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, 4 insertions, 0 deletions

diff --git a/src/pkg/text/template/exec.go b/src/pkg/text/template/exec.go
index a041351448..1739a86179 100644
--- a/src/pkg/text/template/exec.go
+++ b/src/pkg/text/template/exec.go
@@ -564,6 +564,8 @@ func (s *state) evalArg(dot reflect.Value, typ reflect.Type, n parse.Node) refle
 		return s.validateType(s.evalFieldNode(dot, arg, []parse.Node{n}, zero), typ)
 	case *parse.VariableNode:
 		return s.validateType(s.evalVariableNode(dot, arg, nil, zero), typ)
+	case *parse.PipeNode:
+		return s.validateType(s.evalPipeline(dot, arg), typ)
 	}
 	switch typ.Kind() {
 	case reflect.Bool:
@@ -666,6 +668,8 @@ func (s *state) evalEmptyInterface(dot reflect.Value, n parse.Node) reflect.Valu
 		return reflect.ValueOf(n.Text)
 	case *parse.VariableNode:
 		return s.evalVariableNode(dot, n, nil, zero)
+	case *parse.PipeNode:
+		return s.evalPipeline(dot, n)
 	}
 	s.errorf("can't handle assignment of %s to empty interface argument", n)
 	panic("not reached")