text/template: allow grouping of pipelines using parentheses

Based on work by Russ Cox. From his CL:

        This is generally useful but especially helpful when trying
        to use the built-in boolean operators.  It lets you write:

        {{if not (f 1)}} foo {{end}}
        {{if and (f 1) (g 2)}} bar {{end}}
        {{if or (f 1) (g 2)}} quux {{end}}

        instead of

        {{if f 1 | not}} foo {{end}}
        {{if f 1}}{{if g 2}} bar {{end}}{{end}}
        {{$do := 0}}{{if f 1}}{{$do := 1}}{{else if g 2}}{{$do := 1}}{{end}}{{if $do}} quux {{end}}

The result can be a bit LISPy but the benefit in expressiveness and readability
for such a small change justifies it.

I believe no changes are required to html/template.

Fixes #3276.

R=golang-dev, adg, rogpeppe, minux.ma
CC=golang-dev
https://golang.org/cl/6482056

1 files changed, 12 insertions, 0 deletions

diff --git a/src/pkg/text/template/exec_test.go b/src/pkg/text/template/exec_test.go
index 95e0592df8..7f60dcafa5 100644
--- a/src/pkg/text/template/exec_test.go
+++ b/src/pkg/text/template/exec_test.go
@@ -337,6 +337,9 @@ var execTests = []execTest{
 	{"pipeline", "-{{.Method0 | .Method2 .U16}}-", "-Method2: 16 M0-", tVal, true},
 	{"pipeline func", "-{{call .VariadicFunc `llo` | call .VariadicFunc `he` }}-", "-<he+<llo>>-", tVal, true},
 
+	// Parenthesized expressions
+	{"parens in pipeline", "{{printf `%d %d %d` (1) (2 | add 3) (add 4 (add 5 6))}}", "1 5 15", tVal, true},
+
 	// If.
 	{"if true", "{{if true}}TRUE{{end}}", "TRUE", tVal, true},
 	{"if false", "{{if false}}TRUE{{else}}FALSE{{end}}", "FALSE", tVal, true},
@@ -524,6 +527,14 @@ func vfunc(V, *V) string {
 	return "vfunc"
 }
 
+func add(args ...int) int {
+	sum := 0
+	for _, x := range args {
+		sum += x
+	}
+	return sum
+}
+
 func stringer(s fmt.Stringer) string {
 	return s.String()
 }
@@ -531,6 +542,7 @@ func stringer(s fmt.Stringer) string {
 func testExecute(execTests []execTest, template *Template, t *testing.T) {
 	b := new(bytes.Buffer)
 	funcs := FuncMap{
+		"add":      add,
 		"count":    count,
 		"dddArg":   dddArg,
 		"oneArg":   oneArg,