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, 11 insertions, 2 deletions

diff --git a/src/pkg/text/template/parse/parse.go b/src/pkg/text/template/parse/parse.go
index 7ddb6fff1e..6dc2f0fb78 100644
--- a/src/pkg/text/template/parse/parse.go
+++ b/src/pkg/text/template/parse/parse.go
@@ -349,10 +349,13 @@ func (t *Tree) pipeline(context string) (pipe *PipeNode) {
 	pipe = newPipeline(t.lex.lineNumber(), decl)
 	for {
 		switch token := t.next(); token.typ {
-		case itemRightDelim:
+		case itemRightDelim, itemRightParen:
 			if len(pipe.Cmds) == 0 {
 				t.errorf("missing value for %s", context)
 			}
+			if token.typ == itemRightParen {
+				t.backup()
+			}
 			return
 		case itemBool, itemCharConstant, itemComplex, itemDot, itemField, itemIdentifier,
 			itemNumber, itemNil, itemRawString, itemString, itemVariable:
@@ -456,11 +459,17 @@ func (t *Tree) command() *CommandNode {
 Loop:
 	for {
 		switch token := t.next(); token.typ {
-		case itemRightDelim:
+		case itemRightDelim, itemRightParen:
 			t.backup()
 			break Loop
 		case itemPipe:
 			break Loop
+		case itemLeftParen:
+			p := t.pipeline("parenthesized expression")
+			if t.next().typ != itemRightParen {
+				t.errorf("missing right paren in parenthesized expression")
+			}
+			cmd.append(p)
 		case itemError:
 			t.errorf("%s", token.val)
 		case itemIdentifier: