Hiroyuki Yamanaka

Golang Standard library (text.template Execute)

sample

package main

import (
	"log"
	"os"
	"text/template"
)

func main() {
	// Define a template.
	const letter = `
Dear ,

It was a pleasure to see you at the wedding.
It is a shame you couldn't make it to the wedding.
Thank you for the lovely .

Best wishes,
Josie
`

	// Prepare some data to insert into the template.
	type Recipient struct {
		Name, Gift string
		Attended   bool
	}
	var recipients = []Recipient{
		{"Aunt Mildred", "bone china tea set", true},
		{"Uncle John", "moleskin pants", false},
		{"Cousin Rodney", "", false},
	}

	// Create a new template and parse the letter into it.
	t := template.Must(template.New("letter").Parse(letter))

	// Execute the template for each recipient.
	for _, r := range recipients {
		err := t.Execute(os.Stdout, r)
		if err != nil {
			log.Println("executing template:", err)
		}
	}

}

Execute

func (t *Template) Execute(wr io.Writer, data interface{}) error {
	return t.execute(wr, data)
}

Template

type Template struct {
	name string
	*parse.Tree
	*common
	leftDelim  string
	rightDelim string
}

execute

func (t *Template) execute(wr io.Writer, data interface{}) (err error) {
	defer errRecover(&err)
	value, ok := data.(reflect.Value)
	if !ok {
		value = reflect.ValueOf(data)
	}
	state := &state{
		tmpl: t,
		wr:   wr,
		vars: []variable$,
	}
	if t.Tree == nil || t.Root == nil {
		state.errorf("%q is an incomplete or empty template", t.Name())
	}
	state.walk(value, t.Root)
	return
}

errRecover

func errRecover(errp *error) {
	e := recover()
	if e != nil {
		switch err := e.(type) {
		case runtime.Error:
			panic(e)
		case writeError:
			*errp = err.Err // Strip the wrapper.
		case ExecError:
			*errp = err // Keep the wrapper.
		default:
			panic(e)
		}
	}
}

state

type state struct {
	tmpl  *Template
	wr    io.Writer
	node  parse.Node // current node, for errors
	vars  []variable // push-down stack of variable values.
	depth int        // the height of the stack of executing templates.
}

variable

type variable struct {
	name  string
	value reflect.Value
}

parse.Node

• https://golang.org/pkg/text/template/parse/#Node

type Node interface {
	Type() NodeType
	String() string
	Copy() Node
	Position() Pos // byte position of start of node in full original input string
	tree() *Tree
}

state.walk

func (s *state) walk(dot reflect.Value, node parse.Node) {
	s.at(node)
	switch node := node.(type) {
	case *parse.ActionNode:
		// Do not pop variables so they persist until next end.
		// Also, if the action declares variables, don't print the result.
		val := s.evalPipeline(dot, node.Pipe)
		if len(node.Pipe.Decl) == 0 {
			s.printValue(node, val)
		}
	case *parse.IfNode:
		s.walkIfOrWith(parse.NodeIf, dot, node.Pipe, node.List, node.ElseList)
	case *parse.ListNode:
		for _, node := range node.Nodes {
			s.walk(dot, node)
		}
	case *parse.RangeNode:
		s.walkRange(dot, node)
	case *parse.TemplateNode:
		s.walkTemplate(dot, node)
	case *parse.TextNode:
		if _, err := s.wr.Write(node.Text); err != nil {
			s.writeError(err)
		}
	case *parse.WithNode:
		s.walkIfOrWith(parse.NodeWith, dot, node.Pipe, node.List, node.ElseList)
	default:
		s.errorf("unknown node: %s", node)
	}
}

state.at

func (s *state) at(node parse.Node) {
	s.node = node
}

state.evalPipeline

func (s *state) evalPipeline(dot reflect.Value, pipe *parse.PipeNode) (value reflect.Value) {
	if pipe == nil {
		return
	}
	s.at(pipe)
	for _, cmd := range pipe.Cmds {
		value = s.evalCommand(dot, cmd, value) // previous value is this one's final arg.
		// If the object has type interface{}, dig down one level to the thing inside.
		if value.Kind() == reflect.Interface && value.Type().NumMethod() == 0 {
			value = reflect.ValueOf(value.Interface()) // lovely!
		}
	}
	for _, variable := range pipe.Decl {
		s.push(variable.Ident[0], value)
	}
	return value
}