func Return(fnRet func(ctx *Context) (v interface{}, err error)) Ruler {
	return ret(fnRet)
}
Return returns a matching unit that returns fnRet(ctx).
package bpl
import (
	"bufio"
	"fmt"
	"reflect"
	"strings"
	"github.com/qiniu/x/log"
)
type ret func(ctx *Context) (v interface{}, err error)
func (p ret) Match(in *bufio.Reader, ctx *Context) (v interface{}, err error) {
	v, err = p(ctx)
	if err != nil {
		return
	}
	ctx.dom = v
	return
}
func (p ret) RetType() reflect.Type {
	return TyInterface
}
func (p ret) SizeOf() int {
	return -1
}