dot int", "<{{.}}>", "<13>", 13, true},
{"dot uint", "<{{.}}>", "<14>", uint(14), true},
